Aboveground symptoms of stem canker includ … Aboveground symptoms of stem canker include uneven stands, weak shoot growth, and aerial tubers. Foliage may develop yellowing, purpling, and upward curling of leaves. Aboveground symptoms alone are not diagnostic, however, because other diseases can cause similar symptoms. On belowground stems and stolons, Rhizoctonia solani typically causes reddish-brown lesions that often develop into sunken cankers. Stolons can be girdled and killed, resulting in a pruning effect and malformation and abortion of tubers. If tubers in affected fields are left in the ground after vine death, they often develop black scurf, an accumulation of irregular black sclerotia of R. solani on the tuber surface. lerotia of R. solani on the tuber surface.