Symptoms of rugose mosaic vary widely with … Symptoms of rugose mosaic vary widely with virus strain and potato cultivar. Potato virus Y is the most severe of the mosaic viruses. Symptoms include mottling or yellowing of leaflets, leaf crinkling, and sometimes leaf drop. Veins on the underside of leaves often show necrotic areas as black streaks. Infected plants may be stunted. Leaf mottling may be masked at low (below 50?F) or high (above 70?F) temperatures, but at high temperatures the disease can be identified by the crinkling and rugosity of the foliage. A severe crinkling of the leaves occurs when potato viruses Y and X occur in the same plant. o viruses Y and X occur in the same plant.