Net necrosis – Potato leafroll virus
Net necrosis of tubers is the condition resulting from current season infection with potato leafroll virus. A cut through the stem end of the tuber reveals a network of brownish black dots and streaks in the vascular ring and vascular (phloem) strands in the flesh. Often a second longitudinal cut should be made to show that the discolored strands continue throughout the tuber. The symptoms of net necrosis are most likely to be confused with frost necrosis (8) and stem–end browning. Net necrosis differs from frost necrosis in that the dots and streaks of net necrosis are more clearly defined and occur in concentric rings. Stem–end browning, a phenomenon of unknown cause, consists of discolored strands, which do not extend more than l/2 inch from the stem end. Controls include the use of certified seed, selection of varieties that fail to show necrosis, and the use of insecticides to control aphid populations, which transmit the virus.