RELATIONSHIPS OF ENVIRONMENTAL-FACTORS AND INOCULUM LEVELS TO THE INCIDENCE OF POSTBLOOM FRUIT DROP OF CITRUS

Postbloom fruit drop, caused by Colletotrichum gloeosporioides, and environmental variables were monitored in three citrus (Citrus species) orchards from 1989 to 1991. In a navel orange orchard near Arcadia, the incidence of blossom blight was up to 30% in 1989 and up to 75% in 1991, but was less than 10% in other years and at other sites. The area under the curve for blossom blight was related positively to the number of persistent buttons (calyxes and floral disks) and related negatively to fruit counts. Persistent buttons and fruit counts were related negatively. A stepwise multiple regression equation was developed from the 1989 and 1991 data from the navel orange orchard near Arcadia. Inoculum level, measured as the number of diseased flowers 3-4 days before the target date, and the total rainfall from 4 to 8 days before the target date explained 50-65% of the variability in disease incidence. Leaf wetness 4-8 days before the target date was a significant factor, but it explained only a small percentage of the variability. Temperature and relative humidity were not important factors in disease development under these conditions. The equation developed should, with further refinement and validation, provide short-term predictions of disease development and assist growers in the timing of fungicide applications.