© 2007 Plant Management Network. Accepted for publication 7 October 2006. Published 22 January 2007. Effect of a Mustard Green Manure on Potato Yield and Disease Incidence in a Rainfed Environment Peter Sexton, Andrew Plant, and Steven B. Johnson, PO Box 727, University of Maine Cooperative Extension, Presque Isle 04769; and John Jemison, Jr., 495 College Avenue, University of Maine Cooperative Extension, Orono 04469 Corresponding author: Peter Sexton. psexton@umext.maine.edu Sexton, P., Plant, A., Johnson, S. B., and Jemison, J., Jr. 2007. Effect of a mustard green manure on potato yield and disease incidence in a rainfed environment. Online. Crop Management doi:10.1094/CM-2007-0122-02-RS. Abstract The goal of this work was to evaluate the short-term effects of a mustard green manure on potato yield and disease incidence for potatoes grown in a rainfed cropping system. Diseases of interest were black scurf ( Rhizoctonia solani ), powdery scab ( Spongospora subterranea), and white mold ( Sclerotinia sclerotiorum ). On-farm trials with large (60-ft wide) replicated plots were established at three sites in 2003 and 2004, in Maine. Mustard ( Sinapis alba and Brassica juncea blend) was sown at a seed rate of 10 lb/acre, mowed at late flowering, and then disked into the soil. In the following season, tuber yield was measured at each site and compared to that of potatoes grown in barley ( Hordeum vulgare) check plots. Incidence of white mold on leaves and stems, and Rhizoctonia incidence on tubers, were evaluated at one site over two years. On average across all the sites, total tuber yield was 8% greater following mustard green manure versus barley. Mustard green manure was associated with decreased incidence of Rhizoctonia on tubers, and increased incidence of white mold on leaves and stems, in the following potato crop. No effect was observed on incidence of powdery scab. Introduction The goal of this work was to evaluate the short-term effects of a mustard green manure on tuber yield and disease incidence of potatoes in a rainfed cropping system. In the Columbia Basin of Washington and Oregon, where potatoes are grown under irrigation with very high yield potential, green manure mustard has provided similar tuber yields as compared to fumigation with metam sodium (2,7). The putative mode of action for suppression of some soil pathogens following a Brassica sp. green manure is the release of compounds from glucosinolates which occurs when tissue is macerated (10). In response to wounding, glucosinolates are hydrolyzed by the enzyme myrosinase releasing a range of breakdown products. The nature of the breakdown products depends on the type of side chain the parent molecule possessed. Volatiles released from b reakdown of glucosinolates have shown a range of activity against various species of nematodes, insects, weeds, bacteria, and fungi (e.g., 1,3,8,9,11). However, other studies have failed to show a strong response in the levels of soil pests to use of Brassica soil amendments or cover crops (2,13). It should be noted that there is large genotypic variation among Brassica species and cultivars in the amount and kinds of glucosinolates present (up to five fold range in glucosinolate concentration) (6). Also pathogens differ in their susceptibility to glucosinolate breakdown products (11,12). In the northeastern US, potatoes are predominantly grown under rainfed conditions with moderate yield potential. In this region, Rhizoctonia, white mold, and powdery scab are several diseases of concern to potato growers. Previous work suggests that Brassica cover crops may have some efficacy against 22 January 2007 Crop Management