Alireza Akhavan, PhD, AAg, Provincial Plant Disease Specialist, Crops and Irrigation Branch, Regina, and Morgan Coté, AAg, Provincial Irrigation Agrologist, Crops and Irrigation Branch, Outlook
July 2024
Common bean (Phaseolus vulgaris L.) is consumed globally as a great source of protein. Most dry bean production in Saskatchewan occurs in the irrigated area around the Lake Diefenbaker reservoir. Eliminating drought stress, especially in the flowering and pod-filling stages, results in high yield. Several bacterial diseases may affect common beans. These include common bacterial blight (CBB), halo blight and bacterial brown spot. All three are considered important; however, CBB is known to cause economic loss worldwide, and seems to be more common in Saskatchewan.
Common bacterial blight is caused by Xanthomonas axonopodis pv. phaseoli Xap). Despite being seed-transmitted, CBB is considered a foliar disease. Symptoms initially appear as small, water-soaked leaf lesions that enlarge and become necrotic. Often, lesions have a chlorotic boundary. Individual lesions may merge causing plants to look burnt. Symptoms may also be observed on stems, pods, and seeds. Pod lesions are circular with chlorotic margins and may look greasy. As the disease progresses, pod lesions may appear brown to red. Infected seeds can be identified by yellow to brown spots. Germination of diseased seed is reduced, and seeds have low vigour. Sometimes infected seeds have no visible symptoms, look very healthy and can germinate vigorously while they carry many bacterial cells leading to disease development at a later stage.
Overhead irrigation has effects like rain; it can favour bacterial dispersal from colonized to healthy leaves and plants. This is the key factor in increasing the disease in a field. Overhead irrigation and rain may also help the bacterial population to enter the plants through leaf openings and develop symptoms subsequently. Both overhead irrigation and rain form a layer of water on the host plant’s surface, which results in an ideal multiplication site for this pathogenic bacterium. The film of water also helps to increase nutrient release from plant leaves, which can enhance the availability of nutrient sources for the pathogen.
Since CBB is a seed-borne pathogen, using disease-free seed is critical, particularly when growing on cropland with overhead irrigation or in wet areas. This is important because all outbreaks of the disease have resulted from planting infected seeds. Producers must also be careful not to over irrigate. Proper crop rotation practices may also help decrease the pathogen, as the pathogen was shown to be able to stay viable over winter in stubble in some bean-growing areas. Copper-based bactericides can be applied to manage bacterial blights in season, but their effectiveness may be limited when the environmental condition is conducive for the development of the disease.