Studies on a virus causing foliar necrosis of the potato

In a previous communication (Bawden, 1932) two types of necrotic disease of the potato were described under the names of Acronecrosis (Top-necrosis) and Acropetal necrosis, names first suggested by Quanjer (1931). It was there shown that the external and internal symptoms of virus-infected potatoes are closely correlated, and that the former might with advantage be used as the basis of classification, instead of the latter, which are used by Quanjer. Further work has now shown that “ streak ” diseases of the potato exist, which cannot be included in cither of the above groups, and which, in Quanjer’s classification based on the internal symptoms of the stems, would have to be placed in the Anecrotic group. It is the purpose of this paper to describe some of the properties and reactions of a virus which in certain potato varieties causes a severe necrotic disease of the foliage without, apparently, producing any internal changes in the stem. It is proposed to call this disease Foliar necrosis. The following features of Foliar necrosis serve with comparative ease to distinguish it from other necrotic diseases of the potato. Firstly, from Top-necrosis it is distinguished by the direction of spread of the symptoms (in Foliar necrosis it is acropetal, in Top-necrosis basipetal),* by the presence of a pronounced mosaic, and also by the freedom of the main and axillary growing points from destructive necroses. Secondly, it is distinguished from Acropetal necrosis by the essentially interveinal character of the lesions, both mottle and necroses, and by the absence of the elongated brown streaks on veins, petioles, and stems so characteristic of this last disease. Thirdly, it is distinguished from both Top-necrosis and Acropetal necrosis by the absence of any internal lesions in the stems and by the two distinct phases of the disease which follow each other so rapidly in the first year of infection.