POPULATION-DYNAMICS AND SURVIVAL OF STRAINS OF COLLETOTRICHUM-GLOEOSPORIOIDES ON CITRUS IN FLORIDA

The slow-growing orange (SGO) strain of Colletotrichum gloeosporioides causes postbloom fruit drop disease of citrus, whereas the fast-growing gray (FGG) strain is primarily saprophytic on senescent and dead tissue. Propagule densities of the SGO and FGG strains were compared on various tissues under field conditions and on inoculated leaves in the laboratory and greenhouse by assay on a selective medium to compare the survival of the two strains. In field studies in Florida, the SGO strain reached high populations densities on flower petals during bloom. This strain persisted on vegetative tissues, but population densities declined with time after bloom. Propagule densities of the FGG strain were relatively stable throughout the year. When conidial suspensions were sprayed onto foliage of greenhouse-grown seedlings, conidia of both strains germinated to form appressoria, but the percentage of conidia forming appressoria was much higher with the FGG strain. The number of propagules recoverable in leaf washes declined to near zero in 30 days with both strains, but these strains could still be isolated from leaf pieces. When plants that had been sprayed a month earlier with a conidial suspension were treated with water or a flower extract, water alone stimulated germination of some conidia and appressoria of both strains. The flower extract greatly increased germination of appressoria of the SGO but not the FGG strain. The SGO strain produced many new conidia on hyphae from germinating appressoria on the leaf surface in response to treatment with the flower extract, but the FGG strain produced few. The cycle of postbloom fruit drop caused by the SGO strain appears to be as follows: conidia produced in abundance in acervuli on petals are washed onto vegetative tissues, where many germinate to form appressoria and possibly some quiescent infections; appressoria survive on vegetative tissues until the next bloom, and substances from petals stimulate germination of appressoria to form conidia on the surface; these conidia are splash-dispersed to flowers and reinitiate the cycle.