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Effects of Temperature and Wetness Duration on Infection of Pistachio by Botryosphaeria dothidea and Management of Disease by Reducing Duration of Irrigation. Themis J. Michailides, Associate research plant pathologist, University of California, Berkeley/Kearney Agricultural Center, 9240 S. Riverbend Ave., Parlier, CA 93648; David P. Morgan, staff research associate, University of California, Berkeley/Kearney Agricultural Center, 9240 S. Riverbend Ave., Parlier, CA 93648. Phytopathology 82:1399-1406. Accepted for publication 21 July 1992. Copyright 1992 The American Phytopathological Society. DOI: 10.1094/Phyto-82-1399.

The optimum temperatures for growth of Botryosphaeria dothidea, the cause of Botryosphaeria panicle and shoot blight of pistachio, were 2730 C on potato-dextrose agar. The optimum temperatures for pycnidiospore germination were 2436 C. Pycnidiospores that germinated at 3639 C failed to develop colonies. Optimum temperatures for disease development on artificially inoculated fruit were 2733 C and for development of pycnidial initials and pycnidia on fruit, 30 C. The latent period for pycnidia development on pistachio fruit naturally infected by B. dothidea was shorter at 33 C than at 1027 C. Pycnidia did not develop at 6 C and formed on only about 17% of the fruit at 10 C. Field-inoculated pistachio leaves kept wet continuously for 072 h developed no disease at wetness durations of 06 h; durations of 912 h or longer resulted in symptoms and increased disease severity. Ninety-seven and 100% of the inoculated leaves had lesions, approximately 25 and 48% of them showing more than 10 lesions per leaf after 48 and 72 h of wetness duration, respectively. Disease incidence of leaves was best described by a third-order polynomial regression equation. Cluster blight, fruit with pycnidia, and leaf (0.46.0%) and petiole blight (1584.4%) occurred only after 18 h of wetness and increased linearly with wetness durations of 1872 h. In field experiments, two to four additional 12-h wetness periods after an initial 12-h infection period interrupted by 12-h dry periods resulted in greater disease incidence and severity on leaves. Reducing duration of sprinkler irrigation from 24 to 12 h in one orchard in Butte County resulted in reduction of the disease (infected fruit) from 31 to 20% (P = 0.10), from 60 to 48% (P = 0.08), and from 62 to 19% (P < 0.01) in 1989, 1990, and 1991, respectively. In addition, reducing duration of sprinkler irrigation in the same orchard to 12 h resulted in significantly lower incidence and severity of infected leaves compared with leaves from trees irrigated for 24 h. Similarly, in 1991 in a second orchard, reduction of duration of sprinkler irrigation from 48 to 24 h significantly lowered the incidence and the severity of leaves infected by B. dothidea.

Additional keywords: Botryosphaeria ribis, Dothiorella sp., Pistacia vera, quantitative epidemiology.