Identity of a chlorotic ring-spot virus from kale (Brassica oleracea var. acephala)

Abstract

The results obtained from several pathogenicity trials with leaf extracts from a kale (B. oleraceae var. acephala) plant showing mottle and chlorotic ring-spot symptoms, have disclosed thai the physiological disturbance is produced by a virus. The virus can readily be obtained from kale and transmitted mechanically with carborundum to several crucifers, but not to noncrucifers. The reaction of crucifers to inoculation is characterized by initial vein-clearing, vein-banding, systemic mottling, and chlorotic ring-spots on the leaves. Among the crucifers tested, kohlrabi, cauliflower, Chinese cabbage, kale, and rutabaga showed the most conspicuous symptoms of mosaic. Kale, rutabagas, and Chinese cabbage developed chlorotic ring-spot symptoms and mottle; and Chinese cabbage reacted usually with faint mottle and leaf curling. The symptoms were seen clearly at temperatures around 20°C. and gradually became inconspicuous at temperatures below 14° or above 27°C. Brussells sprouts and broccoli were slow in showing mottle symptoms and cabbage appeared to be the least to react to infections, very faint symptoms being produced at the optimum temperatures. In comparing the cauliflower virus from California and the turnip virus from New York, Tompkins (18) found that the turnip virus produced symptoms in Colma cabbage and February cauliflower very similar to those described by Clayton (2) on brussels sprouts and cauliflower, but Tompkins demonstrated that turnip virus from New York caused symptoms on Nicotiana tabacum and N. glutinosa, brussels sprouts, rutabagas, and Chinese cabbage, and no infection on sprouting broccoli, kohlrabi, raddish, and N. langdorsffii, thus showing that the cauliflower mosaic virus is different from the turnip virus and the rutabaga virus described by Clayton from New York. The work done in Cornell with the kale mosaic virus indicates that the kale mosaic virus and the rutabaga mosaic virus described by Clayton are related to the cauliflower mosaic virus described by Tompkins (18) from California. Clayton did not determine the physical properties of the rutabaga virus. The virus from kale does not cause infection in noncrucifers; its physical properties are: Thermal inactivation point around 80°C, dilution tolerance of 1:2000, and aging in vitro approximately 14 days. All these strongly point to the conclusion expressed above. An apparent discrepancy between the kale mosaic virus and that of the cauliflower rests on the temperature range for maximum symptom expression, though not for infection. Tompkins (18) has shown that the cauliflower mosaic virus produces the most marked symptoms in cauliflower at temperatures from 10°C, and that at temperatures from 20-30°C, the symptoms are masked. However, there is the possibility that, even though there is masking of the symptoms, the rate of multiplication of the virus is not suppressed by higher temperatures, if Ave consider the results obtained by Pound and Walker (11) in which they showed that, at temperatures of 16°C, the cauliflower mosaic virus studied produced mottle symptoms in Jersey Queen cabbage in 18 to 21 days, while at higher temperatures the incubation period was gradually decreased, and at 28°C. the time for symptom expression was within 9 to 10 days. The titer of the virus at the different temperatures was not determined, therefore there is no evidence of the relation of time-of-incubation period and virus multiplication at various temperatures. One explanation of our apparent discrepancy as to the epiphytology of this disease could be based on daily fluctuations of the temperatures in the greenhouse, which veils the true relations of the temperature and the incubation and symptom expression of the disease. One thing is obvious, and that is that the temperature ranges used in our work were very wide, and there were opportunities for symptoms to develop either when the temperature was at the lower or at the higher limit of the range, or at any point therein. In order to have a clear picture of the situation it would be necessary to conduct further trials under constant ah temperatures, running parallel tests with the cauliflower mosaic virus for comparative results. In his work with the true cruciferous virus, Clayton found that rutabagas, white and black mustard, Chinese cabbage, turnip, and rape were susceptible; brussels sprouts and cauliflower not easily infected; and cabbage was either resistant or immune. These last results do not seem to be conclusive since Clayton did not use carborundum in his work. At 21 to 27°C. he found that Chinese cabbage and mustard developed streaks. At 12 to 18°C, brussels sprouts and cauliflower recovered completely and the symptoms disappeared either below 13°C. or above 27°C. The difference between the true cauliflower and the cauliflower mosaic viruses is based on the virus-suscept reaction at different temperatures. Tompkins' virus works best at temperatures from 10° to 19°C, and Clayton's at temperatures from 20° to 27°C. The trials conducted here in connection with the kale mosaic virus are too limited to warrant the exact classification of the virus. They do, however, show that the kale mosaic virus belongs to the cauliflower virus category, and is very closely related to the cauliflower mosaic virus described by Tompkins from California. The kale mosaic virus is either the cauliflower mosaic virus or a very close virus entity. Further work on cross-immunity reactions and possibly serological tests could clarify the situation.