EFFECT OF AGE OF POTATO FOLIAGE ON EXPRESSION OF LEAF-ROLL SYMPTOMS

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Abstract
In an extensive experiment conducted under field conditions during four successive years, the symptoms of leaf roll of potatoes, as expressed by potato plants of various ages, were studied. Healthy plants of the varieties Netted Gem, Carter's Early Favorite, and Canus were graft-inoculated with infected scions from Canus at 10-day intervals from June 10 to September 15 and protected from viruliferous aphids by a lethal spray applied frequently. Infection of the plants was verified by the tuber-index method. On young plants leaf-roll symptoms were regularly severe and a high percentage of plants became infected. However, the severity of foliage symptoms and also the percentage of plants infected gradually decreased until about August 15. After this date the infected plants were healthy in appearance but the percentage of them actually infected continued to decline sharply. The data presented show that potato plants infected by the leaf-roll virus after midseason normally do not develop definite foliage symptoms of the leaf-roll disease and, therefore, cannot be detected by ordinary field inspection.

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