AN ANALYSIS OF THE EFFECTS OF INFECTION WITH LEAF-ROLL VIRUS ON THE GROWTH AND YIELD OF POTATO PLANTS, AND OF ITS INTERACTIONS WITH NUTRIENT SUPPLY AND SHADING

The decrease in total dry weight of potato plants (var. Craig's Defiance) caused by infection with leaf-roll virus was almost entirely at the expense of the tubers; the dry weight of the leaves and stems was increased, and that of roots and stolons only slightly decreased. The percentage loss of dry weight of the tubers was decreased by nitrogenous fertilizer, but was unaffected by phosphorus or potassium. Infection greatly decreased net assimilation rate (NAR) between May and early July, but subsequently infected plants had a higher NAR than healthy plants. Infection also decreased leaf area per plant until early July, but it delayed senescence of the leaves, so that in late July and August infected plants had more leaf arca than healthy plants. The more rapid senescence of healthy plants probably accounts for their lower NAR at this time. The mean decrease in NAR by infection was about 20 %, and this was the chief cause of the loss of yield, for the mean decrease in leaf area was only 4 %. Comparisons of intact plants with plants from which either the rolled or the unrolled leaves were removed indicated that the lower NAR of infected plants was mainly due to diminished photosynthetic efficiency of the older, rolled leaves. The effects of nitrogen, phosphorus and potassium on NAR, and their interactions with infection, were small and rarely significant. Nitrogenous fertilizer increased leaf area throughout the growth period; phosphorous increased it in the early stages but hastened senescence of the leaves, so that in August it decreased leaf area; potassium increased leaf area particularly in the later stages of growth. At the times when nitrogen, phosphorus or potassium increased leaf area they usually also increased the effect of infection as this tended to reflect plant size. Except in the early stages of growth infected plants had more leaves than healthy plants, because leaf production continued longer and more lateral branches developed. Shading decreased NAR but increased leaf area; consequently it had no effect on yield. There were no interactions between shading and infection. The loss of yield caused by infection of Up-to-Date plants was smaller than with Craig's Defiance because the leaf area of Up-to-Date was not decreased by infection, and the depression of NAR started later than with Craig's Defiance.