VIRUS DISEASES OF THE POTATO: THE “CARRIER” PROBLEM. ITS RELATION TO SYMPTOMATOLOGY AND COMMERCIAL POTATO GROWING

Abstract

SUMMARY Symptomatology, of which a study of “carriers” is regarded as one aspect, tends to be used as a means of segregating “unit” viruses rather than as a means of furthering plant physiological studies. The effects of changes in environmental conditions of growth, acting through the unstable metabolism of a plant upon a virus, have not been adequately studied. Until this is done, the specificity of the reactions said to be due to a unit virus must remain in doubt and, with it, the whole foundation of the biological methods of segregating viruses. The symptomatological unit is the plant cell, and the clinical picture resulting from infection is the expression of the varying degree to which invaded cells dominate, or are dominated by, a virus. Successive stages in plant dominance are shown by (a) discontinuous or local symptoms; (b) symptomless shoots; (c) individual carrier plants in a variety usually reacting to infection; (d) carrier varieties. The conditions under which the power to suppress symptoms can be broken down are discussed. Masked symptoms are revealed by relatively slight environmental changes, but the breakdown may be the more difficult to achieve as perfect “carrying” is approached. There is no fundamental difference between “masking” and “carrying”. The conditions under which masking can be converted into carrying are also discussed. Evidence is submitted to show that a perfect carrier plant is not only unaffected in vigour as a consequence of carrying a virus, but may thereby exhibit a reduced susceptibility to certain other viruses in the field. Field trials are described in which proof is given that the viruses carried by the varieties King Edward VII (paracrinkle), and Up‐to‐Date (streak and virus X), were not transmitted to intolerant varieties under the most favourable conditions. These carrier varieties are not therefore the serious menace to other varieties they are commonly believed to be.