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

Populations of Tylenchulus semipenetrans and Phytophthora parasitica w ere measured weekly during 27 mo in an orchard of mature grapefruit tr ees on rough lemon rootstock in the central ridge region of Florida. T he study was conducted to identify potential key climatic and host fac tors affecting population changes in both parasites. Patterns of chang e in root maw density and concentrations of root lignin and nonstructu ral carbohydrate suggested annual as well as seasonal variation in the age structure and nutritional value of the fibrous root system. Numbe rs of nematode females on roots and juveniles and males in soil were r elated positively (P = 0.01) to root mass density and root concentrati on of reducing sugars, starch, and total nonstructural carbohydrates. Numbers of nematodes were related inversely (P = 0.01) to soil moistur e and root lignin content. Numbers of fungal propagules in the soil we re related inversely (P = 0.01) to root starch concentration, and the amount of fungal protein in roots (as measured by ELISA) correlated po sitively (P = 0.01) with soil temperature. Multiple linear regression models with three independent variables (soil moisture, root starch co ncentration, and an in vitro index of nematode preference for root exu dates) explained 86 and 84% of the variation in mean monthly populatio n densities of female nematodes on roots and juveniles and males in so il, 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. Ex perimental verification of causality in these relationships would help explain seasonal and annual variation in the parasite burden posed by these two pathogens.