Sensitivity of North American Isolates of Phytophthora erythroseptica and Pythium ultimum to Mefenoxam (Metalaxyl)

Since their introduction in the late 1970s, phenylamide fungicides have been used effectively to control diseases caused by the Peronosporales. These fungicides have a very specific mode of action; inhibiting fungal ribosomal RNA polymerases (7). Because of this specificity, there is a relatively high risk that resistance to the fungicide may appear quickly in the target fungal population (3). Phenylamides were used initially as a straight product (unaccompanied by a second fungicide) and as a result, within 2 years of their introduction, insensitive isolates of Pseudoperonospora cubensis were collected from cucumber in Israel (33), and isolates of Phytophthora infestans insensitive to metalaxyl also were found in potato crops in Ireland (9) and the Netherlands (8). A short time later, resistance was documented in a variety of Oomycete species from diverse crops (grapes, lettuce, tobacco, and turf). Morton and Urech (25) reviewed these and other reports of the initial development of resistance to this class of fungicides. Phenylamide fungicides have been used primarily to control foliar diseases since the initial Environmental Protection Agency (EPA) conditional registration of metalaxyl on tobacco in 1980 (25). Resistance to metalaxyl was first found in North America in 1984 when this fungicide failed to control Pythium turf blight in Pennsylvania (35). Most of the information gathered on resistance in North America has come from studies involving foliar pathosystems. As phenylamides lost their effectiveness in controlling some foliar diseases, their use against soilborne diseases has increased. This poses a potential problem since resistance probably would develop under these conditions as well (41), although this is likely to occur at a slower rate due to the monocyclic nature of most soilborne pathogens. Intrasite and intersite movement of resistance also should be slower in this group. Two important soilborne diseases of potato, pink rot and leak, often collectively known as “water rot”, are caused by the Oomycetes, Phytophthora erythroseptica and Pythium ultimum, respectively. Either disease can be found during or immediately following harvest, especially in areas and in years with high moisture (14,20). Pink rot and leak can be widely distributed in potato fields and storage bins (31,36). Strategies commonly used to manage water rots include crop rotation, planting the potato crop in well-drained soils, avoiding excessive irrigation at the end of the growing season, modifying handling procedures to reduce wounding, and applications of mefenoxam (31,36). Most commonly grown potato cultivars are susceptible to P. erythroseptica and Pythium ultimum (authors, unpublished data) so resistance is not currently a practical control measure for the water rot diseases. Mefenoxam (Ridomil Gold EC and Ultrafluorish EC) is the only fungicide used to control pink rot and leak (26,43). The effectiveness of this fungicide may be compromised if insensitive isolates of P. erythroseptica and Pythium ultimum become widespread. Currently, mefenoxamresistant isolates of P. erythroseptica are known to be present in Maine, New York, and Idaho (13,15,23). This and recent outbreaks of pink rot in the Red River Valley of North Dakota and Minnesota (24) have caused concern among growers and industry representatives. Aside from our preliminary reports (15,16,34,38), a thorough survey of the incidence of resistance to phenylamide fungicides in populations of the water rot fungi has not been conducted. The objective of this study was to determine the level of sensitivity to mefenoxam in isolates of P. erythroseptica and Pythium ultimum collected throughout North America and to monitor the distribution of these isolates.