Role of Natural Seed Infection by the Web Blight Pathogen in Common Bean Seed Damage, Seedling Emergence, and Early Disease Development

The web blight pathogen, Thanatephorus cucumeris (Rhizoctonia solani), was seedborne in both white- and pigmented-seeded genotypes of common bean (Phaseolus vulgaris) and was associated with blemishes and discoloration on the seed coat. The pathogen was consistently isolated from blemished seeds of six genotypes grown in fields where the predominant AG type of the pathogen was AG-I-IB (microsclerotial). Ninety-five percent of the isolates from seeds were AG-1-IB, and the remainder were AGI-IB (macrosclerotial), AG-2, and AG-4. The pathogen was isolated less frequently from unblemished seeds. T cucumeris was isolated from up to 17% of unblemished seeds from black-seeded genotypes, including the reported web blight resistant breeding line HT-7719. All seedborne isolates were pathogenic to bean seedlings. When seedborne in genotypes with white, red mottled, and black seed, T cucumeris significantly (P < 0.05) reduced seedling emergence and establishment. Isolates AG-1-IB and AG-2-2 added to the soil caused only slight injury to bean seedlings, whereas soil isolate AG-4 was very detrimental to seedling emergence and growth.