Some of the Leaf Blights of Corn

Alice L. Robert.

Common corn smut.

Corn is subject to about 30 different leaf diseases in the United States. Fewer than one-third of them cause noticeable damage, even in rather small areas.

Most leaf diseases, or so-called leaf blights, produce spots or streaks on the leaves. The damage may be light to heavy and sporadic or persistent. The diseases injure or kill the leaf tissues and thereby reduce the area of green chlorophyll, which manufactures food for the plant. If considerable leaf areas are killed, the vigor and yield of the plant are reduced, stalks are weakened, and the grain is chaffy and light. The value of the plant for forage is reduced, and so many stalks may be broken that harvesting with machines is difficult.

The leaf diseases have characteristic marks that identify them. Some of them may infect the plants in the seedling stage and kill the entire plant at an early age. They usually enter all parts of the plant and thus are systemic diseases. When they appear later they usually attack only the leaves. Unbalanced supplies of available minerals in the soil may cause streaking, spotting, and discolorations in leaves that sometimes are confused with parasitic diseases. Hereditary factors and unfavorable environmental conditions occasionally produce leaf disorders or discolorations.

Parasitic leaf diseases generally are favored by high humidity and warm temperatures. Wind or insects spread them from one plant to another and from field to field. Fungi or bacteria cause parasitic leaf blights.

LEAF DISEASES caused by fungi occur throughout the corn-producing areas. Five of just more than 20 fungus leaf diseases of corn are serious in the United States. They are characterized by spots or lesions, which usually have fruiting bodies or spores of the causal fungus on the killed parts of the leaves. The lesions may be considerably darkened by the abundance of these spores. Some of the fungi remain in fields from one season to another in corn residues. They generally are spread by wind.

NORTHERN CORN LEAF BLIGHT is caused by the fungus Helminthosporium turcicum. It is found in most of the corn areas of the United States and attacks all types of corn. The disease was first identified in 1878 in this country. It is most generally noticeable in the southern part of the Corn Belt, eastward to the Atlantic coast, and southward into Florida. It has been found as far north as Wisconsin, Minnesota, and South Dakota. The most severe damage is in States that have heavy dews and abundant rainfall, which, with warm summer weather, make ideal conditions for its development and spread. Its occurrence is often sporadic but it may persist through several severe seasons in an area. Since 1942 it has been somewhat serious in some sections of the lower Corn Belt. Heavy infections occurred in Illinois and neighboring States in 1951 and 1952. Fields of sweet corn in Florida were seriously attacked by it in 1952 as the infections were established early on young plants.

Some fields may be severely affected, while others nearby may suffer little damage. Usually the plants become infected as they approach maturity. Losses may range from a trace to as much as 50 percent of the grain yield, depending on the severity of attack and the time the disease becomes established. Sometimes a heavy infection may appear before tasseling and cause severe damage. In a less favorable season the plants may be almost free from disease until they near maturity; then only minor damage results. If much of the green leaf area is killed, starch formation is restricted and the kernels are chaffy. The blighted leaves are less suitable for fodder because of lowered nutritive value. Many of the dead leaves are lost in harvesting.

Heavy rainfall seems to encourage the development of northern corn leaf blight. Its severity might be predicted somewhat from rainfall and temperature records for an area. Heavily infected corn seems to be more susceptible to stalk rots. Early-maturing lines of corn generally are more susceptible than those that mature later.

The lower leaves of the corn plant usually are the first to be attacked by Helminthosporium turcicum. The disease progresses upward as severity increases. Small elliptical spots first appear as dark and grayish-green, water-soaked areas. Later they turn greenish tan. With age they get bigger and become a distinct spindle shape. Individual lesions usually are one-half to three-fourths inch wide and 2 to 3 inches long. They may be 2 inches wide and 6 inches long. Spores develop abundantly on both surfaces of the spots after rain or heavy dews and give a dark-green, velvety look to the lesions, more heavily in the center than near the margin. Several lesions may join and form large areas, that may kill entire leaves and in turn the plants. Heavily infected fields may appear dry and fired.

The fungus lives over winter inside the dried infected leaves of corn residue in the field, produces spores the following spring, and infects the growing corn. Once started, the infection is spread by the wind.

In 1942 no hybrids or inbred lines of corn were known to be resistant to northern corn leaf blight. Since that time plant pathologists and corn breeders have tested all of the important inbred lines of corn and evaluated them for resistance. The experiments were conducted by the Department of Agriculture at Beltsville, Md., and, in cooperation with the Purdue University Agricultural Experiment Station, at Lafayette, Ind.

A few highly resistant inbred lines were found. They are not used in commercial hybrids, but their resistance can be transferred to commercially important lines, thereby making them resistant to the disease. By 1952 some of the inbred lines commonly used in making hybrids for the Corn Belt had been converted and by 1958 they will be available for making hybrid seed for the farmer. Resistance is controlled by a large number of hereditary factors, or genes, that may be transferred from one inbred line to another.

Dry Parzate and Dithane sprays or dusts were used in 1951 and 1952 in Florida on large sweet corn fields and in dent corn seed-breeding fields of some Corn Belt hybrid seed companies. Six to eight applications were necessary for control.

The best control of northern corn leaf blight lies in the development and planting of resistant hybrids.

SOUTHERN CORN LEAF BLIGHT is caused by the fungus Cochliobolus heterostrophus (Helminthosporium maydis). It was first observed in the United States in 1923. Now it occurs throughout the corn areas of the Southern States and reaches northward into Illinois, Indiana, Missouri, and Ohio and eastward to the Atlantic coast. A warm, humid climate favors its development, but it thrives better at higher temperatures than does northern corn leaf blight.

It kills the green tissue of the leaves and reduces the effective leaf area. Yield may be reduced, stalks weakened and the fodder value of the plant damaged. Most of the damage is found in the Southern States, where temperatures and moisture conditions are ideal for its development.

The disease is seen as a grayish-tan to straw-colored spotting, which may extend over most of the leaf surface. The spots may be up to 1 1/2 inches long and one-fourth inch wide. Their sides are more or less parallel, following along leaf veins. Several spots may unite to form large areas of killed tissue when the infection is heavy. Badly damaged fields may appear to be fired. Very susceptible hybrids or varieties have numerous spots even on the upper leaves.

Its fungus also produces microscopic vegetative spores, which are blown from plant to plant to spread the disease. The spores and consequently the spots are light in color, in contrast to the dark spores and spot surfaces of northern corn leaf blight. They are characteristically slender and curved. The perithecial or sexual stage is known, but its importance in epidemics of southern corn leaf blight is not definitely established. The fungus lives over in the vegetative or asexual stage in the dead leaf tissue in the fields, and, when wet, warm weather comes in the spring, vegetative spores are formed and spread to the growing corn. The fungus may live over in the field for at least 2 years. Wind carries the spores from plant to plant.

The only known means of controlling the disease is the use of resistant hybrids. The Department of Agriculture and the agricultural experiment stations in Indiana, North Carolina, and Georgia had discovered by 1950 several sources of resistance that scientists have used to improve inbred lines. Resistance to this disease appears to be inherited in a manner similar to that of northern corn leaf blight, but it is governed by an entirely different set of genes.

ANOTHER DISEASE, commonly called southern corn leaf spot and caused by Helminthosporium carbonum, was identified in 1938 in Indiana. The two known races of the fungus produce different symptoms, although the spores look alike. Neither has been of great economic importance between 1938 and 1953.