Resistance in Peronospora viciae to phenylamide fungicides: reduced efficacy of seed treatments of pea (Pisum sativum) and assessment of alternatives

Metalaxyl seed treatments failed to control downy mildew (caused by Peronos pora viciae) in New Zealand pea crops (Pisum sativum), suggesting that resi stance to the fungicide had developed after several years of use. Glasshous e experiments confirmed that seed treatments with metalaxyl or oxadixyl (sy stemic phenylamide fungicides) were ineffective for downy mildew control in young pea seedlings inoculated with field collections of P. viciae, and th at seed treatments containing cymoxanil or fosetyl-Al protected seedlings f rom infection by metalaxyl-resistant collections. Laboratory bioassays dete rmined that metalaxyl resistance was widespread and common in field collect ions of the pathogen in New Zealand. Estimated (R)-metalaxyl EC(50)s for se nsitive collections were <0.01 mu g/ml, while those for resistant collectio ns were 3-11 mu g/ml. A field experiment confirmed that alternative fungici de seed treatments can be used for control of metalaxyl-resistant P. viciae . A seed treatment formulation containing cymoxanil (a metalaxyl alternativ e), (R)-metalaxyl and fludioxonil increased seedling establishment by 170% relative to untreated seed, gave complete control of downy mildew that was unaffected by (R)-metalaxyl used alone, and resulted in a 216% increase in grain yield. Similar effects on establishment and grain yield were recorded from a fosetyl-Al + thiabendazole + thiram seed treatment, although this t reatment was not as effective for downy mildew control. These results demon strate that metalaxyl-resistant P. viciae in pea crops can be controlled wi th alternative systemic fungicide seed treatments, and emphasise the value of seed treatments for ensuring good seedling establishment and subsequent crop production. Integrated approaches, incorporating appropriate cultural and chemical control strategies, and possibly use of disease-resistant cult ivars, are recommended for management of phenylamide resistance in P. vicia e and control of downy mildew of pea. (C) 2000 Elsevier Science Ltd. All ri ghts reserved.