Symptoms: This disease stunts, but rarely kills plants. Symptoms vary by host, virus strain, and time of infection. Leaves usually have an irregular mosaic pattern of light yellow and green areas, or a band of darker green color may follow the veins while the rest of the leaf remains green. Leaves may also be puckered or malformed, often with a downward cupping of the entire leaflet, or the leaflets may be narrower and longer than normal. Leaves may roll. Occasionally, bright yellow dots may be seen on the leaves. When the plants are infected early, they may be severely stunted and spindly. Seed set is usually affected.
The varieties Kentucky Wonder, Corbett Refugee, Creaseback and varieties bred from them have an entirely different set of symptoms. The top leaflets wilt during the bloom period. Plants appear grayish green in color. Later, the whole plant wilts and eventually dies. The roots appear dark, and dark markings to appear on the lower stem and the petioles. On some varieties, a net-like browning of the veins occurs on the leaves.
Vector: This disease is spread from plant to plant by at least 12 aphid species. Many of these feed on alfalfa, clover, and rye. It has been noticed that beans planted near these crops tend to have Common mosaic worse than those in other fields. Transmission is in a non-persistant manner. This means that the aphids acquire the virus almost immediately from infected plants, but are able to transmit it to healthy plants for only a few days to a week.
Prevention: Use certified disease-free seed! This virus can survive in seed for more than 30 years, and can survive heat treatments hot enought to kill the seed. Do not plant beans near alfalfa, clover, and rye. Resistance is available, and is the most effctive way to control this disease. See current recommendations for aphid control.
Bean Yellow Mosaic is caused by Bean Yellow Mosaic Virus (BYMV, BV-2). This virus can infect beans; gladiolus; sweet, white, and red clovers; peas; lupine; violet; and pumpkin. It was first found in Wisconson in 1931, and is now worldwide. It may cause minor to severe damage.
Symptoms: Symptoms vary greatly depending on the species and age of the host plant and the strain of the virus. Generally, the leaflet that was first infected droops about a week after becoming infected. Small yellow spots about 1/8" in diamter then form on the leaves. These spots grow together, giving the leaflet a mottled yellow and green appearance, which becomes more intense as the plant ages. In some cultivars, this mosaic may be mild; in others, it may be very prominent. If the mosaic is prominent, the other symptoms are more likely to develop. Young leaves may be stiff, glossy, or curled upward, or otherwise malformed. The plants may be stunted, have a bunchy appearance, or flower and fruit later than uninfected plants. In severe cases, dead spots, tip and vein death, and even premature death of the plant may occur.
Different strains of the virus may cause different symptoms. One strain causes purpling at the base of the lower leaves, blackening of the veins, and plant death. Another strain causes very severe mottling, death of the top of the plant, and leaf drop. Yet another strain causes dead patches to appear on the leaves, which may cover large portions of the leaf.
Identification of disease: Increasing intensity of mottling is present as the plant ages.
Vector: This disease is spread from plant to plant by more than 20 aphid species. It is not carried by seed. The aphids include the black bean aphid, which is black to dark olive green in color, with white markings.
Prevention: Plant as far away as possible from clovers, other legumes and gladiolus. Resistance is available, and is the most effective way to control this disease. See current recommendations for aphid control.
By Pamela S. Mercure, IPM Program Assistant, University of Connecticut
References:
Biddle, A.J., S.H. Hutchins, and J.A. Wightman. Pests of Leguminous Crops. Chapter 6 in Vegetable Crop Pests, R.G. McKinlay, ed. CRC Press, Inc, Boca Raton, FL. 1992.
Drijfhout, E. Bean Common Mosaic. pp. 37-39 in Compendium of Bean Diseases. R. Hall, ed. APS Press, St Paul, MN. 1991.
Provvidenti, R. Bean Yellow Mosaic. pp. 46-47 in Compendium of Bean Diseases. R. Hall, ed. APS Press, St Paul, MN. 1991.
Sherf, A.F. and A. A. MacNab. Vegetable Disease and Their Control. John Wiley and Sons, New York. 1986.
Originally published in Grower, New England Vegetable and Small Fruit Newsletter. Vol. 98-5, May 1998. p.5.
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