FURTHER STUDIES IN THE RING‐SPOT DISEASE OF TOBACCO

Abstract

In a recent paper 2 some of the symptoms of the ring-spot disease of tobacco were described and pictured. A careful study of these symptoms led to the belief that ring spot belongs in the virus disease group of plant maladies. The object of the present paper is to report the results of some further studies on the disease. Special attention has been given to symptoms, host range, and means of transmission. Studies have also been made on the keeping qualities of the infectious agent in juices stored at different temperatures. Although ring spot frequently produces chlorosis which causes mottling, it is essentially an infectious necrosis. On tobacco it attacks leaf tissues only. No visible symptoms ever appear on stems or on large leaf veins. It is typically a leaf-spot disease. Chlorotic and necrotic spots occur in the form of small circular areas, rings with green centers, concentric circles, or wavy lines which follow along the leaf mid-ribs and leaf veins. The tissues composing the spots commonly die. In some cases, however, they live indefinitely but are more or less chlorotic. Ring spot is a systemic disease. Lesions usually appear on all or most of the leaves above the point of inoculation. It differs from most virus diseases, however, in that the first symptoms appear at or near the point where a leaf is inoculated. Systemic infection occurs a few days after the first symptoms of local infection appear. Ring-spot disease has been produced repeatedly on definite portions of leaves by applying inoculum to those portions. In one experiment thirty plants were inoculated. One leaf on each of ten plants was inoculated by applying plant juice containing the infectious agent to portions of the blade on one side of the mid-rib. Within four days rings developed on the inoculated portions. Uninoculated portions of these leaves remained healthy, as shown in text figure i. Ten of the plants were inoculated by applying infectious juice to the basal portions of a leaf on each plant. After four days rings developed on the inoculated areas but did not appear on other parts of the inoculated leaves. The other ten plants were inoculated by applying infectious juice to the tip portions of their leaves. Here