I reported on my discovery of this bean problem in my Harvest Monday post. Unfortunately, I was not able to identify the problem plaguing my pole beans (picture above) using the nifty Visual Diagnostic Aid I posted here. So I turned to “teh google” to find what appears to be my problem: a bacterial bean blight called Halo Bean Blight. I have documented what I found below.
There are at least three different kinds of bean blight: Common, Halo, and Fuscous. I think what my plants have is called Halo Blight, so named from the yellow halo that appears around the tiny infected spot on the leaf. The blight is caused by the bacterium Pseudomonas syringae pv. phaseolicola. Infection is caused by infected seed and is spread by splashing water and water aerosols from wind-driven rain. It flourishes in cooler temperatures with lots of moisture and is inhibited by hot, dry weather. That may explain why it has suddenly affected the beans, since both tropical storms Irene and Lee produced lots of wind driven rain and cooler temperatures.
Halo blight is distinguished from common blight by the yellow halo around the small infected spot. The damage starts as a small wet-looking spot on the underside of the leaf that turns reddish brown. A yellow halo of chlorosis then forms around the spot. In common blight, the brown spot is much larger and irregular in shape. The center may dry up and fall out. The yellow around the brown spot is narrow and irregular in shape. So I am pretty sure what is shown above is halo blight. Temperatures in the70s favor the spread of the disease, while higher temperatures inhibit growth. Systemic infections can cause defoliation of the plant.
On the pods, the infection starts as a tiny wet-looking spot that turns brown. In the picture of beans at the top of the page, you can see the “wet-looking” areas on the bean at the bottom, on the left end. The lesion becomes sunken with dark edges and a white or cream colored center. The lesions can then infect the seeds inside the pod, allowing the disease to be passed by infected seed. In severe cases, the pod can become shriveled or misshaped.
What can be done about it? Not much at this point. Now that temps are back in the 80s, I am seeing less damage and actually getting a pretty good crop.It is best to avoid the problem by using disease-free seed. The pole beans were a last-minute impulse item so they were not part of my Fedco order in March. I picked up a package of Blue Lake pole bean seeds at a local farm stand, packaged by a Connecticut seed company. Since no one in my garden or on the blogs have ever seen the problem, I think bean blight is fairly uncommon among home gardeners and mostly affects large field growers. Here is a summary of what I learned:
- The best cure is prevention. Use disease-free seed from a reputable seed company, preferably seed certified to be disease-free. The New York state site advises buying seed grown in western states where it is much drier and disease less likely to occur.
- Seed can be treated with agricultural streptomycin before planting to kill any contamination.
- Copper soap sprays can be effective against halo blight (only) if used as soon as infection starts.
- The blight seems to mostly affect pole varieties, less so bush beans.
- The bacteria is spread by seed and plant residue. The good news is it does not overwinter in the soil. By removing and disposing of all plant material from the bed, I may be able to plant the bed again with beans in a year or two. Commercial farmers have trouble removing all of the contaminated vegetation from their fields so contamination may persist.
- There are no disease resistant varieties of bean.
- As usually recommended for beans, don’t spray the foliage and don’t handle when wet to avoid encouraging and spreading the disease. Sanitize any equipment used in the garden.
- Dispose of any volunteer bean seedlings. Don’t save seed and be careful of accepting seed from a non-certified source such as a seed saver exchange.
- Don’t soak beans before planting. If one bean is infected, it can infect the entire bowl of seeds. Discard any seeds that look discolored or are blemished in any way since they may be blighted.