CHARACTERIZATION OF PHYTOPHTHORA SPP. CAUSING OUTBREAKS OF CITRUS BROWN-ROT IN FLORIDA

Epidemics of citrus brown rot from 1994 to 1997 in the south-central a nd east-coast citrus areas of Florida were characterized and the causa l Phytophthora spp. identified. Two species of Phytophthora, P. palmiv ora and P. nicotianae, were consistently associated with brown rot. Ep idemics caused by P. palmivora appeared to be initiated on immature fr uit dropped on the orchard floor. The soilborne fungus infected and sp orulated on these fruit and was then disseminated to fruit above 1 m i n the canopy In contrast, infection by P. nicotianae, the common cause of root rot, was confined to the lowest 1 m of the canopy. Fruit infe cted by P. palmivora produced large amounts of ellipsoidal sporangia a vailable for splash dispersal, whereas those infected by P. nicotianae produced far fewer spherical sporangia. Isolates from brown rot epide mics were compared with P. nicotianae from citrus in Florida and Texas , P. citrophthora in California, P. palmivora, and selected Phytophtho ra spp, from other hosts. Brown rot symptoms produced by the different pathogenic citrus isolates on inoculated fruit were indistinguishable . Morphology, mating behavior, and isozyme patterns of brown rot isola tes from 1988 to 1997 matched P. palmivora from citrus roots, other ho st plants, and other locations, but were different from characterized isolates of P. citrophthora in California and P. nicotianae in Florida and Texas. Cellulose acetate electrophoresis of the isozyme glucose-6 -phosphate isomerase rapidly identified the causal citrus pathogen fro m infected fruit and soil isolation plates. Although P. palmivora is a n aggressive pathogen of citrus roots, bark, and fruit, populations in orchard soils were low compared with P. nicotianae.