Bacterial Blights: Halo Blight (bacterium – Pseudomonas syringae pv. phaseolicola); Common Blight (bacterium – Xanthomonas campestris pv. phaseoli): Plants infected with the halo blight bacterium form greenish-yellow circles around each lesion. Interior of the lesion turns brown. With age, lesions enlarge and coalesce. The entire leaf finally drops. Stem lesions appear as long, reddish colored spots. When the plant begins to set fruit, lesions are formed at the nodes which girdle the stem. This reduces fruit development. Common blight-infected pods do not exhibit the greenish-yellow halo around the lesion like halo blight lesions. Infected leaves with halo blight turn yellow and slowly die while those with common blight turn brown and drop quickly. Both organisms are seed-borne. Entry into the plant is through the leaf stomata. Rain and damp weather encourage development of these diseases. Common blight is more of a problem in warm weather while halo blight is favored by cool temperatures. Both bacteria can live in the soil for two years on plant residue. To control bacterial blight of beans, seed grown in the western United States should be planted. Avoid spreading the disease by not entering the field when the foliage is wet. Follow a three year rotation.
Anthracnose (fungus – Colletotrichum lindemuthianum): This is a seed-borne fungus which attacks all above ground portions of the plant. Infected seed are marked by dark, sunken lesions that extend through the seed coat. Stem lesions are oval and sunken. The center of the lesion is dark brown with purplish to red borders. In early stages, the fungus develops along the veins and becomes purplish to red in color. In advanced stages, leaves become ragged. Infection of the pods results in small, reddish, elongated spots. Older spots are sunken and have brown to reddish-brown borders. The disease is favored by cool, wet springs and falls. It disappears during hot, dry summers. The fungus can survive in the soil for two years in plant debris. Control is obtained by: (1) the use of disease-free seed, (2) crop rotation, (3) not entering fields when plants are wet, and (4) spraying with fungicides.
Cercospora Leaf Spot (fungi – Cercospora spp.): Lower foliage becomes marked by irregular tan spots one-eighth to one-fourth inch in diameter. Severe infection causes defoliation and plant stunting. It is reported to attack the pod but has only been observed on foliage in Texas. Infection is most severe during periods of extended rainfall, high humidity and temperatures between 75 to 85 degrees F. No resistance exists among varieties. Fungicide sprays should begin at first sign of disease and continued during cool, rainy conditions.
Root Rot (fungus – Rhizoctonia solani): Bean seed may rot in soil or the young seedling may become stunted. A reddish-brown canker is formed on the stem. Cankers may completely girdle the stem or may only partially girdle it, causing severe stunting. Beans should be planted after the soil has warmed to above 69 degreesF. Beans should follow a grass type crop.
Fusarium Root Rot (fungus – Fusarium solani f. sp. phaseoli): Plants infected with Fusarium are characterized by a reddish discoloration of the tap root. Affected plants are stunted with yellow leaves. Young rootlets formed in the area of the lesion are killed. If weather conditions are favorable, a normal crop may be produced. Avoid soils where Fusarium has been a problem. Long rotations (four to five years) will help reduce losses.
Pod Blight (fungus – Diaporthe phaseolorum): Pod blight of lima beans is first observed as brown pustules of irregular shape on the leaves. Lesions grow to one-fourth to three-fourths of an inch in diameter. During the latter part of the growing season, the fungus spreads to nearby pods, where it causes a pale watery spot. The spot enlarges and becomes darker with age. On pods the spot is marked by dark brown to black pustules on the surface arranged in a ring.
Use seed grown in the western United States and use a three- to four-year rotation. Follow a fungicide program to control the disease when it occurs consistently.
Mosaic (virus): Leaves become puckered and mottled with light and dark-green areas. Infected plants become stunted. The virus is seed-borne and can be spread by aphids. Losses can be reduced by growing resistant varieties and following an approved aphid control program.
Curly Top (virus): Infected plants are stunted and have distorted foliage. It is spread by the beet leafhopper. Use resistant varieties and practice good insect control.
Root Knot: (See Root Knot)
Southern Blight: (See Southern Blight)
Cotton Root Rot: (See Cotton Root Rot)
Sun Scald: In the early spring beans are often affected by a condition in which the young leaves turn light tan in color and die. This may happen to the entire leaf or to only a portion of the leaf. The conditions favoring disease development are cool to moderate temperatures, extended periods of high humidity, and cloudy days followed by a bright sunny day. Damage is usually restricted to only a few scattered leaves.
Rust (fungus – Uromyces phaseoli): Small reddish-brown pustules form on lower side of leaves. The fungus overwinters in crop residue. If rust has been severe, a rotation program should be practiced. Resistant varieties should be used when past experience indicates rust to be a problem. Apply approved fungicides at first sign of infection in the fall.
Powdery Mildew (fungus – Erysiphe polygoni): Powdery mildew is character- ized by a white powdery growth on the foliage. Infected pods and foliage become malformed. The fungal spores are spread by wind. Spray with approved fungicides. Powdery mildew seldom becomes an economic problem.
Watery Soft Rot (fungus – Sclerotinia sclerotiorum): The fungus affects the stems, leaves and pods of beans. First signs of infection are small, soft, watery spots that enlarge rapidly under cool, moist conditions. They may enlarge and coalesce and the stem is girdled. Infected pods turn into a soft, watery mass. Following the watery stage the affected tissues dry out and turn brown. Within a short time the brown areas are covered with a dense white fungal growth. With age, the white fungal growth turns gray and is dotted with small, hard black bodies called sclerotia. Most losses occur in shipping. Infected beans tend to stick together.
The disease is favored by temperatures ranging between 60 and 70 degrees F. Long periods of high humidity also favor the development of white mold. Large plants with heavy vine growth encourage disease development.
Sclerotia fall to the ground at maturity where they can lay dormant for as long as ten years. When weather conditions are again favorable, the sclerotia begin growth again. The fungus enters beans directly where pods and leaves come in contact with the developing fungus.
Sclerotia produce small mushroom-like structures which contain thousands of ascospores. The spores are ejected into the air landing on plant parts such as blossoms or decaying leaves and begin to develop. This may be repeated many times resulting in widespread infection.
Every effort should be made to improve air circulation between plants and rows. This can be done by increasing row spacing and decreasing the seeding rate. Excessive applications of nitrogen favor heavy vine growth and should be avoided. During periods of extended cool temperatures and high humidity, fungicides should be applied on a preventive schedule.
Baldhead: Beans emerge and produce cotyledons but nothing is formed above the cotyledonary leaves. This can result from injury from soil insects or fungi, and results from mechanically damaged seed.