Development of Rhizoctonia Black Scurf of Potato in Relation to Tuber Borne Inoculum Density, Dehaulming Methods and Curing Time in Northwestern Alluvial Plains of India

To understand the effect of sclerotial density on tubers, and the method vis-a-vis timing of dehaulming on the development of black scurf, field studies were conducted under artificial epiphytotic conditions. Infection levels of up to 10% tuber coverage by scurf sclerotia (grades 0,1 or 2) did not affect yield, but tubers with more than 10% sclerotial coverage (grade 3 or higher) resulted in significantly lower yields compared to apparently healthy tubers (grade 0). The severity of the scurf was significantly higher when the tubers having up to 5% sclerotial coverage (grade 1) were used for sowing compared to sclerotia-free seeds. Seed tubers with more than 5% sclerotial coverage corresponding to grades 2, 3, and 4 of scurf severity were statistically at par with respect to scurf severity, but these had higher scurf intensity compared to grades 0 and 1 (sclerotial coverage 0–5%). The three test methods of dehaulming viz., cutting the tops, herbicide desiccation, and vine pulling didn’t impact the overall tuber yield, but the timing of dehaulming i.e., 7,14,21, and 28 days before harvesting did impact the yield. There was a significant effect of the time gap between vine kill and harvest on the development of black scurf. Dehaulming conducted at 7 days before harvesting produced significantly higher yields than 14, 21, and 28 days before harvesting. The longer the time gap, the more severe was scurf infection. Using herbicide as a method of dehaulming led to more sclerotial formation compared to cutting and pulling.