Alternaria Leaf and Pod Spot of Snap Bean in Florida¹

Ken Pernezny and R. T. McMillan, Jr.²

Alternaria leaf and pod spot (ALPS) is a very common and potentially destructive disease of snap bean in Florida especially in the southern part of the state. It is also reported to be widespread in other bean-growing areas of the United States, Latin America, and western Europe.

Symptoms

The most serious symptoms of this disease occur on bean pods, because the direct damage to the marketable portion of the plant renders the produce unfit for sale. Lesions on pods usually appear as very small, dark-brown to black flecks. If these flecks are examined with a hand lens, they are seen to be somewhat raised and cone-like. When only a few flecks occur on a pod, the damage may be insufficient to result in cullage at the packinghouse (Fig. 1). However, large numbers of unsightly flecks (Fig. 2) can result in rejection of the entire consignment, especially at lower market prices.

Figure 1. Tiny, black specs associated with light infection of snap bean pods by Alternaria alternata.

Figure 2. Severe symptoms of Alternaria pod spot.

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 (Fig. 3). Faint, concentric rings may occasionally be visible in older lesions. Coalescense of leaf lesions can lead to large, blighted areas and premature leaf drop.

Figure 3. Leaf symptoms of Alternaria leaf and pod spot.

Cause and Disease Cycle

Several species of Alternaria have been reported as causing ALPS. However, the consenus is that A. alternata is probably responsible for most outbreaks in Florida. Generally speaking, this species is a weak pathogen. It is certainly not as aggressive as Alternaria solani that causes the devastating early blight of potato and tomato or A. cucumerina which causes Alternaria leaf spot of cucurbits. Ideal conditions for ALPS include high relative humidity, rainfall, and cool temperatures (60-75 degrees F or 15.5 or 24 degrees C for daytime highs). Under these conditions, ALPS can result in major losses in snap bean. Historically, we have observed the most severe problems with ALPS from January through March in Homestead and the Devil's Garden growing areas of southern Florida.

For scouts and others that have access to a microscope look for multicelled, pigmented spores that have both transverse and longitudinal septa (cell walls). A particularly important feature of A. alternata is the short, "tail" or "beak" of the spore.

Bean crops that are nutritionally stressed are most susceptible. Plants that are deficient in nitrogen and/or potassium are most likely to show ALPS damage.

Narrow row spacings and high seeding rates that lead to high plant densities within rows result in more frequent epidemics of ALPS and greater disease severity.

Control

Consult the University of Florida, IFAS Plant Disease Management Guide and current labels for specific and current fungicide recommendations. Maintain adequate nutrient status of the crops. Avoid close between-row and within-row plant spacings. Fungicide sprays still play a major role in the integrated management of this disease. It is vitally important that effective fungicides be applied when pods are small (pinpod stage) in order to avoid infections that will be evident later as pods mature. Attention should be paid to proper culling of infected pods at the packing house in order to maintain the confidence and goodwill of produce brokers and retailers.

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Footnotes

1. This document is PP-61, one of a series of the Plant Pathology Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Publication date: June 2002. Reviewed: June 2008. Please visit the EDIS Web site at http:/edis.ifas.ufl.edu

2. Ken Pernezny, professor, Plant Pathology, Everglades Research and Education Center, Belle Glade, and R. T. McMillan, Jr., professor emeritus, Plant Pathology, Tropical Research and Education Center, Homestead, Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, 32611.

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U.S. Department of Agriculture, Cooperative Extension Service, University of Florida, IFAS, Florida A. & M. University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Larry Arrington, Dean.