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Influence of Sugar Content and pH on Development of White Rot on Apples. Frank C. Kohn, Jr., Former Graduate Research Assistant, Department of Plant Pathology and Plant Genetics, University of Georgia, Athens 30602. F. F. Hendrix, Professor, Plant Genetics, University of Georgia, Athens 30602. Plant Dis. 67:410-412. Accepted for publication 13 September 1982. Copyright 1983 American Phytopathological Society. DOI: 10.1094/PD-67-410.

Apples were collected at weekly intervals from an orchard in Georgia beginning 3 wk after petal fall and continuing until harvest. At each sampling date, puncture-wounded and unwounded apples were spray-inoculated with Botryosphaeria dothidea conidia at a concentration of about 1 105/ml by using a chromatographic spray bottle at 10 psi. Inoculated fruit were incubated in moist chambers at 30 C and the percentage that developed rot was recorded after 7 days. The sugar content in a subsample of fruit was measured by determining percent soluble solids with a refractometer. The pH of the apple filtrates was also measured. No lesions developed until sugar content reached about 10.5%, which occurred 8 wk before harvest in 1979. Lesion formation continued to increase with increasing sugar content until harvest, when sugar content reached 13.8% and rot incidence was 100%. Regression analysis indicated a significant association (r2 = 72.3%) between sugar content and rot incidence. Greatest mycelial dry weight production in vitro occurred with a 20% sucrose level in the medium.