Streptomycin and fixed copper sprays may help slow the spread and development of bacterial diseases. However, chemical controls for bacterial diseases are less reliable than for many fungal diseases. Previously many mycoplasma-caused diseases were thought to be caused by viruses. Yellows disease is usually spread from one host plant to another by an insect vector. Leafhoppers are common vectors for mycoplasmas.
Viruses multiply only within living cells of host plants.
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.