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Dynamics and Pattern of Latent Infection Caused by Botryosphaeria dothidea on Pistachio Buds

A study was conducted in four commercial pistachio orchards to monitor the presence and pattern of external contamination and latent infections of buds by Botryosphaeria dothidea between 1998 and 2000. Symptomless buds were sampled every 2 to 3 weeks and analyzed either by a washing/crushing method or by direct plating of split (half) or intact buds on lactic acid potato dextrose agar. The proportion of infected buds varied among orchards over time. Levels of latent infections were highest in February and March when they reached as much as 60% in orchards of Glenn and Yolo counties. This period corresponded to the months with the highest rainfall. Buds collected from orchards in Glenn and Yolo counties had, in general, higher incidence of infection than buds from San Joaquin and Merced counties. Buds became infected in June immediately after their formation. Infection incidence on basal segments of buds from male trees was approximately twice (19%) that of apical or middle segments (11%). Plating of split buds resulted in similar levels of incidence as the plating of intact buds. The number of B. dothidea propagules on bud surfaces varied over time and among orchards. In general, the number of propagules per bud was highest in February and March (approximately seven propagules per bud) when the rainfall amount was highest.