Rhizoctonia solani causes stunting of onion (Allium cepa L.) in the coarse, sandy soils of the semiarid Columbia Basin of Oregon and Washington States (Patzek et al., 2013). The disease occurs in the cereal-onion cropping systems of this region when cereals such as winter wheat (Triticum aestivum L.) or barley (Hordeum vulgare L.) are planted as cover or windbreak crops in the fall and then killed with herbicide applications just before seeding onion crops the following spring (Patzek et al., 2013; Sharma-Poudyal et al., 2013a). The dead windbreak cereal crop protects onion seedlings against sand blasting during very windy spring conditions typical of the Columbia Basin (Pelter and Sorensen, 2003). When the cereal crops are tall enough to limit soil erosion for the onion crop in spring, an herbicide is applied to kill the cover crop (Pelter and Sorensen, 2003; Sharma-Poudyal et al., 2013a). The dying cereal roots provide a substrate for saprophytic growth of the fungal pathogen R. solani, particularly strains of anastomosis group (AG) 8, which then rapidly colonize the dying cereal roots, resulting in a significant increase in soilborne inoculum (Babiker et al., 2011; Sharma-Poudyal et al., 2013a). Consequently, this inoculum can lead to infection of the roots of onion seedlings growing adjacent to the dead wheat plants, which can result in significant stunting of onion plants in patches in the field (Sharma-Poudyal et al., 2013a).
Onion seedlings infected with R. solani AG 8 develop pinched-off or “spear-tipped” roots as early as the first true-leaf stage, although the patches of stunted onion plants usually only become visible at the three- to five-true-leaf stage (Sharma-Poudyal et al., 2015). Anecdotal evidence from growers and crop consultants suggests that stunted patches can develop on any of the onion cultivars commonly grown in the Columbia Basin. The patches of stunted seedlings can range from <1 m to >25 m in diameter and, in severe circumstances, patches may comprise up to 15% of an onion field (du Toit, 2009; du Toit et al., 2012; Wicks et al., 2011). Severely infected seedlings remain stunted relative to adjacent healthy plants throughout the season. Although R. solani AG 8 infects onion plants during the seedling phase of growth, the infections do not affect plant stand (Patzek et al., 2013). Therefore, the numbers of bulbs harvested from stunted patches vs. healthy areas of the field do not differ (Sharma-Poudyal et al., 2013b; Sharma-Poudyal et al., 2015) but bulb size and marketable yield are affected adversely (Anstis et al., 2012; du Toit et al., 2012; Sharma-Poudyal et al., 2013b; Sharma-Poudyal et al., 2015). In a yield loss survey in the Columbia Basin in 2012 and 2013, stunted patches in onion crops caused up to 78% reduction in bulb yield within the patches of stunted plants. The pathogen significantly reduced marketable bulb yield within stunted patches compared with adjacent healthy areas of the field for each of six storage-type cultivars evaluated: Cometa (Bayer CropScience Vegetable Seeds, Brooks, OR), Frontier (American Takii, Salinas, CA), Legend (Bejo Seeds, Inc.), Mercury (Seminis Vegetable Seeds, Parma, ID), Tamara (Bejo Seeds, Inc., Oceano, CA), and Redwing (Bejo Seeds, Inc.). The degree of reduction in marketable bulb weight within stunted patches increased as severity of stunting increased. Marketable bulb weight in patches with a stunting severity rating of 1 (majority of onion plants in the patch were stunted ≤33% compared with adjacent plants outside the patch) ranged from 10% to 47%, in patches with a severity rating of 2 (majority of onion plants stunted 33% to 66%) ranged from 24% to 57%, and in patches with a severity rating of 3 (majority of plants stunted >66%) ranged from 40% to 78%. Average reduction in total marketable bulb yield in the stunted patches was 42% (Sharma-Poudyal et al., 2013b; Sharma-Poudyal et al., 2015).
The yield loss study by Sharma-Poudyal et al. (2015) demonstrated that onion cultivars commonly grown in the Columbia Basin can incur significant yield loss from stunting. Due to the increasing prevalence of this disease in the Columbia Basin over the last 10 years, which is one of the major onion producing regions of the United States (USDA NASS, 2012), information on the relative resistance or susceptibility of onion cultivars to this disease could be useful for identifying R. solani AG 8-resistant onion cultivars for planting in high-risk fields in this region. Therefore, the objective of this study was to evaluate onion genotypes for relative resistance to R. solani AG 8. The cultivars or genotypes were obtained from seed companies that commonly produce intermediate- to long-day storage type onions suitable for the semiarid Columbia Basin.
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