Studies of Phylloxera-Suppressive Soils:

Lab and field experiments on grape phylloxera conducted at Davis, California tested the hypothesis that soils in organically managed vineyards suppress phylloxera damage. Soil samples taken from two conventional and three organically managed vineyards were infested with cultured phylloxera to test for differences in suppressiveness and conduciveness of soils. Experiments demonstrated that roots grown in soils of organically managed vineyards have significantly less root rot (30.3%) than that of conventionally managed soils (54.5%). Although phylloxera populations in organic soils tended to be greater, they were inversely correlated with root rot. Field experiments sampled phylloxera-resistant roots in nine conventional and four organic vineyards to assess phylloxera populations and root damage. The vineyard study showed similar trends, organically managed vineyards root rot was (11.8%) compared to the conventionally managed vineyards (27.1%). But unlike the greenhouse experiment, phylloxera numbers were similar in the soil of the organically managed vineyards