A Strain of Clover yellow vein virus that Causes Severe Pod Necrosis Disease in Snap Bean

Snap bean (Phaseolus vulgaris L.) is a major crop in the Great Lakes region of the United States. The most significant production occurs in Wisconsin, Michigan, New York, Illinois, and Pennsylvania. Snap bean crops also are produced in the Pacific Northwest, Georgia, Florida, and Ontario, Canada. In 2005, 124,367 ha of snap bean were harvested with a farm gate value of approximately $400 million (35). Because most snap bean pods are produced for fresh market, freezing, or canning, pods harvested with imperfections due to disease or mechanical injury are discarded, effectively reducing the growers’ yield. Pods harvested with imperfections that exceed allowable thresholds result in rejection by processors of the entire harvest. Snap bean cultivars in the United States are susceptible to many important plant viruses that may constrain production in the absence of resistance. The potential negative effect of viruses on snap bean production was fully evident in 2000 when a severe virus outbreak occurred in Wisconsin and other neighboring snap bean production states. The most frequently detected viruses included Cucumber mosaic virus (CMV) and many distinct strains of Alfalfa mosaic virus (AMV) (8,13). The outbreaks were directly associated with high populations of the soybean aphid (Aphis glycines) in adjacent soybean fields (9). A high incidence of mixed infections of CMV and AMV was associated with necrotic lesions or line patterns on pods (8). Other viruses detected included Bean common mosaic virus (BCMV), Bean yellow mosaic virus (BYMV), Clover yellow mosaic virus, Tobacco streak virus, and White clover mosaic virus. Although BCMV was detected, Bean common mosaic necrosis virus (BCMNV) never was detected in any samples evaluated. Significant virus outbreaks again occurred in 2001 (13) and 2003 to 2005 (R. Larsen, unpublished) in Iowa, Illinois, Kentucky, Michigan, Minnesota, New York, Wisconsin, and the province of Ontario, Canada. The same complex of viruses resulted in yield losses up to 100%, especially in fields of late-season processing and fresh-market snap bean crops. During these growing seasons, snap bean plants in fields were observed showing extensive external and internal pod necrosis, a disease termed “chocolate pod” by local growers. The necrosis frequently affected 75 to 100% of the pod surface. Clover yellow vein virus (ClYVV) (family Potyviridae, genus Potyvirus) was suspected as the causal agent based on preliminary host range response; however, identity of the pathogen was not immediately confirmed. ClYVV causes mild to severe mosaic, leaf malformation and vein necrosis, stunting of the plant, and apical necrosis in highly susceptible cultivars (22). Pods often are mottled, twisted, or malformed. Symptoms in bean plants infected with ClYVV can be highly variable due to the numerous strains of the virus and their interaction with different bean cultivars. Symptoms frequently resemble those caused by the closely related Potyvirus BYMV, making visual distinction between the two viruses difficult. Genes for resistance to ClYVV have been reported in the literature. The cyv gene present in US1140 (23) was transferred to ‘Black Knight’ (27), UI-31 possesses two recessive genes (32), and the desc gene discovered in snap bean cv. Jolanda also was present in snap bean cultivars Imuna and Evolutie (26). The latter three cultivars are not currently grown in the United States. The reactions of these resistant sources against isolates of ClYVV from the Great Lakes region are unknown. To our knowledge, no other commercial snap bean cultivars have been evaluated for resistance or tolerance to ClYVV. The objectives of this work were to validate whether ClYVV was responsible for the chocolate pod disease in snap bean, and to identify sources of resistance to the virus.