| Rose downy mildew, caused by the oomycete Peronospora sparsa , is a sporadic but devastating problem for the bare-root nursery industry in California. This research investigated the epidemiology of the disease in order to develop an integrated control plan. The efficacy of chemical and alternative control measures was evaluated in commercial fields. These trials identified some novel, effective treatments such as the pre-plant treatment of rootstock propagation wood with fungicides or hot water. The most effective control measure was a 10 min dip of the wood in metalaxyl or mefenoxam (100 to 10,000 mg a.i. per liter), which reduced the area under the disease progress curve by up to 76%. The effects of leaf wetness duration, relative humidity, and temperature on the pathogen and its infection and colonization of rose leaves were studied under controlled conditions in the growth chamber. Disease incidence and weather data from natural epidemics were analyzed with logistic regression to develop several predictive models for disease development. The most important variable correlated with natural disease development was leaf wetness, with a duration exceeding 8.4 hours per day over 10 days being conducive. However, studies under controlled conditions revealed that at optimum temperatures of 15 to 20°C, successful infection required as few as two hours of leaf wetness, although longer periods significantly increased disease severity. A preliminary validation of one of the logistic regression models was conducted. In the laboratory, a new DNA-based technique was developed to detect the pathogen within woody rose tissues. This polymerase chain reaction detection technique was used, along with epifluorescent microscopy, to investigate the histology of infected rose tissues. They revealed hyphae, haustoria, and oospores characteristic of the pathogen within stem cortex tissues where they may serve as survival structures to carry the pathogen through unfavorable periods and initiate new epidemics. |
