Management of peach rusty spot epidemics with biorational fungicides

A total of nine biorational fungicides were examined during the 2000 and 2001 growing seasons for their ability to manage rusty spot epidemics, caused by Podosphaera leucotricha, on moderately susceptible 'Jerseyglo' peach. The biorationals studied were the biological control agents Bacillus subtilis, Ampelomyces quisqualis and Phoma glomerata; the plant extracts neem seed oil, cinnamon oil and extracts from leaves of giant knotweed, Reynoutria sachalinensis; the inorganic compounds potassium bicarbonate and potassium dihydrogen phosphate; and chitosan, a deacetylated form of chitin derived from crustacean shells. Disease assessments, consisting of fruit lesion counts, were made at three- to six-day intervals during the course of the epidemics. Three biorational fungicides, B. subtilis, neem seed oil extract and potassium bicarbonate, significantly reduced areas under the disease incidence and lesion density progression curves in both years of the study. The remaining biorational materials reduced disease in only one year or neither year. When the logistic model was fit to the disease progress curves, the three most effective biorationals also significantly reduced the estimated maximum disease level or carrying capacity parameter K. Although these reductions were statistically significant, the amount of disease control was relatively low, ranging from 24 to 34%, versus 94% control for the myclobutanil standard. Thus, B. subtilis, neem seed oil extract and potassium bicarbonate provide insufficient efficacy for commercial management of rusty spot when applied alone. However, since these biorationals consistently provided some levels of control, they may be candidates for use in integrated programs with conventional fungicides.