Early Season Indicators of Postbloom Fruit Drop of Citrus and the Relationship of Disease Incidence and Fruit Production. L. W. Timmer, Professor, University of Florida, IFAS, Citrus Research and Education Center, 700 Experiment Station Road, Lake Alfred 33850. S. E. Zitko, Senior Biologist, University of Florida, IFAS, Citrus Research and Education Center, 700 Experiment Station Road, Lake Alfred 33850. Plant Dis. 79:1017-1020. Accepted for publication 29 June 1995. Copyright 1995 The American Phytopathological Society. DOI: 10.1094/PD-79-1017.

Postbloom fruit drop (PFD) is caused by the slow-growing orange (SGO) strain of Colletotrichum gloeosporioides. When citrus flowers are infected, the fruitlet abscises, leaving persistent calyces (buttons) that remain into the following season. The following three factors were evaluated as early season indicators of postbloom fruit drop for the upcoming season on navel and Valencia oranges at 4 to ll locations from 1992 to 1994: (i) the number of buttons remaining from the previous season as determined in January; (ii) the number of propagules of the pathogen recovered from mature leaves in January to February; and (iii) the percentage of diseased flowers on early bloom in January to February. There was a strong positive correlation between the number of buttons remaining from the previous season and the number in the current season, especially in 1992 and 1994. Buttons from the previous season were negatively but weakly correlated with fruit counts in the current season. Propagule numbers of the SGO strain of C. gloeosporioides from mature leaves were not correlated or were only weakly correlated with current season button or fruit counts. Where early season flowers developed, there was usually a positive correlation between the disease incidence on flowers and the number of buttons formed in the current season. Thus, the number of buttons from the previous year provided the best indication of disease potential in the upcoming season. There was a strong negative linear correlation between the number of buttons in the current year and the number of fruit set on navels and Valencias. It is estimated that about five to six fruit are lost for each 100 buttons formed as a result of PFD.