By Gail Ruhl, Senior Plant Disease Diagnostician, Department
of Botany & Plant Pathology, Purdue University
Patches of brown scorched leaf tissue on garden beans are often associated with intense light injury. This type of injury is called sunscald and occurs everywhere in the United States. Among those beans which are injured by intense sunlight are snap beans, lima beans, wax beans, pinto beans, navy beans, and kidney beans. Sunscald may affect leaves, stems, branches, and pods of bean plants. It is most serious when the plant is
partly defoliated or when there is new succulent tissue. Injury usually occurs when bright sunny days follow warm, cloudy, humid periods.
Sunscald affects all above ground parts of older plants, but the uppermost and outermost leaves generally show the characteristic symptoms. These are the leaves that are usually exposed to the direct rays of the sun. Injury begins on the leaf as a slight browning or bronzing of the epidermis in small patches between the veins, often at the central part of the leaf or sometimes near or at the margin. In later stages the discoloration increases in extent and results in the production of large islands of dead tissue. The tissue becomes thin and brittle and crumbles readily when dry. If unfavorable conditions continue some defoliation may occur. Sunscald may cause tiny brown or reddish spots on pods. Spots may gradually enlarge and develop into short streaks.
Environmental factors which promote this type of injury are intense sunlight and low humidity following periods of high humidity, warm temperatures and cloudy weather. There also appears to be some association with heavy applications of fertilizers which tend to promote spurts of rapid growth. Succulent leaf tissue which rapidly develops during periods of favorable weather is most susceptible to sunscald injury.
The only means of reducing injury is to use areas with well drained soils and adequate but not excessive fertility. Since there is usually little to no yield reduction from minor episodes of sunscald, this injury is considered to be more of a curiosity than a real problem.