Margaret Tuttle McGrath
An explanation for the severity of bacterial brown spot in 2001 is provided by information from research conducted in Wisconsin, where this disease occurs regularly. Intense rain was demonstrated to provide conditions favorable for rapid multiplication of the bacterial brown spot pathogen and for infection. The importance of hard, intense rain was demonstrated by putting a screen over some plants in a field of beans to intercept the raindrops and break their fall. These plants received the same amount of rain as other plants in the field, but the droplets were not moving as fast. After the rainstorm, fewer bacteria were detected on the protected plants and symptoms of bacterial brown spot subsequently developed throughout the field but not on the protected plants. Intense rains may affect the plant in a way that makes nutrients more readily available, removes some sort of toxin or inhibitor, or even helps the bacteria enter the plant. Additionally, intense rains are often associated with thunderstorms, which provide the bacterial brown spot pathogen with an effective means to move out of the atmosphere and to the ground. This pathogen is an ice nucleating bacteria. Thus ice that commonly forms in thunderstorms will develop around these bacteria, then they will fall out of the clouds with the ice. Thunder cells can be local; therefore, where these storms are the source of the pathogen, it is possible for brown spot to be severe in one field but not another field just a few miles away. These rains do not, however, provide good conditions for splash dispersal.
It was also discovered through research conducted in Wisconsin that this pathogen can survive and multiple on healthy plants for a long time without causing disease if conditions are not favorable. Bacteria were detected on most leaves in a bean field before symptoms developed. These epiphytic or resident bacteria are considered the main source of the pathogen for brown spot, rather than bacteria coming from lesions, in contrast with some other bacterial diseases. Thus it is possible for the pathogen to be present in a production area for awhile before disease occurs.
Bacterial brown spot is probably more common in the mid-west because rainfall is usually intense and the bean production area is large compared to the northeast. Intense rains provide favorable conditions, as described in previous paragraphs. The bacterial brown spot pathogen was found commonly, even on non-host plants like corn, in large bean production areas in the mid-west but it was not found on beans planted in gardens outside the production areas. Thus growing beans intensively apparently selects for this pathogen. Additionally, hairy vetch, which is a good host for the bacterial brown spot pathogen, is a common weed in the mid-west.
How do bacteria get to a bean field? Long-distance dispersal occurs via seed and by rain washing airborne bacteria out of the sky onto leaves. A severe epidemic can result from just a few bacteria dispersed into a field if conditions are favorable. Bacteria multiply more quickly than other pathogens and can double their population size in as little as 30 minutes. Seed are more commonly tested for halo blight, which is considered a more devastating disease than brown spot. Bacteria can travel in the atmosphere at least 100 miles. Brown spot bacteria were detected being lifted out of a field of beans in breeze on a typical dry, warm summer day. Short-term dispersal from both host and non-host plants is via rain splash and by insects, equipment, and people.
Bacterial brown spot can be managed with crop rotation, clean seed, resistant varieties, and copper fungicides. Select a field that was not planted to legumes for at least one year (two to three years is better) and that is not next to a field where beans were planted the previous year. One year can be sufficient because this pathogen does not survive well over winter. Vetch and clover are not recommended as winter cover crops where brown spot has occurred because the pathogen can survive on these plants. Select seed that was produced in an area not conducive to bacterial diseases, tested for bacterial pathogens, and/or treated with an antibiotic (AgriStrep 500). Environmental conditions in western US, particularly Idaho, are very dry and thus ideal for seed production. Additionally, seed production fields are furrow irrigated to further reduce the chance for spread in the canopy. Seed is more commonly tested for bacteria causing halo blight than brown spot because seed is a more important source of inoculum for halo blight. AgriStrep 500 can only be applied by commercial seed treaters. Fresh market green bean varieties that are tolerant of bacterial brown spot include Charon, Hialeah, Probe, Espada, and Prosperity. Some are suitable for processing. Yielding ability and horticultural characteristics drive the market and thus need to be considered when selecting a variety. For example, one New York bean grower commented that Hialeah is a nice variety, but he no longer grows it because buyers now want a darker bean. The processing variety Hystyle from Harris Moran is popular in Wisconsin. It is a medium green bean with a sufficiently high level of resistance that growers reportedly are not applying copper fungicides. Wisconsin growers are managing brown spot in varieties that do not have resistance by applying copper fungicides immediately after rain. Copper should be applied before rain where it will not be possible to spray after rain. Do not work when plants are wet, including cultivating or applying pesticides. Scout fields routinely for symptoms, especially between midseason and harvest. With multiple plantings, clean equipment between fields. Plow under bean stubble immediately after harvest.
Figure 1. Symptoms of bacterial brown spot on snap bean leaves are small brown spots that coalesce and are often associated with leaf yellowing. Occasionally affected leaves develop a shot-hole appearance when the spots fall out.
Figure 2. Symptoms of bacterial brown spot on snap bean pods begin as tiny water-soaked spots. They enlarge, become sticky, and develop a tiny white, crusty area in the center, as shown here. Eventually these spots become brown and necrotic, and they may cause the pod to be twisted or bent.
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