Four Enemies of Sorghum Crops

R. W. Leukel, John H. Martin.

The sorghum crops include grain sorghum, sorgo (or sweet sorghum), broomcorn, Sudangrass, and Johnson-grass.

Four general types of diseases attack them: Those that rot the seed or kill the seedlings; those that attack the leaves and lower the value of the plants for forage; those that attack only the heads and so prevent the normal formation of grain; and those that cause root or stalk rots and prevent the normal development of the plant.

Seed rot is most severe when the soil is cold and wet after planting a common condition in the North and in other areas when seed is planted early. Much of the seed then fails to germinate and rots because it is attacked by various seed-borne and soil-inhabiting fungi. To germinate promptly, sorghum seed requires a relatively warm soil, above 70 F. Most seed-rotting fungi thrive at lower temperatures that retard the germination of the seed and give the harmful fungi an opportunity to attack it.

Some fungi (chiefly species of Fusarium, Aspergillus, Rhizopus, Rhizoctonia, a, Penicillium, and Helminthosporium) invade and destroy the endosperm, the starchy tissue of the seed, thus robbing it of the food necessary to produce a strong seedling. Cracks in the seed coats of the kernels give the fungi ready access to the interior and thus aggravate the trouble.

Some fungi, especially species of pythium, attack the young sprout in its early stage and prevent its emergence.

These fungi also attack and rot the primary roots and thus keep the young seedlings from getting enough food materials from the soil to become well established.

Fusarium moniliforme, besides rotting the seed, frequently attacks sorghum seedlings at the surface of the soil soon after they have emerged and causes them to rot or damp-off and fall over. It also may destroy the primary roots of young seedlings. Some races of the fungus are more harmful than others.

Penicillium oxalicum attacks the endosperm and thus arrests germination. It may also kill the seedlings even after they have reached the third- or fourth-leaf stage.

Seed rot and seedling blight may be controlled pretty well by careful selection and treatment of seed and proper cultural practices. Seed should be well matured and properly dried. The seed coat should be as free as possible from cracks and nicks, such as those that are often caused by improper adjustment of the threshing machine. Before being planted, the seed should be treated with a good disinfectant that will protect it from seed-borne fungi and, to a great extent, from the harmful fungi in the soil. The seed should be planted after the soil is warm enough for prompt germination.

SORGHUM LEAF DISEASES may range in severity from small, unimportant Spots or stripes on the leaves to diseased areas covering practically the entire leaf. High temperatures and humid weather generally favor the leaf diseases.

The diseased spots or stripes are usually discolored because of chemical substances or pigments that are produced in the plant cells whenever they are injured. In most varieties of sorghum and Sudangrass this pigment ranges from reddish or brownish purple to almost black. In most broomcorn, kaoliangs, and a few other sorghums, the spots or stripes on the leaves are red. In shallu, Ellis sorgo, and Tift and Sweet Sudangrass they are tan.

Certain environmental conditions or hereditary factors occasionally discolor leaves of sorghum and Sudangrass. Those spots often are confused with symptoms produced by fungus or bacterial diseases. A common condition in sorghum is the presence of intensely colored leaf spots or stripes without any other indication of disease.

Much of this nonparasitic spotting may be due to mechanical injuries from insect punctures, wind, or sand particles. When chlorate weed-killing chemicals or grasshopper bait containing arsenic fall on the leaves of sorghum, they cause a burning effect in irregular but characteristic spots, which resemble those caused by parasitic leaf diseases.

Often, however, the cause of this leaf spotting apparently is a physiological breakdown of the leaf tissues. Occasional plants have leaves so badly discolored that most of their leaf area is involved. The spots may be solid, or they may follow various concentric or irregular patterns. Some of the latter types are hereditary.

Leaf diseases may be caused by bacteria or fungi. Those caused by bacteria usually are characterized by the presence of drops or films of exudate that dry to thin, crustlike scales. Leaf spots caused by fungi have no exudate and usually are more or less roughened, because of the presence of fungal fruiting bodies.

BACTERIAL LEAF DISEASES occur in the United States wherever sorghum is grown. Like most leaf diseases, they are favored by warm (75 to 85 F.), moist weather. The causal organisms may be carried over from one season to another on the seed, on infected plant material in or on the soil, and occasionally on plants that overwinter. They may be spread from one leaf or plant to another by wind, splashing rain, and insects. Infection takes place through breathing pores of leaves.

The bacterial diseases usually do not cause serious losses, because they generally do not develop fully until the plants have reached their full size. During warm, moist seasons, however, they may spread rapidly from the lower to the upper leaves until one-half to two-thirds of the leaf surface is destroyed. The forage value of the crop is impaired and the kernels may not fill properly.

Three bacterial diseases of sorghum are known in the United States: Bacterial stripe, bacterial streak, and bacterial spot.

Bacterial stripe, caused by Pseudomonas andropogonz, the most serious and abundant, attacks grain, forage, and sweet sorghums, broomcorn, and Sudangrass.

Bacterial streak, caused by Xanthomonas holcicola, occurs on the leaves of sorghum and Johnsongrass as narrow, water-soaked, translucent streaks about one-eighth inch wide and 1 to 6 inches long.

Bacterial spot, caused by Xanthomonas syringae, attacks the leaves of sorghum, broomcorn, Sudangrass, Johnsongrass, pearl millet, foxtail millet, and corn. On sorghum the spots appear first on the lower leaves; infection gradually spreads to the upper leaves as the plants approach maturity. Frequently the spots are so numerous that they unite into large diseased areas and kill the whole leaf.

Recommended control measures are sanitation, seed treatment, and the use of resistant varieties. Disposing of old infected plant litter and infected plants that overwinter, along with crop rotation, will reduce the quantity of inoculum present in the fields the next season. Seed treatment before planting will keep the disease from being carried over on the seed. The sorgos as a class seem to be more susceptible to bacterial stripe than are grain sorghums and Sudangrass. The kafirs are resistant to bacterial streak.

EIGHT DISTINCT FUNGUS leaf diseases, caused by as many different fungi, are commonly found on sorghums in the United States: Rough spot, anthracnose, leaf blight, zonate leaf spot, gray leaf spot, target spot, sooty stripe, and rust.

The rough spot disease, caused by Ascochyta sorghina is widespread in Florida, Georgia, Alabama, South Carolina, North Carolina, Louisiana, and Mississippi.

It attacks sorghum, Sudangrass, and Johnsongrass. It is first observed as circular or oblong, light-colored spots. Then usually the red or tan pigment, depending on the variety, becomes apparent as the fungus spreads and injures the leaf tissue. Soon small black specks, the young fruiting bodies of the fungus, develop in the injured spots. The most striking characteristic of rough spot is the abundant development of these fruiting bodies (pycnidia), usually on the surface of the diseased discolored area, but occasionally on green, healthy-appearing parts of the leaf surface.

When the affected areas are rubbed between the fingertips, the sandpaper-like roughness, caused by the hard, raised fruiting bodies, can be detected readily. By the time the leaves die and become dry and papery, the fruiting bodies often are so abundant that they cover most of the leaf surface. Similar lesions occur on the leaf sheaths and occasionally on the stalks.

Sorghum or Sudangrass should not be grown on land where rough spot occurred the preceding season. Seed treatment and the use of available resistant varieties are advisable.

Anthracnose occurs commonly on the leaves of sorghum, Sudangrass, Johnsongrass, and other grasses grown in the humid areas of the South. Most varieties of broomcorn are especially susceptible.

The disease is caused by the fungus Colletotrichum graminicola. It may be carried on the seed and also may live on dead and decaying plant refuse on or in the soil. Infection often causes spots to develop on the leaves when the plants are still in the seedling stage. Later the disease may spread to other leaves as they appear. Usually, however, the leaves are not affected severely until about the middle of the growing season, when the plants have reached the jointing stage.

Infection first appears on the leaves as small, circular or elliptical spots, which later enlarge and may unite to involve large areas of the leaf. The leaf midrib, which is commonly infected along with the leaf blade, is often strikingly discolored. Later the centers of the leaf spots fade to a grayish-tan color; examination with a hand lens reveals the presence of numerous pin-point black specks with short, stiff hairs. Those are the fruiting bodies of the fungus, which, under moist conditions, produce pinkish spore masses. The spores are spread by rain and wind to other leaves, where they start new areas of infection.

Defoliation due to anthracnose reduces the value of the plants for forage and may reduce the sugar content of the stalks in very susceptible varieties. It also may lower the ratio of sucrose to invert sugars.

Clean culture and rotation to avoid planting sorghum in fields cropped the previous year to Sudangrass, sorghum, or Johnsongrass should reduce the losses due to anthracnose. The principal means of control is growing resistant varieties.

Leaf blight, caused by Helminthosporium turcicum, is most prevalent in the warmer humid Atlantic and Gulf Coastal Plains of the Southern and Southeastern States, where it causes serious losses in sorghum and Sudan-grass. It also attacks corn. It probably is the most destructive Sudangrass disease of this area.