EFFECTS OF NUTRITION AND LIGHT INTENSITY ON SYMPTOMS OF LEAF-ROLL VIRUS INFECTION IN THE POTATO PLANT

Increased nitrogen supply to potato plants infected with leaf-roll virus greatly reduced the intensity of leaf symptoms, and caused a smaller and less consistent reduction in phloem necrosis. Application of phosphatic fertilizer initially reduced leaf symptoms, but later increased them; it increased phloem necrosis at all stages of growth. Potash slightly intensified leaf symptoms, but not phloem necrosis. Shading, which approximately halved the light intensity, reduced leaf symptoms but had no effect on phloem necrosis. The masking of leaf symptoms by nitrogen, and by phosphorus in the early stages of growth, was closely correlated with increases in relative leaf growth rate produced by these nutrients. Leaf rolling and phloem necrosis are considered to be independent symptoms not causally related, because they were differentially affected by nutrient supply and shading, and each could occur without the other.