Vegetable Crops

Field Diseases of Beans and Lima Beans

W. J. Zaumeyer, H. Rex Thomas.

More than 50 diseases attack beans and lima beans in this country. In a season when weather favors their development, they may cause losses to American farmers of 15 million dollars.

No region of the United States is free of all bean diseases. Root rots cause serious trouble wherever crop rotations are not practiced. Frequent rains during the growing season are likely to lead to heavy losses from bacterial blights, especially if seed that carries the blight organisms is planted. Bean rust often causes serious losses in parts of the South and West. Virus diseases occur in all bean-growing districts. Downy mildew of lima beans often gives considerable trouble in the North Atlantic States.

About 30 years ago, when anthracnose and bacterial blights were causing heavy losses of snap beans, research revealed that the organisms causing those diseases, which are seed-borne, do not thrive in areas where low rainfall and high temperatures prevail during the growing season. Consequently the business of growing seed of snap beans moved from the East and Middle West to the drier intermountain region. Seed growers in parts of that region and in the Pacific States need have little fear of large losses from the two diseases, and crops grown from their seed all over the United States are less apt to be diseased than those grown from seed produced east of the Rockies.

Another line of research that has enabled farmers to grow healthier bean crops is the development of fungicides for control of certain diseases. Still another is the development of better systems of crop rotation and better methods of field sanitation. Possibly the most progressive step of all has been the development of disease-resistant varieties. A number of new varieties, each resistant to several diseases, have been introduced.

THE BACTERIAL BLIGHTS are among the most important diseases of beans wherever the crop is subject to frequent rains during the growing season. If occasional showers occur during the early part of the growing season the diseases are usually noted even though the total rainfall is not excessive. Only in certain sections of the Intermountain and Pacific Coast States, such as southern Idaho, eastern Washington, and California, are beans grown relatively free of these diseases. Practically all beans grown for seed purposes therefore are produced in a rather small area in the West. The severity of the diseases in any place varies with the weather and the presence of infected seeds.

The bacterial blights include common blight, fuscous blight, halo blight, and bacterial wilt. Each is caused by a different bacterial organism, but all, except the wilt organism, produce similar symptoms on bean plants. Under certain conditions the bacterial wilt organism produces a wilting of the plant unlike that caused by any of the other organisms. Under other conditions some of the wilt symptoms are like those of other bacteria] diseases.

Water-soaked spots on the leaves usually are the first visible evidence of blight. The spots enlarge, turn brown, and finally kill the leaf.

Halo blight makes a halolike zone of greenish-yellow tissue around each water-soaked spot in cool weather. Leaves of newly infected plants are yellow.

Leaves of plants infected with the common or the fuscous blight organism turn brown rapidly and look burned. Positive identification of the two diseases cannot be made without laboratory tests.

The necrotic leaf lesions made by the bacterial wilt organism are hard to tell from those caused by the common and fuscous blight organisms. From a combination of all the symptoms of bacterial wilt, this disease can usually be told from the other three diseases.

Water-soaked spots similar to those on the leaves are produced on the pods by all of the organisms, except the wilt organism. The spots grow larger, and the tissue around each is reddish brown or brick red. The spots finally dry and are often covered by a dried bacterial ooze. Commonly the bacteria invade the upper suture of the pod, cause a water soaking of the tissue on both sides of it, and later infect the seed. On light-colored bean seed a yellowish discoloration may be visible at the hilum, the point of attachment to the pod, but on colored seed the infection is hard to detect. Badly infected seeds usually are shriveled and yellow. Slightly infected seeds may appear healthy.

Upper left: Stem anthracnose on lima bean pods. Upper right: Downy mildew on pods of lima bean. This organism requires a high humidity to cause infection. Lower left: Bacterial blight infection of pea leaves. The bacteria are spread from plant to plant by rain and hail. Lower right: Red node, a virus disease of bean.

The only evidence of infection of pods by the bacterial wilt organism is a slight discoloration of the upper suture, but the seeds inside them often are yellow and shrunken. When the infected seeds are planted, the plant becomes infected internally, and the bacteria spread through it. General stunting and death may result. Some of the seedlings may grow to fairly good size, but usually a lesion, known as stem girdle or joint rot, occurs at the node where the cotyledons were attached. The affected stem, further strained by the increasing weight of the top, breaks at the node.

The bacteria that cause common blight (Xanthomonas phaseoli), fuscous blight (X. phaseoli var. fuscans), and halo blight (Pseudomonas phaseolicola) enter the plants through the stomata of the leaves, stems, and pods. The wilt organism (Corynebacterium flaccumfaciens) enters through wounds. Rain, hail, driving winds, damp weather, and even sprinkler irrigation favor the spread of the organisms.

Plants grown from infected seed frequently develop lesions on the cotyledons and stems. From those spots bacteria are splashed by rain or hail to leaves of other plants. A few diseased plants scattered through a field as a result of infected seed may be the source of a general outbreak if weather conditions are favorable for the spread of the organisms. Warm weather favors common and fuscous blights and bacterial wilt. Cool weather favors halo blight.

The bacterial blight organisms can also live over winter on diseased bean refuse in the soil, possibly for 2 years.

The most effective control measure against bacterial blights is the use of disease-free seed. One should use only seed grown in areas where rainfall is scant and blight epidemics do not occur.

Because the blight organisms can overwinter in the soil, beans should not be planted for at least 2 years in fields that have been infected. Nor should bean straw from a crop that showed blight be applied to land that is to be planted to beans. Infected bean straw used for bedding farm animals may continue to harbor bacteria, and the use of such straw on fields to be planted to beans is unwise.