THE BIOLOGY AND PATHOLOGY OF THE FUNGUS RHIZOPUS STOLONIFER, CAUSE OFBLACK MOLD DISEASE OF TABLE GRAPES IN ISRAEL

Rhizopus stolonifer causes severe losses to Israeli grape exports. The fungus develops on mature berries in the field, during storage at tem peratures above 0 degrees C and during shelf-life. It was isolated fro m naturally contaminated soils throughout the year, and from fruits af ter mid-June. The airborne spore population increased in vineyards at the time of fruit maturation and was related to the proximity of stone -fruit orchards. The size of this population was highly correlated wit h disease incidence and thus may be a satisfactory tool for disease pr ediction. Intact young berries were more resistant than mature ones to Rhizopus inoculation, in both the vineyard and in the laboratory. Mec hanical wounding, or chloroform dips, dramatically increased the susce ptibility of young berries. The decline in acidity and the increase in the soluble solids content during growth and maturation of the berry were accompanied by an increase in susceptibility to inoculation. Scan ning electron microscopy showed the fruit surface to be covered with w hite amorphous structures. At higher magnifications small cracks (appr oximately 1 mu m in length) were observed on the surface but in no cas e was fungal penetration noticed through these cracks. In artificially inoculated fruits the fungus germinated near the wound and developed vigorously inside and around the fissure, producing sporangia. The you ng sporangium is covered by a smooth external layer which is compresse d against the sporangiospores. After lysis of the cover, the young hex agonal sporangiospores can be observed. With maturation, the columella collapses and the mature sporangiospores are dispersed. In naturally infected berries, an important path for fungal penetration is the inco mplete connection between the berry and the pedicle. Heavily infested berries become disconnected from their pedicles. Two chemicals applied to the surrounding atmosphere, orthophenyl phenate (as a fog) and cal cium hypochlorite (as chlorine vapour), significantly decreased postha rvest decay in artificially inoculated berries.