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Title: Metalaxyl-M-Resistant Pythium Species in Potato Cropping Systems in the Pacific Northwest, USA

Author
item Porter, Lyndon
item HAMM, PHILLIP - OREGON ST UNIV
item DAVID, NICHOLAS - OREGON ST UNIV
item GIECK, STACY - OREGON ST UNIV
item MILLER, JEFFERY - OREGON ST UNIV

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/1/2009
Publication Date: 4/3/2009
Citation: Porter, L., Hamm, P., David, N., Gieck, S., Miller, J. 2009. Metalaxyl-M-Resistant Pythium Species in Potato Cropping Systems in the Pacific Northwest, USA. Plant Disease. Published online DOI 10. A 1007/s 12230-009-0985-z.

Interpretive Summary: Species of the plant pathogenic fungi of Pythium can cause major problems in potato cropping systems by directly causing a water rot of potato known as Pythium leak. Pythium leak can cause the rot of potato seed pieces, whole potatoes in the field, and rot of potatoes at harvest or in storage facilities. In addition, in the Pacific Northwest (PNW), species of Pythium can cause major seed and seedling rots of crops planted in rotations with potatoes. The fungicide metalaxyl has been used for years to successfully manage Pythium leak and Pythium related seed and seedling rots. The number of fungicides available to manage plant pathogenic Pythium is limited, therefore it is important to avoid the overuse of metalaxyl to reduce the likelihood of Pythium species developing resistance against this very effective fungicide. In the present study Pythium species were sampled from soil and infected tubers to determine the presence of metalaxyl-resistant Pythium species in potatoes and soils in potato cropping systems in the PNW of the USA. Soil samples taken from 312 individual fields in Idaho (140), Oregon (59), and Washington (113) were assessed using agar treated with metalaxyl to identified the presence of MR isolates of Pythium and 1.4, 42.4 and 32.7% of the fields from these states contained metalaxyl-resistant (MR) Pythium isolates, respectively. MR Pythium isolates consisted of greater than 50% of the Pythium soil population in 10 of 64 fields where resistant isolates were detected. Ten species of Pythium were recovered from soil samples, of which P. ultimum (62), P. spinosum (1) and an unidentified Pythium sp. (1) were resistant to metalaxyl. MR P. ultimum isolates were pathogenic on potato tubers in laboratory tests. Implications for disease management where MR isolates are found is discussed.

Technical Abstract: Several Pythium spp. causing leak on potato (Solanum tuberosum) are managed by the systemic fungicide metalaxyl-M (mefenoxam). Metalaxyl-M-resistant (MR) isolates of Pythium spp. have been identified in potato production areas of the U.S, but detailed information on the occurrence of resistance is lacking from the Pacific Northwest (PNW). Therefore the incidence of MR isolates of Pythium spp. in potato cropping systems in the PNW was determined from 2004 to 2006. Soil samples taken from 312 individual fields in Idaho (140), Oregon (59), and Washington (113) were assayed using metalaxyl-M amended agar for the presence of MR isolates of Pythium and 1.4, 42.4 and 32.7% of the fields from these states contained MR Pythium isolates, respectively. MR Pythium isolates consisted of greater than 50% of the Pythium soil population in 10 of 64 fields where resistant isolates were detected. Ten species of Pythium were recovered from soil samples, of which P. ultimum (62), P. spinosum (1) and an unidentified Pythium sp. (1) were resistant to metalaxyl. MR P. ultimum isolates were pathogenic on potato tubers in laboratory tests. Implications for disease management where MR isolates are found is discussed.