Development of Rhizoctonia solani on Stems, Stolons and Tubers of Potatoes I. Effect of Inoculum Source

Rhizoctonia solani may affect potato growth, yield and grade through lesions on stems and stolons and through development of black scurf on daughter tubers. R. solani inoculum can be found on seed potatoes and in the soil, although the relative importance of each inoculum source is unknown. Field studies at Parma and Aberdeen, Idaho, were conducted in 2004 and 2005 to evaluate the importance of each source of inoculum on the subsequent development of this disease. Seed of cultivars Ranger Russet (2004) and Russet Burbank (2005) was washed and sorted into three (2004) and two (2005) levels of black scurf. Prior to planting, the plots were inoculated with R. solani cultures mixed with vermiculite at low, medium and high rates. Each level of seed inoculum was planted at each level of soil inoculum. Significantly greater levels of disease on stems and stolons was consistently found on plants grown from high inoculum seed compared to low inoculum seed. However, significant effects of soil inoculum level on stem and stolon disease were rarely seen. In contrast, both seed and soil inoculum level influenced the development of black scurf on daughter tubers. The largest response to soil inoculum level was seen when seed inoculum was low.