Am. J. Pot Res (2009) 86:315–326 DOI 10.1007/s12230-009-9085-z Metalaxyl-M-Resistant Pythium Species in Potato Production Areas of the Pacific Northwest of the U.S.A. Lyndon D. Porter & Philip B. Hamm & Nicholas L. David & Stacy L. Gieck & Jeffery S. Miller & Babette Gundersen & Debra A. Inglis Published online: 3 April 2009 # Potato Association of America 2009 Abstract Several Pythium species causing leak on potato information is lacking on the distribution of MR isolates in are managed by the systemic fungicide metalaxyl-M. the Pacific Northwest. Soil samples from numerous fields Metalaxyl-M-resistant (MR) isolates of Pythium spp. have (312) cropped to potatoes in Idaho (140), Oregon (59), and been identified in potato production areas of the U.S.A., but Washington (113) were assayed using metalaxyl-M- amended agar for the presence of MR isolates of Pythium in 2004 to 2006. Altogether, 1.4%, 42.4% and 32.7% of the L. D. Porter (*) fields from these states, respectively, were positive for MR Vegetable and Forage Crops Research Unit, USDA-ARS, Pythium. Isolates of Pythium ultimum that were highly 24106 N. Bunn Road, Prosser, WA 99350, USA resistant to metalaxyl were recovered from 53 fields e-mail: [email protected] representing ID, OR, and WA. Greater than 50% of the : Pythium soil population consisted of MR isolates in ten of P. B. Hamm S. L. Gieck 64 fields from Oregon and Washington. Nine species of Department of Botany & Plant Pathology, Hermiston Agricultural Research and Extension Center, Pythium were recovered from soil samples, of which MR P. Oregon State University, ultimum and P. spinosum were identified. Isolates of MR P. 2121 South First Street, ultimum recovered from soil were pathogenic on potato Hermiston, OR 97838, USA tubers and may pose a serious threat to the management of P. B. Hamm Pythium leak and seed rot of diverse crops rotated with e-mail: [email protected] potato. N. L. David Department of Plant Sciences, Resumen Varias especies de Pythium que causan la North Dakota State University, pudrición acuosa en papa son controladas por el fungicida NDSU, Dept. # 7670, sistémico metalaxilo-M. Cepas de metalaxilo-M-resistente P.O. Box 6050, Fargo, ND 58108, USA e-mail: [email protected] (MR) de Pythium spp. han sido identificadas en áreas de producción de papa de los Estados Unidos, pero no hay J. S. Miller información de la distribución de cepas MR en el Pacifico Miller Research LLC, Noroeste. Muestras de suelo de numerosos campos (312) 1175 E. 800 N, Rupert, ID 83350, USA cultivados con papa, en Idaho (140), Oregon (59), y e-mail: [email protected] Washington (113) fueron ensayadas utilizando agar : metalaxilo-M-enmendado para la presencia de Pythium B. Gundersen D. A. Inglis MR del 2004 al 2006. En total, 1.4%, 42.4% y 32.7% de NWREC, Washington State University, 16650 State Route 536, los campos de estos estados, respectivamente, dieron Mount Vernon, WA 98273, USA positivo a Pythium MR. Cepas de Pythium ultimum B. Gundersen altamente resistentes al metalaxilo fueron recuperadas de e-mail: [email protected] 53 campos representando a Idaho, Oregon y Washington. D. A. Inglis Más del 50% de la población de Pythium del suelo, e-mail: [email protected] consistió de cepas MR en 10 de los 64 campos de Oregon 316 Am. J. Pot Res (2009) 86:315–326 y Washington. Nueve especies de Pythium fueron recuper- Metalaxyl is a racemic fungicide that contains both R- and adas de muestras de suelo, de las cuales fueron identifica- S-enantiomers. Metalaxyl-M which contains 98% of the R- das P. ultimum y P. spinosum MR. Cepas de P. ultimum enantiomer (Nuninger et al. 1996) replaced metalaxyl as the resistente al metalaxilo-M recuperadas del suelo fueron active ingredient in Ridomil Gold EC (Syngenta Crop patogénicas en tubérculos de papa y pueden representar una Protection, Greensboro, NC) in 1997 and continues to be seria amenaza en el manejo de la pudrición acuosa por commonly used to manage Pythium and other oomycete Pythium y la pudrición de la semilla en diversos campos pathogens on potatoes. Metalaxyl-M is considered to be rotados con papa. more effective than the S-enantiomer in controlling oomy- cete plant pathogens (Hubele et al. 1983). The efficacy of Keywords Mefenoxam . Fungicide resistance . metalaxyl-M against Pythium leak has been called into Pythium paroecandrum . Pythium inflatum question under challenge inoculations because wounding appears to break the peripheral tuber barrier of metalaxyl and allows Pythium infection and leak symptoms to occur Introduction (Taylor et al. 2004). However, certain metalaxyl-M appli- cation methods have demonstrated some leak control, but it Several Pythium spp. are soilborne oomycete plant patho- is questionable whether the cost-benefit ratio of using gens that can cause major problems in potato production by metalaxyl strictly to manage Pythium leak is economically rotting potato seed pieces, and tubers in the field, at harvest favorable to potato growers (Taylor et al. 2004). or in storage facilities (Powelson et al. 1993; Salas and Resistance to metalaxyl in oomycete pathogens was first Secor 2001). Pythium ultimum is considered to be the reported in isolates of Pseudoperonospora cubensis recov- primary Pythium species causing Pythium leak on potato ered from greenhouse-grown cucumber plants in Israel in (Salas and Secor 2001). P. ultimum reportedly requires a 1980 (Reuveni et al. 1980) and under field conditions in wound to enter a potato tuber (Taylor et al. 2004), making isolates of P. infestans isolated from potatoes in 1981 in both tubers highly vulnerable to infection during harvest, Ireland (Dowley and O’Sullivan 1981) and the Netherlands transport and loading of potatoes into storage facilities. (Davidse 1981). Development of resistance to metalaxyl in In the Pacific Northwest (PNW; Idaho, Oregon and commercial agricultural fields or orchards has been con- Washington), potatoes are rotated with a diverse array of firmed in at least seven species of Phytophthora (Bruin and crops including: peas, carrots, corn, beans, onions and Edgington 1981;FerrinandKabashima1991; Taylor et al. cereals that are susceptible to Pythium seed and seedling rot 2002; Timmer et al. 1998; Chauhan and Singh 1987;and (Higginbotham et al. 2004; Kraft and Burke 1971; Pscheidt Seemuller and Sun 1989); and in six other genera in the and Ocamb 2007; Paulitz and Adams 2003; Broders et al. order Peronosporales including Pythium (Taylor et al. 2002; 2007; Davis and Nunez 1999; Sumner et al. 1997; Hendrix Mazzola et al. 2002; Falloon et al. 2000; Wiglesworth et al. and Campbell 1973). The systemic fungicide metalaxyl-M 1988; Schettini et al. 1991; Herzog and Schuepp 1985; is commonly used as a foliar or in-furrow application to Molinero-ruiz 2003; Mazzola et al. 2002; White et al. 1988; manage Pythium leak and pink rot on potato, cavity spot on Cook et al. 1983;Hammetal.2004). Since the development carrot, and as a seed treatment to manage Pythium pre- of metalaxyl resistance is common among oomycete plant emergence damping off on most vegetable seed and cereal pathogens, it is important to assess the Pythium population in crops grown in crop rotation with potato in the PNW potato production areas where metalaxyl is used, to (Pscheidt and Ocamb 2007). Therefore in the PNW, determine the current and future opportunity to effectively soilborne populations of Pythium spp. can be exposed to use this fungicide not only to help manage Pythium leak on metalaxyl in the soil on an annual basis. In some cases, potato but Pythium damping off on other crops in rotation where growers are planting two or more crops in the same with potato. Development of metalaxyl resistance in the field within the same growing season (i.e. peas and corn), Pythium population is particularly important in the PNW or where repeated foliar applications are used, isolates of where 550,500 acres of potatoes were grown in 2007 Pythium spp. may be exposed to metalaxyl-M multiple accounting for 56.6% of the commercial potato production times in a single growing season. in the USA (USDA-NASS 2008). Metalaxyl is a highly effective systemic fungicide with a Metalaxyl-resistant (MR) isolates of P. ultimum were single-site mode of action that inhibits ribosomal RNA previously recovered from 1 of 11 and 1 of 5 tubers with polymerases (Davidse et al. 1983) of several oomycete leak-like symptoms in Washington and Idaho in 1998 and pathogens. Metalaxyl has been used in the PNW since 1982 2000, respectively (Taylor et al. 2002). MR isolates of P. to manage oomycete pathogens on potatoes such as ultimum were also recovered from 5 of 57 infected tubers with Phytophthora infestans (cause of late blight), Phytophthora leak-like symptoms from Minnesota in 2000 (Taylor et al. erythroseptica (cause of pink rot), and Pythium ultimum. 2002), and from potato tubers located near Hermiston, OR in Am. J. Pot Res (2009) 86:315–326 317 2002 displaying abnormal development of severe symptoms all cropped to potatoes at the time samples were taken from prior to harvest (Hamm et al. 2004). Lack of additional the fields. A standard soil probe (2.9 cm diameter) was used effective fungicides to manage this tuber and seed-rotting to collect the subsamples from each sample point from the pathogen and unusually severe symptoms associated with MR surface to a soil depth of 30.5 cm. Pythium population isolates in Oregon prior to harvest without evidence of densities for each soil sample were assessed for both MR wounding, make development of MR isolates of P. ultimum Pythium isolates and total colony forming units of Pythium an important issue in the PNW.