Fungicide Control of Phytophthora ramorum on Rhododendron

Commercial rhododendron plants have been the most important hosts of Phytophthora ramorum in Europe. As part of the European Union (EU) emergency phytosanitary measures 2002/757/EU and 2004/426/EU all commercial rhododendron-growing premises are surveyed for P. ramorum. Detection of P. ramorum leads to quarantine measures, including destruction of plants, and could lead to considerable financial damage for the companies involved. Growers are taking all possible measures to avoid P. ramorum, including preventive fungicide treatments. However, because little information is available on the effect of the different oomycete fungicides on P. ramorum, there is a danger of conducting sub-optimal protective fungicide treatments. The objective of this research was to evaluate the efficacy of different oomycete fungicides against P. ramorum. Active ingredients from most of the oomycete fungicides on the Belgian market were screened for their in vitro effect on the mycelial growth of four P. ramorum isolates. Based on the results from the in vitro experiments, a selection of these fungicides was tested against P. ramorum on rhododendron plants. Application time and application method were included as variables. A wide range of in vitro fungicide activity was observed. Metalaxyl, dimethomorph, and benthiavalicarb-isopropyl showed complete inhibition of mycelial growth at 1 μg ml or less. Cymoxanil, etridiazole, and mancozeb caused complete growth inhibition at 1 to 100 μg ml. Chlorothalonil, Cuoxychloride, famoxadone, fluazinam, and cyazofamid did not completely inhibit growth at 100 μg ml. Fosetyl-Al and propamocarb did not cause growth inhibition at 100 μg ml. Fungicide effects were independent of the strain of P. ramorum used, except for one strain, which showed a decreased sensitivity to metalaxyl. A subset of the fungicides was tested on plants. Overall, fungicides that performed best on plants were metalaxyl, cyazofamid, and benthiavalicarb-isopropyl. Dimethomorph and fosetyl-Al had intermediate effects. Cymoxanil and mancozeb were the least effective of the products tested. Protective effects were best 1 A version of this paper was presented at the Sudden Oak Death Second Science Symposium: The State of Our Knowledge, January 18-21, 2005, Monterey, California. 2 Agricultural Research Center (CLO), Dept. of Crop Protection (DGB), Burg. Van Gansberghelaan 96, 9820 Merelbeke, Belgium. 3 Corresponding author: [email protected] GENERAL TECHNICAL REPORT PSW-GTR-196 242 when the lower surface of the leaf was covered with the fungicide, consistent with the observation that infection of non-wounded leaves inoculated with zoospores takes mostly place through the lower surface of the leaves. Curative fungicide treatments 2 days after zoospore inoculation were much less effective than protective treatments (1 day before zoospore inoculation). This research shows that protective applications of specific fungicides can contribute to effective control strategies of P. ramorum on rhododendron. Considering that we identified a strain with decreased activity against metalaxyl, it seems advisable to limit the number of consecutive uses of products with the same mode of action. However, growers may face few options in alternating fungicides due to the limited number of products with use permits on rhododendron.