SEASONAL PATTERNS ASSOCIATED WITH TYLENCHULUS-SEMIPENETRANS AND PHYTOPHTHORA-PARASITICA IN THE CITRUS RHIZOSPHERE

Populations of Tylenchulus semipenetrans and Phytophthora parasitica were measured weekly during 27 mo in an orchard of mature grapefruit trees on rough lemon rootstock in the central ridge region of Florida. The study was conducted to identify potential key climatic and host factors affecting population changes in both parasites. Patterns of change in root maw density and concentrations of root lignin and nonstructural carbohydrate suggested annual as well as seasonal variation in the age structure and nutritional value of the fibrous root system. Numbers of nematode females on roots and juveniles and males in soil were related positively (P = 0.01) to root mass density and root concentration of reducing sugars, starch, and total nonstructural carbohydrates. Numbers of nematodes were related inversely (P = 0.01) to soil moisture and root lignin content. Numbers of fungal propagules in the soil were related inversely (P = 0.01) to root starch concentration, and the amount of fungal protein in roots (as measured by ELISA) correlated positively (P = 0.01) with soil temperature. Multiple linear regression models with three independent variables (soil moisture, root starch concentration, and an in vitro index of nematode preference for root exudates) explained 86 and 84% of the variation in mean monthly population densities of female nematodes on roots and juveniles and males in soil, respectively. The average monthly levels of P. parasitica protein in roots were best fit (R2 = 0.76) by linear models incorporating soil moisture, temperature, and concentrations of ketone sugars in roots. Root mass density and concentration of ketone sugars explained 86% of the monthly variation in P. parasitica propagule densities in soil. Experimental verification of causality in these relationships would help explain seasonal and annual variation in the parasite burden posed by these two pathogens.