Influence of Wetness Period and Temperature on Infection and Development of Shot-Hole Disease of Almond Caused byWilsonomyces carpophilus

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Abstract
Temperature and duration of wetness period during the inoculation period influenced the development of shot-hole disease on almond leaves caused by Wilsonomyces carpophilus (Stigmina carpophila). In controlled environment studies, a 14-hr wetness period resulted in 0.1 and 45.0 lesions per leaf after 10 days at 8 and 22 C, respectively. Extended wetness periods during the infection period increased the number of lesions per leaf regardless of the temperature. Numbers of lesions increased as temperature was increased from 8 to 22 C for wetness periods greater than 12 hr. At 8 C, 4.7 and 45.6 lesions per leaf developed after wetness periods of 30 and 48 hr, respectively, and, at 15 C, 110.0 lesions developed at 15 C after a wetness period of 28 hr. Temperature after the infection period influenced symptom expression, the rate of lesion development, and lesion abscission, but not the number of lesions formed. Lesion abscission, or shot-hole, was significantly higher at 22 C (72.0% of lesions showing abscission) than at 8 C (0.3%) and 15 C (3.9%). Additional wetness period after an initial infection period resulted in a greater number of lesions and a higher disease index after 8 days at 8, 15, and 22 C and the formation of sporodochia after 16 days at 22 C. Field studies under ambient day and night temperatures and wetness periods of 10-12 hr and 14-16 hr resulted in an average of 6.6 and 16.0 lesions per leaf, respectively. In field studies, additional wetness periods also resulted in an increase in leaf infections and higher disease index. Germination percentages of conidia of W. carpophilus averaged 85.3% in 4 hr and 99.2% in 8 hr at temperatures between 10 and 30 C. Isolates tested had similar pathogenic reactions on almond leaves of cultivars Ne Plus Ultra (chamber tests), Carmel, Mission, and Nonpareil (chamber and field tests). A preliminary predictive model for disease was developed based on temperature and wetness period duration when inoculum was present.