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Managing the almond and stone fruit replant disease complex with less soil fumigant

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Authors

Greg T. Browne , USDA ARS, UC Davis
Bruce D. Lampinen, UCCE, UC Davis
Brent A. Holtz, UCCE San Joaquin County
David A. Doll, UCCE Merced County
Shrinivasa K. Upadhyaya, UC Davis
Leigh S. Schmidt, USDA ARS, UC Davis
Ravindra G. Bhat, UC Davis
Vasu Udompetaikul, UC Davis, King Mongkut's Institute of Technology
Robert W. Coates, UC Davis
Bradley D. Hanson, UCCE, UC Davis
Karen M. Klonsky, UCCE, UC Davis
Suduan Gao, USDA-ARS
Dong Wang, USDA-ARS
Matt Gillis, TriCal Inc.
James S. Gerik, USDA-ARS
R. Scott Johnson, UC Kearney Agricultural Research and Extension Center

Publication Information

California Agriculture 67(3):128-138. https://doi.org/10.3733/ca.v067n03p128

Published online July 01, 2013

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Abstract

As much as one-third of California's almond and stone fruit acreage is infested with potentially debilitating plant parasitic nematodes, and even more of the land is impacted by Prunus replant disease (PRD), a poorly understood soilborne disease complex that suppresses early growth and cumulative yield in replanted almond and peach orchards. Preplant soil fumigation has controlled these key replant problems, but the traditional fumigant of choice, methyl bromide, has been phased out, and other soil fumigants are increasingly regulated and expensive. We tested fumigant and nonfumigant alternatives to methyl bromide in multiple-year replant trials. Costs and benefits were evaluated for alternative fumigants applied by shanks in conventional strip and full-coverage treatments and applied by shanks or drip in novel spot treatments that targeted tree planting sites. Short-term sudangrass rotation and prudent rootstock selection were examined as nonfumigant approaches to managing PRD. Trial results indicated that integrations of the treatments may acceptably control PRD with relatively little soil fumigant.

Full text

Approximately 1 million acres of California's best agricultural land are devoted to production of almonds and stone fruits (USDA 2011), and sustained high production from this land requires that the orchards be replanted every 15 to 25 years, depending on the production system. Research has documented myriad problems that can suppress growth and productivity in such replanted orchards (Bent et al. 2009; Browne et al. 2006; Larsen 1995; McKenry 1996, 1999; Westerdahl and McKenry 2002). Abiotic soil factors related to previous crop production, such as compaction, salinity, suboptimal pH, nutritional imbalances and herbicide residues, can compromise the performance of replanted orchards, but many of these problems can be avoided or remedied without great difficulty or expense.

Biotic replant problems, including plant parasitic nematodes and Prunus replant disease (PRD), can pose more of a challenge. Plant parasitic nematodes infest as much as one-third of California's almond and stone fruit acreage (McKenry and Kretsch 1987) and have the potential to compromise all phases of an orchard's productive life by inflicting root damage. Several rootstocks for almonds and stone fruit have shown genetic resistance to root knot nematodes, but little resistance has been demonstrated against the other two major nematode pests affecting these crops, the ring nematode and the root lesion nematode (McKenry 2007). PRD, which is much more widespread than nematode damage on almonds and stone fruits, is a poorly understood soilborne disease complex that suppresses early growth and cumulative yield in replanted almond and peach orchards (Bent et al. 2009; Browne et al. 2006). It afflicts successive generations of almonds and stone fruit planted at the same location and is associated with poor health of the trees’ fine roots and incidence of several plant-parasitic fungi and oomycetes. The severity of the disease varies greatly among orchards, but it is observed most commonly on loam, sandy loam, and sand soil textures in California. PRD can occur on its own or in combination with other replant problems.

Preplant soil fumigation has been an effective means of control for biological replant problems, but fumigant usage today is being challenged on several fronts, including the phase-out of methyl bromide (US EPA 2012), township caps on the use of the fumigant 1,3-dichloropropene (1,3-D) (Carpenter et al. 2001), volatile organic compound regulations under the U.S. Clean Air Act (Cal-DPR 2012), and increasingly restrictive buffer zones (see pages 122 127 for explanation and more details on each of these topics). Due to required buffer zones (to protect bystanders from unintended exposure), many fields have large areas that cannot be treated using conventional fumigation.

Rootstocks for almonds and stone fruits were tested for their resistance to the Prunus replant disease complex near Parlier, CA. Shown are a plot of PRD-affected rootstocks in nonfumigated replant soil, left, and a plot of relatively healthy rootstocks grown in soil preplant fumigated with 1,3-D:Pic 63:65 (Telone C35), right.

Reducing dependence on fumigation

Over the long term, breeding of rootstocks that broadly resist or tolerate soilborne pathogens and development of cultural practices that effectively remediate replant problems may remove dependence on soil fumigation. In this article we report on the effectiveness of options currently available for control of the most widespread almond and stone fruit replant problem, PRD. We examine the potential contributions of optimized soil fumigation methods, crop rotation and rootstock selection to the integrated management of PRD and reduced fumigant use.

Almond replant trials

As part of our research, we established two almond replant trials in Madera County focused on fumigant-based options for control of PRD. The trials were designed to help optimize soil fumigation practices by identifying fumigant formulations that are particularly effective for control of the disease complex and by determining the effectiveness of different fumigant rates and novel fumigant delivery methods. Regarding the latter emphasis, GPS-based software and hardware systems were developed recently to deliver spot fumigation treatments by tractor to tree planting sites (Coates et al. 2007; Udompetaikul et al. in press). The new spot treatment system was designed for planning, mapping and treating all tree sites in a replacement orchard and is considered to be much safer and faster than spot fumigation treatments applied with a hand-held probe. Spot treatment can reduce the amount of fumigant required to treat an orchard acre by 50% to 90%, but evaluations of the GPS-controlled tractor application system were needed.

Two orchards in California, one near Firebaugh and the other near Madera, were selected for replanting experiments. The Firebaugh trial included soils of Dinuba fine sandy loam, El Peco fine sandy loam and Fresno fine sandy loam, whereas the Madera trial included El Peco, Fresno, and Lewis sandy loams and Tujunga loamy sand. Lands for the Firebaugh and Madera replant trials were cleared of old almond orchards grown on ‘Nemaguard’ rootstock in the summers of 2006 and 2007, respectively, using conventional practices. After removal, the old trees were chipped (the removed tree residue was ground up by a tub grinder and hauled away for energy generation or other uses). To reduce soil compaction, the cleared lands were ripped to a depth of 5 to 6 feet and then smoothed. In preparation for soil fumigation, the lands were then sprinkler irrigated with about 1.5 inches of water to reduce the potential for fumigant emissions to escape into the atmosphere.

Fumigants were applied to the soil in October 2006 for the Firebaugh trial and October 2007 for the Madera trial. The fumigant formulations were:

  • methyl bromide (MB), 98%; chloropicrin (Pic), 2%, as a warning agent (MBC Concentrate, TriCal Inc.)

  • 1,3-D, 98% (Telone II)

  • chloropicrin (Pic), 99% (Tri-Clor)

  • mixtures of 1,3-D:Pic, including 63:35 (Telone C35) and 39:60, (Pic-Clor 60)

  • iodomethane (IM):Pic 50:50 (Midas)

In each orchard, all preplant soil fumigation treatments were applied by TriCal Inc. (Hollister, CA) to plots that would accommodate a width of three tree rows (66 feet) and a length of 10 tree spaces (140 to 170 feet). The MB treatments were applied with a conventional MB rig (TriCal Inc.), and the system injected fumigant at soil depths of 18 to 20 inches through two shanks spaced 60 inches apart; one pass was made for each tree row, effectively treating a 10-foot-wide strip. The other fumigant treatments were applied with a Telone rig (TriCal Inc.), which also injected fumigants at soil depths of 18 to 20 inches, but through three or five shanks (depending on the treatment). The shanks were spaced 20 inches apart and tipped with horizontal “wing” attachments. Fumigant was released from two points 8 inches apart, one behind each wing tip. The rig was used to apply three types of treatments: single-pass strip treatments, in which fumigant was applied only to 8.3-foot-wide strips centered over future tree rows; full-coverage treatments, in which the entire area of a replicate plot received fumigant; and spot treatments, in which either 8.3-foot-wide by 8-foot-long (Firebaugh trial) or 5-foot-wide by 7-foot-long (Madera trial) rectangular areas centered over future tree sites were treated.

The spot treatments were administered via a Telone rig retrofitted with GPS-based software and hardware to rapidly turn shank injections off and on as the tractor traveled down the future tree rows with the shanks remaining in the soil (Coates et al. 2007; Upadhyaya et al. 2009; Udompetaikul et al. in press). Before the spot applications began, the software was used to create a virtual map of each orchard's future tree sites according to desired row and tree spacings and planting patterns (rectangular and diamond planting patterns were used in the Firebaugh and Madera trials, respectively), and the desired width and length of the zones to be fumigated around each mapped tree planting site were selected.

The control plots were ripped with Telone rig shanks but received no fumigant. Each treatment was applied to several replicate plots (six at Firebaugh and five at Madera). The plots were randomized in a complete block design.

The Firebaugh trial was replanted in January 2007, and the Madera trial in January 2008. In each replicate plot, a center row was replanted to ‘Nonpareil’ almond and the two adjacent rows were replanted to other varieties selected for cross-pollination. In all cases, the rootstock for ‘Nonpareil’ was ‘Nemaguard’ peach. Efficacy of the treatments was assessed according to the percentage of incident photosynthetically active radiation (PAR) intercepted by the ‘Nonpareil’ tree canopies in midsummer and nut yields collected starting in the third growing season and annually thereafter. To measure the PAR interception, we used a new mobile platform that provides a good estimate of the yield potential of tree canopies (Lampinen et al. 2012).

Almond replant trial results

In both the Firebaugh and Madera trials, most of the preplant soil fumigation treatments showed enhanced canopy growth through the first and second yield years (the third and fourth growing seasons after planting, respectively) when compared to the nonfumigated control (table 1; P = 0.002 to < 0.0001 for effect fumigant treatment).

TABLE 1. Results summary, almond replant trials in Madera County

At Firebaugh, compared to the control, preplant strip treatments with MB and 1,3-D boosted PAR interception by 20% and 39%, respectively, in yield year 1 (table 1). Thereafter, these fumigation treatments had little effect on PAR interception. Other fumigant treatments at Firebaugh, including Pic and combinations of Pic with 1,3-D or IM, were generally more effective than the MB and 1,3-D treatments, boosting mean PAR interception by 56% to 97% in yield year 1 and 11% to 22% in yield year 2 compared to the control. By yield year 3 (the fifth growing season after planting), however, none of the treatments affected PAR interception (table 1; P = 0.24).

First-year impact of Prunus replant disease at the Firebaugh replant trial; stunted trees in the foreground row were planted in plot of nonfumigated replant soil, while trees in the background rows were planted in preplant fumigated soil.

In the Madera trial, PAR responses to fumigation were generally more similar among the treatments than in the Firebaugh trial (table 1). At Madera, increases in PAR interception due to preplant fumigation ranged from 34% to 68% in yield year 1 and 35% to 69% in yield year 2 compared to the nontreated control (table 1). The increases in PAR interception between yield years 1 and 2 were generally less at Madera than at Firebaugh. Pressure bomb readings taken in yield years 1 and 2 at Madera suggested that tree water stress was responsible for the lesser growth.

First-year impact of Prunus replant disease at the Madera replant trial; stunted trees in the foreground were planted in plot of nonfumigated replant soil, while larger trees in the background of the same row were in plot of preplant fumigated soil.

In both trials, using the assumption of a net price (i.e., the price after subtraction of nut hauling, hulling and marketing costs) of $2 per pound of nut meats, increases in PAR interception translated into profitable yield increases for all treatments except MB (table 1). The high cost of the MB treatment was not offset by the relatively poor yield increases it generated. By yield year 2, the MB treatment reduced cumulative net returns by $1,120 and $552 per acre in the Firebaugh and Madera trials, respectively, compared to the control. The full-coverage treatment with 1,3-D:Pic 63:35 resulted in the second greatest and greatest cumulative nut yields over the harvests monitored in the Firebaugh and Madera trials, respectively, but the high cost of the treatment kept the net returns relatively low compared to several other MB-alternative treatments (table 1).

Across both trials, the strip treatments with Pic and combinations of 1,3-D:Pic (63:35 and 39:60) generally afforded greater net returns than other treatments. Although the GPS-controlled spot treatments generated lower net returns than some of the strip treatments, the spot treatments provided greater returns than the strip treatment with 1,3-D alone, which has been an almond and stone fruit industry standard. In terms of dollars of net revenue per pound of fumigant, the spot treatments were generally more efficient than strip or full-coverage treatments (table 1). When a net price of $1.70 per kernel pound was assumed (instead of $2 per pound, for the sake of comparison), all of the MB-alternative treatments still increased net crop revenues, but the returns were again negative for the MB treatment and relatively low for the 1,3-D:Pic 63:35 full-coverage treatment. We intend to continue annual PAR and yield measurements in the Madera and Firebaugh trials. Yields have not converged among the treatments, suggesting that their economic value will continue to sort out over time.

Soil sampling from all replicate plots of the control, MB strip and 1,3-D:Pic 63:35 broadcast treatments detected negligible to small nematode populations in 2009 and 2012. Specifically, in 2009 at Firebaugh, we detected one ring nematode per half pint (250 milliliters) of soil from one MB-treated plot, and no lesion, ring or root knot nematodes from other plots; at Madera, there were three lesion nematodes per half pint (250 milliliters) of soil from one control plot, and no lesion, ring or root knot nematodes from other plots. In 2012 at Firebaugh, we detected no lesion, ring or root knot nematodes; at Madera, we detected 164 and 348 lesion nematodes per half pint (250 milliliters) in two respective control plots, and no lesion, ring or root knot nematodes in other plots. These results suggest that PRD was the dominant replant problem in these trials, but it is possible that plant parasitic nematode populations will build and have future economic impacts.

Despite the long-term uncertainties, our trials indicate that effective preplant soil fumigation can be an essential step in maximizing net revenues in replanted almond orchards, at least when ‘Nemaguard’ rootstock is used in the replanted orchard and PRD is active. Furthermore, our findings suggest that at orchard sites at risk for PRD and not infested with plant parasitic nematodes, growers can increase net revenues by using strip treatments with Pic or mixtures of Pic with 1,3-D instead of treatments with 1,3-D alone. Finally, the efficacies and efficiencies of GPS-controlled spot fumigation treatments indicate that they may have important applications where site or air quality sensitivities permit use of only very low rates of fumigant per acre.

Microplot replant trials

We conducted microplot trials to explore the potential of fallowing and crop rotation to remediate PRD. It was found in replanted apple orchards in Washington state that preplant rotation with wheat as a green manure lessened the severity of apple replant disease (Mazzola and Gu 2000; Mazzola and Mullinix 2005). Also, certain crops such as ‘Piper’ sudangrass have been recommended during fallow periods for suppression of nematode populations (Westerdahl et al. 2010). We investigated the potential for using short-term crop rotation and fallowing to reduce the severity of PRD in California.

For this purpose, microplots were constructed at the San Joaquin Valley Agricultural Sciences Center (SJVASC), U.S. Department of Agriculture–Agricultural Research Service (USDA-ARS), Parlier. The microplots consisted of sections of concrete pipe (24 inches in diameter by 48 inches long) inserted vertically into soil, with the rims protruding approximately 8 inches above the soil surface. The microplots were spaced 3 feet apart, edge to edge, and were filled with Hanford sandy loam soil that had been excavated from 0.3- to 2.5-foot depths in an adjacent peach orchard where trees had expressed PRD.

To test plant response to different soil treatments including preplant crop rotation, researchers established soil microplots by installing 4-foot lengths of 24-inch-diameter concrete pipe vertically into the soil and filling the pipes with soil from a nearby orchard affected with Prunus replant disease.

The soil in the microplots was planted with trees on ‘Nemaguard’ rootstock to maintain PRD induction potential, and the plants were watered with drip irrigation. Soil assays indicated that the soil did not have significant numbers of damaging plant parasitic nematodes.

Eight different treatments were imposed on the microplots in a randomized complete block design; there were five replicate microplots per treatment. The treatments were chosen to simulate remediation options of potential interest to almond and stone fruit growers during orchard replanting (table 2). For example, growers may choose to schedule orchard replacement to accommodate dry fallowing of the land for several months or years before replanting, or, alternatively, to replant quickly, without an extended fallow period. Also, whether or not fallowing is involved, growers typically have the option to fumigate the soil or leave it untreated before replanting. Fallowing and fumigation options were represented in treatments 1 to 4 (table 2). When an orchard-free period is observed before replanting, a rotation crop may be used. We selected treatments 5 to 8 to test some of the crop rotation options (table 2).

TABLE 2. Preplant treatments applied to Parlier microplots filled with soil from a peach orchard affected by Prunus replant disease

Treatment options 1 and 2 have the potential to be completed without losing a season of almond or peach production. Treatments 3 through 8 would typically require the loss of a crop cycle, unless a spring-harvested stone fruit variety was being replaced. If potted trees were to be used for the orchard replanting, it would be possible to complete the rotation with wheat alone (treatment 7) without loss of an almond or stone fruit cropping cycle (potted trees can be planted in late spring). Planting bareroot trees after the wheat rotation would require an undesirable delay. Unless kept in cold storage, bareroot trees are optimally planted by early February.

Details of the microplot trials were as follows: Three separate (repeat) experiments were completed. All three experiments had the same treatments, but the experiments were started successively, one year apart. In each experiment, the summer and fall portions of treatments 1 through 8 were imposed beginning in June of the year the experiment began (nearly 1 year before the microplots would be replanted with ‘Nemaguard’ peach plants.) The summer and fall portions of treatments 1 through 8 were continued until the following November, 4 months before replanting (table 2). During this period, the treatments involved maintaining growth of trees on ‘Nemaguard’ rootstock, dry fallowing (the soil was kept bare by hand-weeding) or growing hybrid corn or ‘Piper’ sudangrass (table 2). The ‘Nemaguard,’ corn and sudangrass plants were drip-irrigated to meet evapotranspiration needs, but the fallowed plots were not irrigated. All plots (including those fallowed) were fertilized periodically with equal amounts of ammonium sulfate fertilizer.

Near the end of the preplant period, in early November, the scions of trees on ‘Nemaguard’ rootstock (in treatments 1 and 2) and the tops of the sudangrass plants (treatments 6 and 8) were removed and then discarded outside the microplots. Also, the corn stalks (treatment 5) were chopped into pieces 2 to 3 inches long and kept within the microplots. The ‘Nemaguard’ and sudangrass root system residues and the corn roots and stubble were turned into the top foot of soil in their respective plots using a shovel to simulate thorough disking. Soil in all other plots was turned in the same manner, and the wheat was planted in its plots (treatments 7 and 8). In mid-November, the soil fumigation treatments were imposed on the appropriate plots using a microfumigation rig; MB plus Pic (50:50 formulation) was injected at 400 pounds per acre at 1 foot below the soil surface. At the end of the winter-spring period, soil in all plots, including those with wheat, was turned over repeatedly to a depth of 1 foot with a shovel to simulate disking.

In each of the three repeat experiments, we assessed efficacy of the preplant remediation treatments by replanting the microplots with ‘Nemaguard’ peach seedlings in the following March (i.e., for each experiment, nearly a year after the experiment's beginning) and measuring accumulated shoot weights of the seedlings the following November. The ‘Nemaguard’ seedlings were watered by drip irrigation to meet evapotranspiration demand and fertilized periodically with ammonium sulfate. All plots received the same irrigation and fertilization schedule, except in cases where soil moisture became excessive due to reduced water use by PRD-affected plants; in such cases, irrigation was briefly withheld from overly wet plots until soil moisture levels were similar among all plots.

Microplot replant trial results

In the three successive microplot trials (fig. 1, experiments 1, 2 and 3), several relatively consistent effects emerged, including the following:

  • Preplant fumigation with MB plus Pic (50:50) consistently improved growth of replanted ‘Nemaguard’ peach seedlings, with or without extra preplant fallowing (fig. 1, treatments 1–4).

  • The extra 5 months of preplant fallowing alone (fig. 1, treatment 3) did not significantly improve ‘Nemaguard’ growth, compared to the nonfallowed, nonfumigated control (treatment 1).

  • A summer rotation with ‘Piper’ sudangrass (fig. 1, treatment 6) significantly improved growth of replanted ‘Nemaguard,’ as compared to fallowed and non-fallowed controls (treatments 1 and 3), but the degree of benefit did not consistently match that achieved by fumigation.

  • Rotations involving corn or wheat (fig. 1, treatments 5, 7 and 8) were sometimes beneficial, as compared to the controls (treatments 1 and 3).

Fig. 1. Effects of preplant fallowing, crop rotation and fumigation on growth on ‘Nemaguard’ peach rootstock in microplot trials near Parlier. Experiments 1, 2 and 3 above were started in June of three successive years (2002, 2003 and 2004). For each experiment, treatment numbers are shown in the top row of x-axis labels; the second row of labels represents the corresponding cropping status of the treatments from June to November (Pe = peach, Fa = fallow, Co = corn, Su = sudangrass); the third row of labels indicates subsequent fumigation treatment (NF = nonfumigated, F = fumigated) and the fourth row of labels indicates subsequent cropping status from November to March (Fa = fallow, Wh = wheat). Vertical bars are 95% confidence intervals.

These results suggest that some crop rotations, and particularly a summer rotation with ‘Piper’ sudangrass, may help growers reduce the severity of PRD and thereby reduce the need for soil fumigation. Orchard validation of some of the microplot findings was completed in a peach replant trial, as described below.

Peach replant trial

Favorable responses to spot and strip fumigation treatments in the almond orchard replant trials and to crop rotation in the microplot trials led to validations in a peach orchard replant trial. For the experiment, plums on ‘Nemaguard’ rootstock were removed from a block at the SJVASC in early July 2007. The land was ripped to a depth of 2 to 3 feet, leveled, pre-irrigated and divided into five main plots, each of which was split in half. Each half of the five main plots measured 72 feet by 140 feet. One half was kept fallow (i.e., maintained relatively weed-free by a combination of cultivation and post-emergence herbicide treatments), while the other half was planted to ‘Piper’ sudangrass as a green manure crop. The sudangrass was grown for 2 months under sprinkler irrigation, then shredded and disked into the ground; the disking operation was extended across the whole field in preparation for preplant soil fumigation treatments.

Soil fumigation treatments were applied in late October 2007. The treatments were assigned randomly to 20-foot-wide by 144-foot-long strip plots that ran across both halves of each main plot (i.e., the halves that had been cropped with sudangrass and those that were fallowed). Each of the fumigation treat

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Author notes

The authors gratefully acknowledge support and assistance from the Pacific Area-Wide Pest Management Program, USDA-ARS, the Almond Board of California, the UC Agricultural Experiment Station, TriCal Inc., Paramount Farming, Agriland Farming, Bauer Farming, Harry Berberian and Sons, Donald Ewy, Mike Stanghellini, Burchell Nursery Inc., and Duarte Nursery Inc.

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Citations

Managing for soil health can suppress pests
Amanda Hodson and Edwin Lewis 2016. California Agriculture 70(3):137
http://dx.doi.org/10.3733/ca.2016a0005

Anaerobic soil disinfestation: A chemical-independent approach to pre-plant control of plant pathogens
S L Strauss and D A Kluepfel 2015. Journal of Integrative Agriculture 14(11):2309
http://dx.doi.org/10.1016/S2095-3119(15)61118-2

Tractor-mounted, GPS-based spot fumigation system manages Prunus replant disease
V. Udompetaikul et al. 2013. California Agriculture 67(4):222
http://dx.doi.org/10.3733/ca.v067n04p222

Organically acceptable practices to improve replant success of temperate tree-fruit crops
Thomas Forge et al. 2016. Scientia Horticulturae 200:205
http://dx.doi.org/10.1016/j.scienta.2016.01.002

Managing the almond and stone fruit replant disease complex with less soil fumigant

Greg T. Browne, Bruce D. Lampinen, Brent A. Holtz, David A. Doll, Shrinivasa K. Upadhyaya, Leigh S. Schmidt, Ravindra G. Bhat, Vasu Udompetaikul, Robert W. Coates, Bradley D. Hanson, Karen M. Klonsky, Suduan Gao, Dong Wang, Matt Gillis, James S. Gerik, R. Scott Johnson
Webmaster Email: wsuckow@ucanr.edu

Managing the almond and stone fruit replant disease complex with less soil fumigant

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Authors

Greg T. Browne , USDA ARS, UC Davis
Bruce D. Lampinen, UCCE, UC Davis
Brent A. Holtz, UCCE San Joaquin County
David A. Doll, UCCE Merced County
Shrinivasa K. Upadhyaya, UC Davis
Leigh S. Schmidt, USDA ARS, UC Davis
Ravindra G. Bhat, UC Davis
Vasu Udompetaikul, UC Davis, King Mongkut's Institute of Technology
Robert W. Coates, UC Davis
Bradley D. Hanson, UCCE, UC Davis
Karen M. Klonsky, UCCE, UC Davis
Suduan Gao, USDA-ARS
Dong Wang, USDA-ARS
Matt Gillis, TriCal Inc.
James S. Gerik, USDA-ARS
R. Scott Johnson, UC Kearney Agricultural Research and Extension Center

Publication Information

California Agriculture 67(3):128-138. https://doi.org/10.3733/ca.v067n03p128

Published online July 01, 2013

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Abstract

As much as one-third of California's almond and stone fruit acreage is infested with potentially debilitating plant parasitic nematodes, and even more of the land is impacted by Prunus replant disease (PRD), a poorly understood soilborne disease complex that suppresses early growth and cumulative yield in replanted almond and peach orchards. Preplant soil fumigation has controlled these key replant problems, but the traditional fumigant of choice, methyl bromide, has been phased out, and other soil fumigants are increasingly regulated and expensive. We tested fumigant and nonfumigant alternatives to methyl bromide in multiple-year replant trials. Costs and benefits were evaluated for alternative fumigants applied by shanks in conventional strip and full-coverage treatments and applied by shanks or drip in novel spot treatments that targeted tree planting sites. Short-term sudangrass rotation and prudent rootstock selection were examined as nonfumigant approaches to managing PRD. Trial results indicated that integrations of the treatments may acceptably control PRD with relatively little soil fumigant.

Full text

Approximately 1 million acres of California's best agricultural land are devoted to production of almonds and stone fruits (USDA 2011), and sustained high production from this land requires that the orchards be replanted every 15 to 25 years, depending on the production system. Research has documented myriad problems that can suppress growth and productivity in such replanted orchards (Bent et al. 2009; Browne et al. 2006; Larsen 1995; McKenry 1996, 1999; Westerdahl and McKenry 2002). Abiotic soil factors related to previous crop production, such as compaction, salinity, suboptimal pH, nutritional imbalances and herbicide residues, can compromise the performance of replanted orchards, but many of these problems can be avoided or remedied without great difficulty or expense.

Biotic replant problems, including plant parasitic nematodes and Prunus replant disease (PRD), can pose more of a challenge. Plant parasitic nematodes infest as much as one-third of California's almond and stone fruit acreage (McKenry and Kretsch 1987) and have the potential to compromise all phases of an orchard's productive life by inflicting root damage. Several rootstocks for almonds and stone fruit have shown genetic resistance to root knot nematodes, but little resistance has been demonstrated against the other two major nematode pests affecting these crops, the ring nematode and the root lesion nematode (McKenry 2007). PRD, which is much more widespread than nematode damage on almonds and stone fruits, is a poorly understood soilborne disease complex that suppresses early growth and cumulative yield in replanted almond and peach orchards (Bent et al. 2009; Browne et al. 2006). It afflicts successive generations of almonds and stone fruit planted at the same location and is associated with poor health of the trees’ fine roots and incidence of several plant-parasitic fungi and oomycetes. The severity of the disease varies greatly among orchards, but it is observed most commonly on loam, sandy loam, and sand soil textures in California. PRD can occur on its own or in combination with other replant problems.

Preplant soil fumigation has been an effective means of control for biological replant problems, but fumigant usage today is being challenged on several fronts, including the phase-out of methyl bromide (US EPA 2012), township caps on the use of the fumigant 1,3-dichloropropene (1,3-D) (Carpenter et al. 2001), volatile organic compound regulations under the U.S. Clean Air Act (Cal-DPR 2012), and increasingly restrictive buffer zones (see pages 122 127 for explanation and more details on each of these topics). Due to required buffer zones (to protect bystanders from unintended exposure), many fields have large areas that cannot be treated using conventional fumigation.

Rootstocks for almonds and stone fruits were tested for their resistance to the Prunus replant disease complex near Parlier, CA. Shown are a plot of PRD-affected rootstocks in nonfumigated replant soil, left, and a plot of relatively healthy rootstocks grown in soil preplant fumigated with 1,3-D:Pic 63:65 (Telone C35), right.

Reducing dependence on fumigation

Over the long term, breeding of rootstocks that broadly resist or tolerate soilborne pathogens and development of cultural practices that effectively remediate replant problems may remove dependence on soil fumigation. In this article we report on the effectiveness of options currently available for control of the most widespread almond and stone fruit replant problem, PRD. We examine the potential contributions of optimized soil fumigation methods, crop rotation and rootstock selection to the integrated management of PRD and reduced fumigant use.

Almond replant trials

As part of our research, we established two almond replant trials in Madera County focused on fumigant-based options for control of PRD. The trials were designed to help optimize soil fumigation practices by identifying fumigant formulations that are particularly effective for control of the disease complex and by determining the effectiveness of different fumigant rates and novel fumigant delivery methods. Regarding the latter emphasis, GPS-based software and hardware systems were developed recently to deliver spot fumigation treatments by tractor to tree planting sites (Coates et al. 2007; Udompetaikul et al. in press). The new spot treatment system was designed for planning, mapping and treating all tree sites in a replacement orchard and is considered to be much safer and faster than spot fumigation treatments applied with a hand-held probe. Spot treatment can reduce the amount of fumigant required to treat an orchard acre by 50% to 90%, but evaluations of the GPS-controlled tractor application system were needed.

Two orchards in California, one near Firebaugh and the other near Madera, were selected for replanting experiments. The Firebaugh trial included soils of Dinuba fine sandy loam, El Peco fine sandy loam and Fresno fine sandy loam, whereas the Madera trial included El Peco, Fresno, and Lewis sandy loams and Tujunga loamy sand. Lands for the Firebaugh and Madera replant trials were cleared of old almond orchards grown on ‘Nemaguard’ rootstock in the summers of 2006 and 2007, respectively, using conventional practices. After removal, the old trees were chipped (the removed tree residue was ground up by a tub grinder and hauled away for energy generation or other uses). To reduce soil compaction, the cleared lands were ripped to a depth of 5 to 6 feet and then smoothed. In preparation for soil fumigation, the lands were then sprinkler irrigated with about 1.5 inches of water to reduce the potential for fumigant emissions to escape into the atmosphere.

Fumigants were applied to the soil in October 2006 for the Firebaugh trial and October 2007 for the Madera trial. The fumigant formulations were:

  • methyl bromide (MB), 98%; chloropicrin (Pic), 2%, as a warning agent (MBC Concentrate, TriCal Inc.)

  • 1,3-D, 98% (Telone II)

  • chloropicrin (Pic), 99% (Tri-Clor)

  • mixtures of 1,3-D:Pic, including 63:35 (Telone C35) and 39:60, (Pic-Clor 60)

  • iodomethane (IM):Pic 50:50 (Midas)

In each orchard, all preplant soil fumigation treatments were applied by TriCal Inc. (Hollister, CA) to plots that would accommodate a width of three tree rows (66 feet) and a length of 10 tree spaces (140 to 170 feet). The MB treatments were applied with a conventional MB rig (TriCal Inc.), and the system injected fumigant at soil depths of 18 to 20 inches through two shanks spaced 60 inches apart; one pass was made for each tree row, effectively treating a 10-foot-wide strip. The other fumigant treatments were applied with a Telone rig (TriCal Inc.), which also injected fumigants at soil depths of 18 to 20 inches, but through three or five shanks (depending on the treatment). The shanks were spaced 20 inches apart and tipped with horizontal “wing” attachments. Fumigant was released from two points 8 inches apart, one behind each wing tip. The rig was used to apply three types of treatments: single-pass strip treatments, in which fumigant was applied only to 8.3-foot-wide strips centered over future tree rows; full-coverage treatments, in which the entire area of a replicate plot received fumigant; and spot treatments, in which either 8.3-foot-wide by 8-foot-long (Firebaugh trial) or 5-foot-wide by 7-foot-long (Madera trial) rectangular areas centered over future tree sites were treated.

The spot treatments were administered via a Telone rig retrofitted with GPS-based software and hardware to rapidly turn shank injections off and on as the tractor traveled down the future tree rows with the shanks remaining in the soil (Coates et al. 2007; Upadhyaya et al. 2009; Udompetaikul et al. in press). Before the spot applications began, the software was used to create a virtual map of each orchard's future tree sites according to desired row and tree spacings and planting patterns (rectangular and diamond planting patterns were used in the Firebaugh and Madera trials, respectively), and the desired width and length of the zones to be fumigated around each mapped tree planting site were selected.

The control plots were ripped with Telone rig shanks but received no fumigant. Each treatment was applied to several replicate plots (six at Firebaugh and five at Madera). The plots were randomized in a complete block design.

The Firebaugh trial was replanted in January 2007, and the Madera trial in January 2008. In each replicate plot, a center row was replanted to ‘Nonpareil’ almond and the two adjacent rows were replanted to other varieties selected for cross-pollination. In all cases, the rootstock for ‘Nonpareil’ was ‘Nemaguard’ peach. Efficacy of the treatments was assessed according to the percentage of incident photosynthetically active radiation (PAR) intercepted by the ‘Nonpareil’ tree canopies in midsummer and nut yields collected starting in the third growing season and annually thereafter. To measure the PAR interception, we used a new mobile platform that provides a good estimate of the yield potential of tree canopies (Lampinen et al. 2012).

Almond replant trial results

In both the Firebaugh and Madera trials, most of the preplant soil fumigation treatments showed enhanced canopy growth through the first and second yield years (the third and fourth growing seasons after planting, respectively) when compared to the nonfumigated control (table 1; P = 0.002 to < 0.0001 for effect fumigant treatment).

TABLE 1. Results summary, almond replant trials in Madera County

At Firebaugh, compared to the control, preplant strip treatments with MB and 1,3-D boosted PAR interception by 20% and 39%, respectively, in yield year 1 (table 1). Thereafter, these fumigation treatments had little effect on PAR interception. Other fumigant treatments at Firebaugh, including Pic and combinations of Pic with 1,3-D or IM, were generally more effective than the MB and 1,3-D treatments, boosting mean PAR interception by 56% to 97% in yield year 1 and 11% to 22% in yield year 2 compared to the control. By yield year 3 (the fifth growing season after planting), however, none of the treatments affected PAR interception (table 1; P = 0.24).

First-year impact of Prunus replant disease at the Firebaugh replant trial; stunted trees in the foreground row were planted in plot of nonfumigated replant soil, while trees in the background rows were planted in preplant fumigated soil.

In the Madera trial, PAR responses to fumigation were generally more similar among the treatments than in the Firebaugh trial (table 1). At Madera, increases in PAR interception due to preplant fumigation ranged from 34% to 68% in yield year 1 and 35% to 69% in yield year 2 compared to the nontreated control (table 1). The increases in PAR interception between yield years 1 and 2 were generally less at Madera than at Firebaugh. Pressure bomb readings taken in yield years 1 and 2 at Madera suggested that tree water stress was responsible for the lesser growth.

First-year impact of Prunus replant disease at the Madera replant trial; stunted trees in the foreground were planted in plot of nonfumigated replant soil, while larger trees in the background of the same row were in plot of preplant fumigated soil.

In both trials, using the assumption of a net price (i.e., the price after subtraction of nut hauling, hulling and marketing costs) of $2 per pound of nut meats, increases in PAR interception translated into profitable yield increases for all treatments except MB (table 1). The high cost of the MB treatment was not offset by the relatively poor yield increases it generated. By yield year 2, the MB treatment reduced cumulative net returns by $1,120 and $552 per acre in the Firebaugh and Madera trials, respectively, compared to the control. The full-coverage treatment with 1,3-D:Pic 63:35 resulted in the second greatest and greatest cumulative nut yields over the harvests monitored in the Firebaugh and Madera trials, respectively, but the high cost of the treatment kept the net returns relatively low compared to several other MB-alternative treatments (table 1).

Across both trials, the strip treatments with Pic and combinations of 1,3-D:Pic (63:35 and 39:60) generally afforded greater net returns than other treatments. Although the GPS-controlled spot treatments generated lower net returns than some of the strip treatments, the spot treatments provided greater returns than the strip treatment with 1,3-D alone, which has been an almond and stone fruit industry standard. In terms of dollars of net revenue per pound of fumigant, the spot treatments were generally more efficient than strip or full-coverage treatments (table 1). When a net price of $1.70 per kernel pound was assumed (instead of $2 per pound, for the sake of comparison), all of the MB-alternative treatments still increased net crop revenues, but the returns were again negative for the MB treatment and relatively low for the 1,3-D:Pic 63:35 full-coverage treatment. We intend to continue annual PAR and yield measurements in the Madera and Firebaugh trials. Yields have not converged among the treatments, suggesting that their economic value will continue to sort out over time.

Soil sampling from all replicate plots of the control, MB strip and 1,3-D:Pic 63:35 broadcast treatments detected negligible to small nematode populations in 2009 and 2012. Specifically, in 2009 at Firebaugh, we detected one ring nematode per half pint (250 milliliters) of soil from one MB-treated plot, and no lesion, ring or root knot nematodes from other plots; at Madera, there were three lesion nematodes per half pint (250 milliliters) of soil from one control plot, and no lesion, ring or root knot nematodes from other plots. In 2012 at Firebaugh, we detected no lesion, ring or root knot nematodes; at Madera, we detected 164 and 348 lesion nematodes per half pint (250 milliliters) in two respective control plots, and no lesion, ring or root knot nematodes in other plots. These results suggest that PRD was the dominant replant problem in these trials, but it is possible that plant parasitic nematode populations will build and have future economic impacts.

Despite the long-term uncertainties, our trials indicate that effective preplant soil fumigation can be an essential step in maximizing net revenues in replanted almond orchards, at least when ‘Nemaguard’ rootstock is used in the replanted orchard and PRD is active. Furthermore, our findings suggest that at orchard sites at risk for PRD and not infested with plant parasitic nematodes, growers can increase net revenues by using strip treatments with Pic or mixtures of Pic with 1,3-D instead of treatments with 1,3-D alone. Finally, the efficacies and efficiencies of GPS-controlled spot fumigation treatments indicate that they may have important applications where site or air quality sensitivities permit use of only very low rates of fumigant per acre.

Microplot replant trials

We conducted microplot trials to explore the potential of fallowing and crop rotation to remediate PRD. It was found in replanted apple orchards in Washington state that preplant rotation with wheat as a green manure lessened the severity of apple replant disease (Mazzola and Gu 2000; Mazzola and Mullinix 2005). Also, certain crops such as ‘Piper’ sudangrass have been recommended during fallow periods for suppression of nematode populations (Westerdahl et al. 2010). We investigated the potential for using short-term crop rotation and fallowing to reduce the severity of PRD in California.

For this purpose, microplots were constructed at the San Joaquin Valley Agricultural Sciences Center (SJVASC), U.S. Department of Agriculture–Agricultural Research Service (USDA-ARS), Parlier. The microplots consisted of sections of concrete pipe (24 inches in diameter by 48 inches long) inserted vertically into soil, with the rims protruding approximately 8 inches above the soil surface. The microplots were spaced 3 feet apart, edge to edge, and were filled with Hanford sandy loam soil that had been excavated from 0.3- to 2.5-foot depths in an adjacent peach orchard where trees had expressed PRD.

To test plant response to different soil treatments including preplant crop rotation, researchers established soil microplots by installing 4-foot lengths of 24-inch-diameter concrete pipe vertically into the soil and filling the pipes with soil from a nearby orchard affected with Prunus replant disease.

The soil in the microplots was planted with trees on ‘Nemaguard’ rootstock to maintain PRD induction potential, and the plants were watered with drip irrigation. Soil assays indicated that the soil did not have significant numbers of damaging plant parasitic nematodes.

Eight different treatments were imposed on the microplots in a randomized complete block design; there were five replicate microplots per treatment. The treatments were chosen to simulate remediation options of potential interest to almond and stone fruit growers during orchard replanting (table 2). For example, growers may choose to schedule orchard replacement to accommodate dry fallowing of the land for several months or years before replanting, or, alternatively, to replant quickly, without an extended fallow period. Also, whether or not fallowing is involved, growers typically have the option to fumigate the soil or leave it untreated before replanting. Fallowing and fumigation options were represented in treatments 1 to 4 (table 2). When an orchard-free period is observed before replanting, a rotation crop may be used. We selected treatments 5 to 8 to test some of the crop rotation options (table 2).

TABLE 2. Preplant treatments applied to Parlier microplots filled with soil from a peach orchard affected by Prunus replant disease

Treatment options 1 and 2 have the potential to be completed without losing a season of almond or peach production. Treatments 3 through 8 would typically require the loss of a crop cycle, unless a spring-harvested stone fruit variety was being replaced. If potted trees were to be used for the orchard replanting, it would be possible to complete the rotation with wheat alone (treatment 7) without loss of an almond or stone fruit cropping cycle (potted trees can be planted in late spring). Planting bareroot trees after the wheat rotation would require an undesirable delay. Unless kept in cold storage, bareroot trees are optimally planted by early February.

Details of the microplot trials were as follows: Three separate (repeat) experiments were completed. All three experiments had the same treatments, but the experiments were started successively, one year apart. In each experiment, the summer and fall portions of treatments 1 through 8 were imposed beginning in June of the year the experiment began (nearly 1 year before the microplots would be replanted with ‘Nemaguard’ peach plants.) The summer and fall portions of treatments 1 through 8 were continued until the following November, 4 months before replanting (table 2). During this period, the treatments involved maintaining growth of trees on ‘Nemaguard’ rootstock, dry fallowing (the soil was kept bare by hand-weeding) or growing hybrid corn or ‘Piper’ sudangrass (table 2). The ‘Nemaguard,’ corn and sudangrass plants were drip-irrigated to meet evapotranspiration needs, but the fallowed plots were not irrigated. All plots (including those fallowed) were fertilized periodically with equal amounts of ammonium sulfate fertilizer.

Near the end of the preplant period, in early November, the scions of trees on ‘Nemaguard’ rootstock (in treatments 1 and 2) and the tops of the sudangrass plants (treatments 6 and 8) were removed and then discarded outside the microplots. Also, the corn stalks (treatment 5) were chopped into pieces 2 to 3 inches long and kept within the microplots. The ‘Nemaguard’ and sudangrass root system residues and the corn roots and stubble were turned into the top foot of soil in their respective plots using a shovel to simulate thorough disking. Soil in all other plots was turned in the same manner, and the wheat was planted in its plots (treatments 7 and 8). In mid-November, the soil fumigation treatments were imposed on the appropriate plots using a microfumigation rig; MB plus Pic (50:50 formulation) was injected at 400 pounds per acre at 1 foot below the soil surface. At the end of the winter-spring period, soil in all plots, including those with wheat, was turned over repeatedly to a depth of 1 foot with a shovel to simulate disking.

In each of the three repeat experiments, we assessed efficacy of the preplant remediation treatments by replanting the microplots with ‘Nemaguard’ peach seedlings in the following March (i.e., for each experiment, nearly a year after the experiment's beginning) and measuring accumulated shoot weights of the seedlings the following November. The ‘Nemaguard’ seedlings were watered by drip irrigation to meet evapotranspiration demand and fertilized periodically with ammonium sulfate. All plots received the same irrigation and fertilization schedule, except in cases where soil moisture became excessive due to reduced water use by PRD-affected plants; in such cases, irrigation was briefly withheld from overly wet plots until soil moisture levels were similar among all plots.

Microplot replant trial results

In the three successive microplot trials (fig. 1, experiments 1, 2 and 3), several relatively consistent effects emerged, including the following:

  • Preplant fumigation with MB plus Pic (50:50) consistently improved growth of replanted ‘Nemaguard’ peach seedlings, with or without extra preplant fallowing (fig. 1, treatments 1–4).

  • The extra 5 months of preplant fallowing alone (fig. 1, treatment 3) did not significantly improve ‘Nemaguard’ growth, compared to the nonfallowed, nonfumigated control (treatment 1).

  • A summer rotation with ‘Piper’ sudangrass (fig. 1, treatment 6) significantly improved growth of replanted ‘Nemaguard,’ as compared to fallowed and non-fallowed controls (treatments 1 and 3), but the degree of benefit did not consistently match that achieved by fumigation.

  • Rotations involving corn or wheat (fig. 1, treatments 5, 7 and 8) were sometimes beneficial, as compared to the controls (treatments 1 and 3).

Fig. 1. Effects of preplant fallowing, crop rotation and fumigation on growth on ‘Nemaguard’ peach rootstock in microplot trials near Parlier. Experiments 1, 2 and 3 above were started in June of three successive years (2002, 2003 and 2004). For each experiment, treatment numbers are shown in the top row of x-axis labels; the second row of labels represents the corresponding cropping status of the treatments from June to November (Pe = peach, Fa = fallow, Co = corn, Su = sudangrass); the third row of labels indicates subsequent fumigation treatment (NF = nonfumigated, F = fumigated) and the fourth row of labels indicates subsequent cropping status from November to March (Fa = fallow, Wh = wheat). Vertical bars are 95% confidence intervals.

These results suggest that some crop rotations, and particularly a summer rotation with ‘Piper’ sudangrass, may help growers reduce the severity of PRD and thereby reduce the need for soil fumigation. Orchard validation of some of the microplot findings was completed in a peach replant trial, as described below.

Peach replant trial

Favorable responses to spot and strip fumigation treatments in the almond orchard replant trials and to crop rotation in the microplot trials led to validations in a peach orchard replant trial. For the experiment, plums on ‘Nemaguard’ rootstock were removed from a block at the SJVASC in early July 2007. The land was ripped to a depth of 2 to 3 feet, leveled, pre-irrigated and divided into five main plots, each of which was split in half. Each half of the five main plots measured 72 feet by 140 feet. One half was kept fallow (i.e., maintained relatively weed-free by a combination of cultivation and post-emergence herbicide treatments), while the other half was planted to ‘Piper’ sudangrass as a green manure crop. The sudangrass was grown for 2 months under sprinkler irrigation, then shredded and disked into the ground; the disking operation was extended across the whole field in preparation for preplant soil fumigation treatments.

Soil fumigation treatments were applied in late October 2007. The treatments were assigned randomly to 20-foot-wide by 144-foot-long strip plots that ran across both halves of each main plot (i.e., the halves that had been cropped with sudangrass and those that were fallowed). Each of the fumigation treat

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Author notes

The authors gratefully acknowledge support and assistance from the Pacific Area-Wide Pest Management Program, USDA-ARS, the Almond Board of California, the UC Agricultural Experiment Station, TriCal Inc., Paramount Farming, Agriland Farming, Bauer Farming, Harry Berberian and Sons, Donald Ewy, Mike Stanghellini, Burchell Nursery Inc., and Duarte Nursery Inc.

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Citations

Managing for soil health can suppress pests
Amanda Hodson and Edwin Lewis 2016. California Agriculture 70(3):137
http://dx.doi.org/10.3733/ca.2016a0005

Anaerobic soil disinfestation: A chemical-independent approach to pre-plant control of plant pathogens
S L Strauss and D A Kluepfel 2015. Journal of Integrative Agriculture 14(11):2309
http://dx.doi.org/10.1016/S2095-3119(15)61118-2

Tractor-mounted, GPS-based spot fumigation system manages Prunus replant disease
V. Udompetaikul et al. 2013. California Agriculture 67(4):222
http://dx.doi.org/10.3733/ca.v067n04p222

Organically acceptable practices to improve replant success of temperate tree-fruit crops
Thomas Forge et al. 2016. Scientia Horticulturae 200:205
http://dx.doi.org/10.1016/j.scienta.2016.01.002


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