Alternaria leaf and pod spot is a common and destructive disease of snap bean in Florida, especially in the southern part of the state. It also is widespread in other bean-growing areas of the U.S., Latin America, and Western Europe.
The most serious symptoms of this disease occur on bean pods, since damage to the marketable portion of the plant can render the produce unfit for sale. Lesions on pods usually appear as very small, dark-brown to black flecks. When examined with a hand lens, these flecks are somewhat raised and cone-like. When only a few flecks occur on a pod, the damage may be insufficient to result in rejection at the packinghouse. Large numbers of unsightly flecks, however, can result in rejection of the entire lot, especially at lower market prices.
Leaf symptoms first appear as small, water-soaked flecks that rapidly develop into circular to irregular spots with pale-brown centers and reddish-brown borders.
Faint, concentric rings may occasionally be visible in older lesions. As the disease progresses, leaf lesions may merge together leading to large, blighted areas and premature leaf drop.
Survival and Spread
Several species of Alternaria have been reported as attacking beans, but the consensus is that Alternaria alternata is probably responsible for most outbreaks in Florida. Normally, this species is a weak pathogen and not as aggressive as Alternaria solani, which, causes the devastating early blight of potato and tomato. Ideal conditions for the development of Alternaria leaf spot include high relative humidity, rainfall, and cool temperatures (60°F to 75°F for daytime highs). Under these conditions, Alternaria leaf spot can result in major losses in snap bean. Severe outbreaks of the disease can be expected from January through March in Homestead, Belle Glade, and Devil’s Garden growing areas.
For scouts and others with access to a microscope, the multicelled, pigmented spores that have both transverse and longitudinal septa (cell walls) and a short “tail” or “beak” are diagnostic of the disease.
Management of Alternaria leaf and pod spot consists of maintaining adequate crop nutrition and avoidance of close between-row and within-row plant spacing.
Fungicides also play a major role in the integrated management of this disease.
Beans that are nutritionally deficient in nitrogen and/or potassium are most susceptible as are those planted at high densities, which can result in more frequent disease incidence and greater disease severity.
It is particularly important that effective fungicides be applied when pods are small (pin pod stage) in order to avoid infections that will be evident later as pods mature.
Strobulurin fungicides have given good results, but should be applied according to the label and rotated with materials with other modes of action to avoid potential problems with resistance.
Strict attention paid to culling of infected pods at the packinghouse can help maintain the confidence of buyers and brokers.