By Robert Harveson Bacterial Wilt of Dry Beans - New or Revisited Disease?
Bacterial wilt, caused by the gram positive, aerobic bacterium, Curtobacterium flaccumfaciens pv. flaccumfaciens (Cff), was first reported from a South Dakota navy bean field in 1922. It then became one of the most problematic bacterial diseases in the USA, particularly throughout the irrigated high plains and Midwest. Bacterial wilt was commonly found in dry bean production in western Nebraska during the 1960’s and early 1970’s but has not been considered to be an important disease in Nebraska since that time. However, new observations have been published from North Dakota and Alberta, Canada in 1995 and 2002, respectively based on the presence of discolored seeds found in processing plants after harvest. More recently, the disease was found in two Nebraska (Scotts Bluff Co.) Great Northern fields in 2003, and was widely observed through the Nebraska Panhandle from multiple (> 40) fields during 2004. Affected fields consisted of beans from multiple market classes, including yellows, Great Northerns, and pintos. During 2005 it was additionally identified from more than 150 fields in Nebraska, Colorado, and Wyoming. Symptoms Field symptoms consist of leaf wilting during periods of warm, dry weather or periods of moisture stress. This occurs because of the pathogen’s presence within the vascular system, which blocks water movement from roots into the foliage. Plants often recover during evening when temperatures lower, but wilt again during the heat of day. Disease is generally more severe on young plants or those exposed to some form of stress. Seedlings are particularly susceptible, and if attacked when 2-3 inches tall usually are killed. Symptoms on adult plants are less pronounced and disease generally develops and progresses more slowly. During the last several years, infected plants in Nebraska have additionally exhibited symptoms consisting of interveinal, necrotic lesions surrounded by bright yellow borders. These symptoms may be confused with those caused by common blight (Xanthomonas axonopodis pv. phaseoli.), but the bacterial wilt lesions tend to be more irregular. Additionally, water soaking of leaves is not generally observed with wilt, as it is in common blight and halo blight (Pseudomonas syingae pv phaseoli) infections. If plants survive to produce mature seed, they are often discolored (results of bacterial infection and colonization of seeds). Seeds may be infected, even while pods appear to remain healthy, and is due to pathogen moving into developing seeds through the vascular system. White-seeded cultivars are particularly prone to quality reductions due to the conspicuously colored seed coats from systemic infections. Several variants of the pathogen have been reported including those that produce yellow, orange, and purple pigments that readily stain white seeds. Infection and Survival Cff is seedborne, and infected seeds represent the major source of inoculum and means for dispersal, both long and short distance. The pathogen can be transmitted both within and on the outside of seeds. It can overwinter on infected residue or weeds, but survival in soil by itself is poor. It has been documented to occur for only 2 years between bean crops grown in rotation with wheat. Cff can also survive for at least 2 years in seeds on the ground. Due to a strong resistance to drying, the pathogen can remain viable up to 24 years in seed stored under optimum conditions in the laboratory. Initial infection occurs when the pathogen enters the vascular system through either infected seed or through wounds on leaves or stems. Disease is not thought to develop via entry into the numerous natural pores in leaves called stomata – unlike common and halo blights. However, disease develops more rapidly and becomes more severe following hailstorms, or when temperatures exceed 90 F. Wilting of plants is more pronounced during moisture stress, and secondary spread occurs in a similar manner for that of common and halo blights. The most important hosts for bacterial wilt are Phaseolus spp., especially P. vulgaris, (common bean), but natural infections also occur on peas, lima beans, cowpeas, soybeans, and lablab beans. Management and Importance The resistant cultivar ‘Emerson’ was developed many years ago by the University of Nebraska specifically for controlling bacterial wilt, but breeding for resistance to this disease was later discontinued. It was determined that using seed sanitation, increasing disease-free seed in dry climates, and using proper crop rotations effectively stopped the introduction of the pathogen while eventually eliminating the pathogen from fields where it was present. Similar control measures used for other bacterial diseases, such as applications of copper, and seed treatments with antibiotics like streptomycin would also be moderately effective for reducing yield losses in current crops, but the key was to limit introducing and maintaining the pathogen in dry bean fields. Losses in both yield and quality have been demonstrated. Some heavily infested fields in Nebraska during 2004 were estimated to yield up to two thirds less than would be expected of a typical crop. Numbers of discolored seeds collected from surviving, but infected plants were found to average 25%, while the incidence of discolored seeds harvested from entire fields were determined to approach 10% in some instances. It is not known why the disease has suddenly appeared again in Nebraska dry bean production over the last 2 years, but it does warrant some concern. This pathogen is considered to be an A2 quarantine pest for Europe, and is subject to phytosanitary regulations for some countries. This means that quarantine regulations require that bean seeds must originate from areas where Cff is absent, or where the seed crop has been found free of the bacterium during the last growing season. The same regulations could also potentially be placed on exportation of beans for human consumption. At the very least, the high incidence of discolored Great Northern seeds would induce an economic loss for growers by way of reduced quality.
|