Intumescence of ornamental sweet potato By Dr. Steve Nameth Department of Plant Pathology, The Ohio State University Columbus, OH 43210 Intumescence is a proliferation of callus tissue on the surface of leaves of certain sweat potato cultivars. Intumescences are white at first, granular, and less than 1-2 mm high (Figures 1) (Figures 2). They usually form simultaneously and are scattered uniformly over the leaf surface, but sometimes they form preferentially over the major veins. They occur on either surface of the leaf but are more prevalent on the upper surface. Within a few days they turn brown and dry and collapse to a fraction of their original size, giving the leaf surface a rough, sandpapery appearance. At this time no pathogen has been associated with this plant disorder. It develops spontaneously under conditions of high humidity and low light intensity. High temperatures may also be necessary. The disorder is noticed in the greenhouse most often early in the growing season when the weather first becomes warm and humid. It may develop more frequently in the greenhouses than in the field, because of the high humidity common in greenhouses. I may be very difficult to avoid this problem especially when the conditions are ideal for the disorder to develop.