species that colonize potato, with the green peach aphid being the most efficient. The insect vector is absolutely essential to spread because mechanical transmission, like that which occurs when the leaves of an infected plant rub on a healthy one, does not occur with PLRV. The infection process is actually quite complicated with this virus. First the aphid must acquire the virus by feeding on a PLRV infected plant. Then the virus must circulate from the gut of the aphid, through the circulatory system until it finally gets into the salivary glands, from which it can be excreted when the aphid feeds on healthy plants. Only after this has happened can the aphid spread the virus. This sequence of events may require 24 hr. or more to occur. Unfortunately, once an aphid becomes infected, it remains so for the rest of its life. Spread of the virus between plants within a field and between fields can be done by the winged forms of the aphid but most spread within a field, especially from infected plants to nearby, healthy ones, is accomplished by the wingless forms. Seed certification programs allow only a very small level of PLRV in certified seed. In Idaho, for instance, during the second field inspection the allowable amount of PLRV is only 0.05% for G4 seed, 0.01% for G3 and G2 and none at all allowed in nuclear and G1. Very small percentages of PLRV in seed potatoes do not normally pose any risk for the commercial producer. However, even very small percentages of virus can be a problem if green peach aphids appear very early and in abundance. In years that are very favorable for insects, like the 1996 season was, the aphid population can become so large that even a very low percentage of PLRV infected seed could result in sufficient spread to cause a problem, because as the aphid population increases, so does the probability that they will encounter an infected plant. Control of aphids with insecticide application is the only means of managing this problem in production years that are highly favorable for aphids. Seed borne infection generally results in small, stunted, badly impaired plants which have reduced yield both in tuber numbers and in tuber size. Large tubers that show the typical net necrosis symptom may well be the result of current season infection. PLRV infects other hosts including moonflower, Datura stramonium