Viruses are spread by insects, nematodes and humans. Symptoms include vein banding, mosaic, flecking or spotting on foliage and abnormal growth, similar to herbicide damage. Tobacco mosaic virus on tomatoes and leaf mosaic on dahlias are common viral diseases. Control is limited to removing and destroying infected plants. They most commonly feed on plant roots, but some nematodes invade leaf tissue.
Nematodes suck out liquid nutrients and inject damaging materials into plants.
They injure plant cells or change normal plant growth processes. Symptoms of nematodes include swelling of stems or roots, irregular branching, deformed leaves, lack of blossoming and galls on roots. Nematodes can facilitate the entry of viruses and fungi into plants. Control efforts are difficult and provide limited success. Chemical controls must be done by professional pesticide applicators.