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Abstract

Studies of two viruses causing mosaic diseases in Soybean.

Abstract

A full account is given of studies carried out in Illinois on two viruses causing soy-bean mosaic, namely, Soja virus 1 [soy-bean mosaic virus] and Phaseolus virus 2 [bean yellow mosaic virus: R.A.M., iii, p. 626; xiii, p. 488; xxiv, p. 44, et passim]. Scarcely any work has been published on soy-bean mosaic in the United States since 1924, but a number of pathologists have observed its presence in all the major areas of production.
On the ' standard' Bansei variety the first symptom appearing six to 14 days after mechanical inoculation with the soy-bean mosaic virus is a transient, yellowish vein-clearing in the minor veins of the growing trifoliate leaves. The rugosity usually present on the third trifoliate leaf formed after inoculation increases in severity on successive leaves and eventually develops into dark green vesicles, either scattered or aligned on either side of the major leaf veins. Vein-clearing of the mature leaves is common. The leaf margins frequently curve downwards at the sides and upwards at the tip. The leaves coarsen, becoming leathery to the touch, and somewhat brittle at maturity. The affected plants are stunted, and bear abnormally few pods, some of which may be laterally curved or twisted, with only scanty pubescence, and often infertile. With the exception of Ogden, the reactions of the oil-type varieties tested was much less severe than those of BanSei.
The initial manifestations of bean yellow mosaic virus are essentially identical with those already described, but more persistent. A pronounced yellow mottle is the typical feature of the successively developing leaves, the chlorotic areas being either scattered or forming indefinite bands along the sides of the major veins. Under certain conditions vivid yellow islets appear on the lamina. As the foliage matures, rusty, necrotic spots are formed in the chlorotic areas. The size of the affected plants was not appreciably reduced, while the pods were of healthy appearance and their numbers approximately normal. There were no marked differences in the response of vegetable-and oil-type soy-beans or of the several varieties tested to infection by bean yellow mosaic virus.
Air temperature exerted a considerable effect on the development of soy-bean mosaic virus symptoms, which were severe at 18.5° C. and largely masked at 29.5° [ibid., ii, p. 77]. The symptom expression of bean yellow mosaic virus was not materially modified by air temperature. The incubation period of both viruses varied widely with the prevailing temperature, soy-bean mosaic symptoms appearing in four days at 29.5°, in six at 24°, and in 14 at 18.5°, while the corresponding times for bean yellow mosaic were five, seven, and 12 days, respectively.
The soy-bean mosaic virus did not induce symptoms in any of the other legumes tested in host range studies, but it was recovered from the symptomless inoculated leaves of the Burpee's Stringless, Stringless Green Pod, and Stringless Green Refugee garden beans (Phaseolus vulgaris) after 23 to 27 days. Bean yellow mosaic virus, however, caused systemic mottling of white and yellow sweet clover (Meli-lotus alba and M. officinalis), crimson clover, Alaska and Telephone peas, and all garden bean varieties tested, and was recovered from these plants. The first symptom observed on certain bean varieties was a drooping of the leaflets at the pulvini; then small, halo-like spots appeared on the blades, expanded, and coalesced to form a prominent yellow mottling, which gradually became more distinct. At maturity the leaflets were cupped downwards and presented a somewhat glossy aspect. The plants were not markedly stunted and the pods were unaffected.
The thermal inactivation point of soy-bean mosaic virus was found to be between 64° and 66° and its longevity in vitro extended over four days. The thermal in-activation point of bean yellow mosaic virus lay between 54° and 56° and it withstood three to four days' ageing in vitro. In neither case could the dilution end points be reliably established.
The transmission of soy-bean mosaic virus through the seed has been demonstrated [ibid., iii, p. 626; xiii, p. 489; xx, p. 444], but yellow bean mosaic virus was not transmitted in the author's experiments in 6, 685 seedlings grown from the seed of inoculated and naturally infected soy-beans, or in 107 from the seed of inoculated garden beans. Soy-bean mosaic virus was conveyed from diseased plants to healthy seedlings by the pea and peach aphids (Macrosiphum pisi[Acyrthosiphon pisum] and Myzus persicae). Bean yellow mosaic virus was transmitted from diseased garden bean plants to healthy seedlings in only one out of nine tests with Macrosiphum pisi.