Race 1 produces circular or oval spots up to 1 inch in diameter. At first they are yellowish green, but as they age they become reddish tan and profusely covered with dark spores, which give them a velvety appearance. Often the spores are arranged in concentric rings in the lesions. The fungus infects the sheaths, forming larger round areas that may become black with spores. Ears and husks are also attacked. Kernels become a carbon black. In some ears only a part of the kernels may be blackened; in others all are black. Race 1 affects all visible parts of the plant and causes it to die early. When plants are infected in the young or seedling stage, they may become completely covered with spots and die before tasseling. Young seedlings may become infected from the seed.

Race 2 produces leaf symptoms that are easily confused with southern corn leaf blight. Spots are tan or light brown and angular. They elongate slightly with the leaf and are one-eighth to one-fourth inch wide and up to 1 inch long. In heavy infection they coalesce to form large deadened areas, which often kill the lower leaves. This race also causes a blackening of the ears. The sporulation it causes on the spots is not heavy.

Losses from southern corn leaf spot can be controlled by the development of resistant strains. Susceptibility to race I is of a rather simple genetic nature, and resistance can be easily obtained. Several of the useful inbred lines of corn have been converted from susceptible to nonsusceptible lines by cross-breeding. The mode of inheritance of reaction pertaining to race 2 is not known.

GRAY LEAF SPOT of corn occurs throughout the Southern States. In 1943 it was prevalent in Kentucky and Tennessee. Since that time heavy infections have been found in Georgia and Virginia. The disease may be caused by two fungi, Cercospora sorghi and C. zeae-maydis, which sometimes occur on the same leaf.

On hasty examination this spotting may be confused with that caused by southern corn leaf blight. The spots are narrower, however; some are less than one-sixteenth inch wide and are one-fourth to 1 1/2 inches long. They begin as light-sepia spots, but as they age they bleach to an ashen gray with narrow light-sepia borders. The spores of the fungi are very light, and are long and narrow with several cross walls. The mode if inheritance of resistance to attack by these fungi is not known.

BACTERIAL LEAF DISEASES are found all over the United States. Like other leaf diseases, they are favored by humid, warm weather. They are spread by insects, wind, and contaminated material in the soil. Their severity may vary from year to year.

Three bacterial leaf diseases of corn are known in the United States. Only two, bacterial wilt and bacterial leaf blight, cause much concern.

BACTERIAL WILT, also known as Stewart's disease, is caused by Bacterium stewartii and is the most important of the bacterial leaf diseases of corn. It attacks sweet, dent, and pop corn. Generally it does more damage to sweet corn, particularly the early, yellow, sweeter varieties. It was first found in New York in 1895 and occurs to some extent each year in the Eastern States, throughout the Corn Belt, and in fields of sweet corn in the South.

Up to 1940 bacterial wilt caused extensive damage to sweet corn and heavy losses to canners. In the 1930's it appeared also in a somewhat different form in field corn as a leaf blight on dent corn in Indiana and Illinois. Losses as high as 20 percent were sustained in many dent corn fields in the Corn Belt. The introduction of sweet corn as a commercial green market crop in Florida also brought the menace of bacterial wilt.

Canners and green marketers turned to the use of late-maturing resistant white varieties as Country Gentleman and Stowell Evergreen. They have since replaced the susceptible corn with resistant hybrids, and losses have been reduced. One of the outstanding resistant hybrids and one of the first to be used is the yellow Golden Cross Bantam. A few of the others used in the United States are Aristogold, Hoosier Gold, Golden Harvest, Ioana, Marcross, Carmelcross, and Iochief. Their use offers substantial control in sweet corn. The disease is not so serious to field corn, but it may cause appreciable damage if conditions are favorable.

Stewart's disease is a typical vascular disease of sweet corn. It is caused by a nonmotile bacterium that grows into a small yellow colony on artificial culture media. In the corn plant, bacteria develop inside the vascular bundles, plug them full, and induce wilting as if the plant were suffering from lack of water. The whole young plant may wilt and die early even though plenty of water is in the soil. Those that do not die become stunted and do not produce normal ears. The leaves develop long, pale-green streaks with wavy margins, and they soon wilt and die. In a severely affected plant bacteria ooze out as tiny yellow droplets from the bundles in the stalk if it is cut across. Other infected parts also will ooze the bacteria. If cut across and placed in a drop of water, clouding will be seen. All parts of the plant may be infected.

Infection that takes place after the plant is firmly established comes about through wounds made by flea beetles as they feed on the leaves. Infection may spread through the bundles of the leaves into the other parts of the vascular system of the plant and cause severe damage.

Dent corn is generally more resistant than sweet corn to bacterial wilt. Rarely is the infection so severe that the plants die before they tassel, but that may occur in a few inbred lines. Large numbers of streaks may develop on the leaves as the corn approaches maturity. They usually develop from beetle feeding points, and, in susceptible lines, they increase to cause a severe leaf blight. Streaks are long and pale green with irregular or wavy margins. They turn lighter, die, and eventually kill large portions of the leaf in severe cases. Sometimes a heavily infected cornfield looks as if it had suffered from frost. Susceptibility to stalk rot is increased when a large area of leaf surface is destroyed.

Bacterial wilt may be carried over inside the seed. This is another source of infection for young plants. The bacteria also may live over winter in the soil, in manure, or in old cornstalks and leaves left in the field. Infection of a new crop seldom starts from these.

The bacteria must enter through a wound in the plant to cause disease and initial infections generally start from injuries caused by flea beetles in feeding. These tiny beetles carry bacteria within their bodies over winter, come out in April and May, start feeding on the young corn plants, and thus introduce the bacteria.

A correlation exists between the severity of winter and the amount of bacterial wilt the following spring. Mild winters tend to maintain large beetle populations, and wilt may reach epidemic proportions the following year. Bacterial wilt is less severe after a cold winter, apparently because fewer beetles survive. As new generations of beetles appear in the spring, they feed on infected corn leaves, harbor the bacterium in their bodies, and spread the disease to other plants throughout the growing season.

Sprays and dusts do not control the disease. Seed treatment is ineffective. Spraying of the young plants with DDT as the first beetles emerge in the spring has reduced the beetle population on experimental plots and thus lessened the bacterial wilt spread. The use of resistant hybrids is the recommended control.

BACTERIAL LEAF BLIGHT of corn, caused by Pseudomonas alboprecipitans, has occurred sporadically in several Central and Southern States since 1928. It was recognized first in Alabama and soon thereafter was found in Virginia, Georgia, Texas, Kansas, and Nebraska. Since then it has been found in other States.

The disease affects some varieties of dent, sweet, and pop corn. The same bacterium causes a disease on yellow bristlegrass (Setaria lutescens), a common weed in cornfields. It is not known if it spreads from yellow bristlegrass to the corn. There is no evidence that the disease is transmitted naturally by seeds. Temperatures between 85 and 95 F. favor infection.

The disease usually is seen in spots in the field as a leaf blight, that may cause stalks and tops to rot as it develops in the plant.

The lesions are scattered irregularly over the leaves. They appear as sharply delimited necrotic stripes from one-eighth to one-fourth inch wide and from 1 to 16 inches long.

The stripes at first are water-soaked and olive green. Later the color changes to a light buff, with delicate margins of sepia to cinnamon buff. They appear somewhat translucent. In rather heavy infections, the stripes may merge into extensive areas of dead tissue, sometimes over the entire width of the leaf. As the lesions become older they may break lengthwise and give a shredded appearance to leaves, especially if they are badly diseased and exposed to rainy and windy weather.

Small sections of the stripes cut and placed in a drop of water show a clouding, which is caused by a streaming out of the bacteria from the diseased tissues.

The stalk rot that sometimes accompanies this leaf blight is in the upper part of the plant, usually at or just above the node where the ear is produced. The outside of the stalk may show reddish-brown streaks and inside the stalk is a dark brown to black rot that decays the pith of nodes and inter-nodes leaving the fibrous bundles as long loose shreds. As the rot progresses, the tops of the corn plants fade and die and often the tassels fail to appear. Many infected plants are dwarfed, show bleached tops, and produce sterile multiple ears.

The bacterium that causes this disease is a small motile one that develops on artificial culture media as a small white colony, sometimes surrounded by a definite clearing or halo.

BACTERIAL LEAF SPOT of corn is caused by Pseudomonas syringae (Bacterium holci) and sometimes results in the firing of the lower leaves of young plants. It was first observed in Iowa in 1916 but has never become serious. It is seen on the lower leaves of the corn plant as round, elliptical, or irregular, water-soaked lesions ranging from one-eighth to one-half inch in diameter. The spots at first are dark green and water-soaked, but later become light brown, with a darker brown or reddish-brown narrow border. A yellowish halo may be seen around some of the larger spots. In extremely rainy weather there may be an infection of the margins and tips of the lower leaves, which causes them to become brown and die.

ALICE L. ROBERT, a native of Louisiana, has been a pathologist in the division of cereal crops and diseases, Bureau of Plant Industry, Soils, and Agricultural Engineering, since 1938. She is a graduate of Louisiana State University and conducts research in corn diseases at Beltsville, Md.