These parasites are the smallest of microorganisms causing plant disease. Viruses are extremely small (they can be seen only with the aid of an electron microscope), being composed of nothing more than a protein shell containing a small amount of genetic material. Plant viruses usually survive year-round in perennial weeds and trees or insect vectors, and are transmitted to other plants through the feeding activities of certain insects, usually aphids, through grafting and by mechanical means. Mycoplasma-like organisms (mycoplasmas, spiroplasmas and rickettsias) are generally considered to be somewhere between bacteria and viruses in size, shape and function. These organisms are the cause of 'yellows' type diseases and are transmitted to plants during leafhopper feeding. Some of the more common symptoms of virus and MLO diseases are described here. Mosaic is characterized by a light green to yellow leaf mottling, usually accompanied by abnormal leaf growth. Vein banding is noticeable as a light green to yellow band around the leaf veins which may turn dark with time. Ring spot causes rings of light green to yellow alternating bands with the normal green of the leaf. Yellows is apparent when the entire plant or some parts turn uniformly yellow. Stunting usually accompanies all of the previous symptoms, or stunting may be the only symptom. Some virus diseases do not show any visible symptoms other than a yield reduction. Many virus and mycoplasma diseases are transmitted from plant to plant by insects, some are transmitted by grafting and others are transmitted by mechanical means. These diseases are difficult to control. Some are controlled by resistant varieties, the use of virus free seed or planting stock, eradicating the alternate host and by controlling the insects that transmit the diseases.