Studies on the importance and control of potato virus x

The success of seed certification schemes in controlling the aphis-transmitted viruses that cause leaf roll and rugose mosaic has greatly increased the relative importance of potato virus X, in spite of the value of such schemes in reducing the prevalence of the most virulent strains. Virus X occurs throughout commercial stocks of most varieties and is responsible for many of the uncertainties and difficulties encountered in field inspections, for the production of leaf symptoms by many strains depends on the weather. Also, the predominating virus strain in a stock may change from season to season. Methods of testing for the presence of avirulent strains in potatoes are described. These include transmission to suitable indicator plants, plant-protection tests, and serological methods. The virus content of different parts of the plant varies widely, but is everywhere high enough for transmission to indicator plants at all times of the year. The virus content of immature tubers is too low to give a precipitin reaction, but extracts from developing sprouts contain nearly as much virus as those from foliage, and can be used reliably. Infection of virus-free stocks of the varieties Majestic and Arran Banner with four different strains of virus X reduced their yields by amounts varying from 5 to 24%. Scotch Stock Seed Majestic, however, cropped better than the virus-free seed, and the need for considering additional factors to freedom from viruses when selecting clonal lines for propagation is stressed. Virus X was not transmitted by cutting healthy tubers with a knife previously used to cut infected ones, and cut tubers whose storage parenchyma was rubbed with infective sap gave rise to virus-free plants. Infection occurred when sprouts were rubbed with infective sap, and the virus also spread from infected to healthy sprouted tubers stored in the same sack. Spread of the virus under natural conditions is slow, and it is concluded that the maintenance of virus-free stocks under commercial conditions is feasible, provided that precautions are taken to prevent contact with infected plants. The relative importance of volunteer plants and other sources of infection is discussed, and methods of testing the virus-free stocks are suggested.