In the first article of a two-part progress report on the relative susceptibility of avocado root-stocks to chlorosis evidence was presented to the effect that young avocado trees on Guatemalan rootstocks are less tolerant to a type of chlorosis—yellowing of leaves—than trees on Mexican stocks. The evidence was based mainly on information obtained in two rootstocks plots, one located in Santa Barbara County and one in Orange County. In the former 70% and in the latter 78% of the trees on Guatemalan stocks became chlorotic about one year after planting, while only 1% of the trees on Mexican stocks in both plots showed the disorder. In November 1951, 40% of the chlorotic trees on Guatemalan stocks in the Santa Barbara County plot and 35% in the Orange County plot were either dead or seemingly beyond recovery. Since then the condition of the surviving chlorotic trees has fluctuated. In November 1952, it was uncertain as to what percentage would develop into normal trees.

The above progress report is based on Research Project No. 1458.

Avocado growers, A. J. Borchard, E. C. Bor-chard, M. R. Walker, R. T. Hodges, and F. Blower co-operated in the investigations reported above.