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VEGETABLE CROPS Bacterial Blights of Beans Page: 729.10 Date: 1-1979 COOPERATIVE EXTENSION • NEW YORK STATE • CORNELL UNIVERSITY by Arden Sherf Dept. of Plant Pathology, Cornell University

Common Blight
(Xanthomonas phaseoli)

Characteristic leaf symptoms of common blight consist of irregular areas of brown dry tissues surrounded by a narrow lemon yellow border (fig. 1). These lesions frequently occur at the leaf margins. Pods have sunken circular spots, at first water-soaked, but later dry, with a reddish brown narrow border. The bacteria invade the seeds and remain dormant until germination begins. Even a trace of infected seed when planted can initiate severe infection of entire fields. The bacteria exude in the leaf and pod spots and are spread mainly by splashing and blowing rain. Warm, humid conditions favor development of the disease.

Halo Blight
(Pseudomonas phaseolicola)

Halo blight is the most important bacterial disease of beans in New York State and is the main reason seed must be imported from the dry West. The most characteristic symptoms occur on the bean leaves. Small, water-soaked spots, resembling pinpricks, develop on the undersides of the leaves. These spots soon turn reddish brown, and the tissues surrounding the spots gradually become yellow green (fig. 2). The zone of yellowed tissues resembles a halo, thus the name halo blight. Temperatures of 70° F favor halo development, whereas 80° F and higher temperatures inhibit halo formation. Severe infections resulting from seed contamination may give internal systemic infection, exhibited by yellowing and stunting. These plants defoliate, wilt, and die early, but serve as important reservoirs of bacteria to be spread to neighboring plants. Halo blight pod symptoms appear first as tiny, water-soaked pinpricks on the surface. These gradually enlarge to form dark, sunken spots of varying sizes, which may have a white or cream ooze in their centers. Such spots cause severe quality loss in both fresh market and processed beans and may result in rejection of an entire field. Cool, moist conditions favor spread and development of halo blight. The bacteria are spread by splashing rains, hail, overhead irrigation water, windblown soil, rabbits, deer, and especially cultivators and sprayers.

Controls 

Since both these disease can be seedborne, the use of western grown, preferably certified seed is the most important measure. Planting beans in ground where beans have not been grown for 2 to 3 years will eliminate the threat of debris infection. Suspending field operations when leaves are wet reduces chances of field spread. Plowing down harvested bean stubble in the fall hastens the destruction of the organisms. There are no resistant varieties of snap beans; however, Redkote, a release from Cornell, is a red kidney bean variety with good tolerance to the most important race of halo blight. Spraying with fixed copper is of some benefit if applied at first signs of the disease (halo blight only).

Figure 1.  Common blight (Xanthomonas phaseoli) of beans	Figure 2.  Halo blight (Pseudomonas phaseolicola) of beans

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