Influence of Water Stress on Susceptibility of Nonwounded Peach Bark to Botryosphaeria dothidea. P. L. Pusey, Plant Pathologist, USDA-ARS, Southeastern Fruit and Tree Nut Research Laboratory, P.O. Box 87, Byron, GA 31008. . Plant Dis. 73:1000-1003. Accepted for publication 7 July 1989. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1989. DOI: 10.1094/PD-73-1000.

Botryosphaeria dothidea invades bark of peach trees through wounds and lenticels. The effect of water stress on disease development at lenticels was studied in 1-yr-old peach trees that were inoculated by applying suspensions of conidia on nonwounded stems. From May to September 1985, inoculated trees were irrigated daily or had water withheld for 2, 4, or 6 days in each 8-day cycle. At the end of the test, trees with the 6-day withholding had higher mean values for number of gum exudation sites and number of lesions and a higher percentage of bark necrosis than trees in the other treatment groups (P = 0.05). In 1986, the effect of severe water stress at or near the time of inoculation was determined by stopping daily irrigation of trees 2 days before inoculation and starting irrigation again after leaf water potential had dropped to below –3.0 MPa. This was done with four different sets of trees from April to October. In another test with a similar objective, trees in various stages of wilt in May 1986 were inoculated and then subjected to 96 ± 2% relative humidity for 7 days to maintain water potential levels. In neither of the latter two types of tests was water stress or water potential at the time of inoculation shown to be related to disease development. To determine the effect of severe water stress after inoculation, the trees inoculated in April or June 1986 had daily irrigation interrupted after varying periods. Water was withheld until leaf water potential was below –3.0 MPa. Based on observations in November, stress imposed 2–6 mo after inoculation resulted in a dramatic increase in number of lesions and in percentage of bark necrosis. Stress imposed during the period of August to October consistently caused an increase in disease severity. This has significance because commercial peach orchards (even those equipped with irrigation systems) generally are not irrigated after fruit harvest, which occurs between May and August.