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Effects of Some Weather Factors and Fusicladium effusum Conidium Dispersal on Pecan Scab Occurrence. A. J. Latham, Department of Botany, Plant Pathology, and Microbiology, Auburn University, Auburn, AL 36849; Phytopathology 72:1339-1345. Accepted for publication 26 March 1982. Copyright 1982 The American Phytopathological Society. DOI: 10.1094/Phyto-72-1339.

Dispersal of Fusicladium effusum conidia was diurnal with peak numbers trapped at 1200 hours. Few conidia were dispersed at night when pecan leaves either were wet, the relative humidity (RH) was 100%, or there was no wind. Highest daily numbers of conidia were recorded during the last week of May and first 2 wk of June. Numbers of airborne conidia were correlated positively with wind velocity, temperature, and rain, and negatively with RH during each or several of the months of May, June, and part of July in 1974, 1975, and 1980. Highly significant correlations were found between numbers of airborne conidia trapped 8 days after the date lesions were counted during 1974 and 1975. Characteristics and/or factors associated with the scab epidemic of 1975 were scab lesion counts at the fourth week of April were nearly equivalent to those made during the fourth week of May in nonepidemic years, number of airborne F. effusum conidia trapped were 63% higher than in a nonepidemic year, rainfall occurred weekly during May and June. The development of scab lesions 79 days following a 12-hr leaf wetness-infection period caused by rain revealed the inadequacy of a scab forecasting theory based upon 100 hr of leaf wetness before fungicide application.

Additional keywords: epidemiology, disease forecasting.