Back to Extension publications listingJohnny Dan Gay, Extension Plant Pathologist
Diseases cause widespread, severe damage to the Georgia snap bean crop every year. Serious seedling diseases result in poor stands. Damaging stem, foliage and pod diseases further reduce yields. Disease control is necessary to produce the best quality and highest yields of both processing and fresh market snap beans.
Diseases causing the greatest losses to snap beans in Georgia are seedling rots, nematodes, rust, anthracnose, bacterial blight and mosaic.
In recent years it has been almost impossible to get a good stand of late planted snap beans in Georgia. Most beans planted after mid-June begin to die about the time they come up to a stand. A number of different soil-borne fungi may attack beans.
Pythium has been most commonly associated with stand failure of fall beans. However, Rhizoctonia and Fusarium may also cause seedling disease. Bean seedlings may be attacked from the time they first begin to germinate until they are several weeks old. They are much more susceptible in the young, tender, succulent stage.
In addition to the above mentioned fungi, Sclerotium rolfsii (the southern blight fungus) has been involved in the loss of stands also. Deep plowing of debris and following the recommendations given below should help control the incidence of seedling disease caused by this fungus.
Rhizoctonia and Pythium cause young been seedlings to wilt and collapse or damp-off from water-soaked rotting of the stem near the soil line. With Pythium, the stem has a more watery rot that is colorless to dark brown. The slimy outer tissue of the stem slips easily from the central core. With Rhizoctonia, the stem initially appears water-soaked, but may dry and turn brown or form reddish-brown to brick red, slightly sunken cankers on the stem. The plants may recover, but they will be stunted.
Research at the Coastal Plain Experiment Station has shown that seedling diseases can be reduced considerably by following these recommendations.
Seed Treatment: Treat seed with Arasan unless they were treated when purchased. To treat with Arasan, place one pound of bean seed in a quart jar and add 1/4 teaspoonful of Arasan. Then shake the container until good coverage is obtained. To treat 50 pounds of seed, add 4 tablespoonsful of Arasan in a rotating drum seed treater or other suitable mixing container and mix until seed are coated with the fungicide. Use Ridomil PC 11G in-furrow at seeding. Rotation, litter destruction and Ridomil PC 11G.
Sanitation: Do not throw infested soil against the bean stems during cultivation. Throwing soil around the stems of beans increases the incidence of seedling disease even when PC 11G is used.
Several different nematodes, including root-knot (Meloidogyne spp.), lesion (Pratylenchus spp.), sting (Belonolaimus spp.), stubby-root (Trichodorus spp.) and cyst (Heterodera spp.) may affect snap beans. All parasitic nematodes cause similar above ground symptoms on beans.
Root-knot is probably the most common nematode and causes the most damage to snap beans in Georgia. Due to the buildup of nematodes during the summer months, fall beans are more apt to be severely affected than are spring beans. Recommended bean varieties are susceptible to root-knot nematodes. If a high population of root-knot is present in the soil, beans may be stunted, produce low yields or be killed. Root-knot damage is usually more severe on light, sandy soils.
Usually, root-knot nematodes are not evenly distributed over a field. Thus, symptoms may appear in round to irregular patches in a field. Or the most noticeable symptom may be irregular plant height and vigor.
Nematodes damage the roots. Above ground symptoms are those of a plant with a damaged root system: pale green to yellow foliage, stunting and abnormal wilting during the warmer part of the day. Below ground symptoms furnish the positive proof of root-knot problems. Affected roots have swellings or galls, ranging from very small pinhead size to one-half inch or more in diameter on the larger roots. These galls are enlargements of the root tissue and cannot be detached without breaking the root.
If the roots are not knotted, but the above ground symptoms indicate a nematode problem, ask your county agent to take a soil sample in the root zone of the affected plants. He will send it to the Plant Disease Clinic, Four Towers Building, Cooperative Extension Service, The University of Georgia, Athens, GA, to be checked for other types of nematodes which may also damage beans.
Rotation: Rotations have some value if a single species of nematodes is present. If several genera or species are in a field, it is difficult to find a rotational crop that will not favor the buildup of at least one kind of nematode. Do not plant fall beans behind a root-knot susceptible crop, such as cucumbers, tobacco, cotton, squash, tomatoes, cantaloupes, peas or beans. Leaving the land fallow during the summer or planting rye in the field in the fall should reduce the root-knot nematode population.
Fumigation: The application of a fumigant type nematicide in the row before planting is the best way to control nematodes in beans. Fumigants can be applied by a gravity-flow applicator, and should be placed 6 to 8 inches deep in the row. Fumigants require a two-week waiting period between application and planting. The non-fumigant Mocap may be used at seeding. Mocap should not be used under heavy root-knot pressure situations.
Rust, caused by the fungus Uromyces phaseoli var. typica, attacks all above ground parts of the bean plant, but is most commonly seen on the underside of the leaves.
The rust fungus is not seed-borne, but overwinters on old bean plants. Spores produced on old bean plants are spread to new bean foliage by the wind. Early symptoms of the disease may be seen approximately five days after spores land on the leaves. A new crop of spores is produced about every 10 days. The development of rust is favored by cloudy, humid weather and an optimum temperature around 75 degrees F.
The first symptoms of rust on the foliage are very small, white, slightly raised spots or pimples which may be surrounded by a yellow halo. The white pimples later become raised, reddish-brown pustules. These rupture and release a powdery mass of spores (seed-like bodies) which give a rust color to the fingers if rubbed across an infected leaf. As many as 2,000 individual spots may be found on a single leaf. Heavily infected leaves usually turn yellow, shrivel and fall, resulting in premature defoliation.
Spray or Dust: The timely application of Bravo, Evade or Sulfur will protect susceptible bean foliage from rust infection. To spray, use 13/8 to 3 pints of Bravo 720 or 2 1/4 pints of Evade in enough water to thoroughly cover the top and bottom leaf surfaces.
Make the first application at the first sign of rust infection on a few plants. Repeat at weekly intervals until rust is no longer a problem. One to three applications are usually adequate.
Crop Rotation: The rust fungus will live for one year on old bean plant material left in the field. Beans planted in these fields can become infected earlier and be more severely damaged by rust. Therefore, a two year rotation with crops other than beans is important.
Resistant Varieties: Most of the recommended commercial varieties do not have resistance to rust. However, the Extender variety has resistance to many races. Genuine White Halfrunner, Mountaineer and Old Dutch, which are grown primarily in home gardens, are also resistant to many races of rust.
Anthracnose, caused by the fungus Colletotrichum lindemuthianum, is favored by cool, wet weather. It may cease to be active during hot, dry weather.It is more common in North Georgia than in other parts of the state, and occurs more commonly in home gardens where locally grown seed are used.
This fungus overwinters inside bean seed and diseased bean plants left in the field after harvest. This fungus can live in the seed as long as they remain viable. Fungus spores will survive in old bean debris under field conditions for more than two years. Once the disease is brought into a field on the seed, it can be spread by splashing rain and insects, or by people and equipment when the beans are wet.
The disease may show up on the leaves, stems or pods of bean plants. Dark brown to black, oval-shaped cankers with purple borders often occur on the bean stems and leaf veins. However, the most easily recognized symptom of anthracnose occurs on the pods as small, rust to reddish colored spots. This is the first evidence of the disease. These spots enlarge, turn dark brown to black and sink into the pods. A brownish, sometimes slightly raised border develops around each sunken spot, whose center may be covered with a pinkish ooze during wet weather.
Disease-Free Seed: Seed grown in dry regions of the west are usually free of the anthracnose fungus. They should always be used in preference to locally produced seed. Seed produced under wet, humid weather conditions quite often have the fungus present inside the seedcoat.
Crop Rotation: Do not grow beans on the same soil more often than once every three years. This fungus survives in old plant material in the field, but is not usually spread to new uninfected sites from the old, infected debris.
Sanitation: Do not work in bean fields when plants are wet. The seed-like spores, present in the diseased spots as a sticky mass, are more easily spread from diseased to healthy plants when plant parts are wet.
Spraying: In commercial plantings where disease-free seed and crop rotation are used, spraying is seldom necessary. The spray for rust will also protect against anthracnose. Although anthracnose is rarely a problem, it is advisable to use at least one application of Bravo or Evade in the pin bean stage to prevent other diseases.
Bacterial blight, caused by the bacterium Xanthomonas phaseoli, overwinters inside bean seed and diseased bean plant debris in the soil. The bacteria survive only one year in old bean plants after the crop is harvested. The disease organism may be introduced by seed into a new field. It is spread from plant to plant by splashing rain and equipment used in the field when the plants are wet.
The symptoms first appear as small, water-soaked or somewhat transparent spots on the underside of the leaves. These spots enlarge and run together. As they develop, the centers become dry and reddish or brown, surrounded by a narrow yellow halo. On very susceptible varieties, the spots expand until the leaves appear scorched or sunscalded. These leaves soon become ragged and torn by the wind and rain. They later wither and drop off.
Pod lesions start as round, water-soaked dots that enlarge, run together, dry and form sunken blotches, surrounded by a reddish or brick-red margin. Entire pods may shrivel. The bacterium causing this disease may also attack the stems of young seedlings, causing water-soaked, sunken areas that enlarge and develop into reddish streaks. Infected areas may have a yellow ooze or whitish crust during wet or humid weather.
Crop Rotation: Do not plant beans for at least two years behind beans or soybeans.
Sanitation: Plow under all infested plant material immediately after harvest. Do not cultivate or handle plants when they are wet with dew or rain -- this may spread the bacteria from plant to plant.
Disease-Free Seed: Plant only disease-free seed grown in the blight-free regions of the western United States. Rainfall and humidity are normally too low for the causal bacteria to infect the plants there.
Spray: Copper fungicides applied will control common bacterial blight.
A number of viruses, including common bean mosaic, Southern bean mosaic and yellow bean mosaic, may affect snap beans. Common bean mosaic and Southern bean mosaic may be seed transmitted. All three virus diseases may be spread by insects. Yellow bean mosaic is usually spread from clover fields to beans by aphids, and is found only on pole beans in North Georgia. It is usually widespread only where beans are planted after clover or adjacent to clover fields. Mosaic is considered a minor bean disease in Georgia, except in North Georgia near clover fields.
Symptoms of the different virus diseases are very similar and difficult to separate. All the mosaic virus diseases cause some stunting and reduced yields. The affected bean leaves have irregular, light green or yellow areas merging with darker green patches, which produce the familiar mottling or mosaic effect. The mottling of contrasting yellow and green areas serves as a means of distinguishing yellow bean mosaic from common bean mosaic.
Plants infected by yellow bean mosaic are more dwarfed and bunchy. Bean pods on infected plants are quite often undersized and sometimes curled. They may contain fewer beans than those produced on normal plants. Leaves may be long and narrow and show some puckering or downward cuppings at the margin. The overall symptoms may differ slightly with the variety and age of the plants and, to some extent, with growing conditions.
Rotation: Fields planted to beans or red clover should not be planted back to beans the following year.
Sanitation: Clean up all weed host plants adjacent to the bean field that could serve as a reservoir for the virus. Do not plant within 700 feet of red or white clover fields.
Certified Seed: Since common bean mosaic and Southern bean mosaic may be seed transmitted, use certified seed.
Resistant Varieties: Using resistant varieties is of doubtful value in the case of yellow bean
mosaic. There are several varieties which have some degree of resistance to common bean
mosaic. These include Bush Green Pod, Wade, Extender, Improved Tendergreen,
Harvester, Resistant Cherokee, Bush Wax and Stringless Blue Lake.
Pesticide Precautions
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