There are four fungi to which stalk rot has been attributed. While no one of them is definitely known to be the sole cause, each may have a part. The diseases believed to be caused by them are known as charcoal rot, fusarium stalk rot, colletotrichum stalk rot, and rhizoctonia stalk rot.

Stalk rot may follow a period of drought, extreme heat, or other unfavorable conditions that weaken the plant. The disease is favored also by injuries to the stem, crown, or roots caused by cultivation implements, insects, wind, and hail, or any other agency that makes an opening for the entrance of destructive fungi and bacteria.

DEFINITE METHODS for control of the four stalk rots are not known, although resistant varieties offer the chief hope of reducing losses caused by them. Some varieties appear to be resistant to charcoal rot and to colletotrichum stalk rot, but none has been observed to be resistant to the other stalk rots. Rotation and other cultural practices may prove helpful, as will also the control of insects that attack the stalks of sorghum plants and leave openings through which stalk rot fungi gain ready entrance.

Charcoal rot, caused by the fungus Macrophomina phaseoli (or Sclerotium bataticola), is the most destructive of the stalk rots. It is unpredictable and more or less sporadic in its appearance. It is associated with crop sequence and soil and weather conditions that subject the crop to extreme heat or drought at a critical stage in its development.

Usually injury is not apparent before the plants approach maturity, when many poorly filled heads are evident along with lightweight kernels and premature ripening and drying of the stalks, many of which are lodged. The diseased stalks may be soft and discolored at the base, the pith is disintegrated, and the separated vascular fibers have a shredded appearance. After a period of dry, warm weather, the fibers become covered with small black sclerotia formed by the fungus. When the roots and stubble decay in the field, the sclerotia become incorporated with the soil where they may germinate later and infect the roots of any one of 30 or more different crops.

Another soil-borne fungus, Rhizoctonia solani, which attacks cotton and several other crops, also may cause a stalk rot of sorghum. This stalk rot differs from charcoal rot in that it first attacks the pith, giving it a reddish color, while the vascular fibers remain white. Large brown sclerotia are formed on the outside of the stalk.

The colletotrichum stalk rot, which is severe on broomcorn, is caused by the same organism, colletotrichum graminicola, that causes anthracnose on the leaves. The stalk rot phase usually is preceded by the anthracnose stage.

The fungus enters the stalk directly through the rind and spreads rapidly throughout the conducting tubes and vessels of the plant. That interferes with the movement of water and food material and results in inferior heads and seeds. Diseased stalks frequently break over at the base or at a point several joints above the ground, making harvesting difficult. That and the poor quality of broomcorn brush mean that many fields of broomcorn remain unharvested. Control measures are based on clean culture, crop rotation, and the use of resistant varieties.

TREATMENT OF SORGHUM SEED every year with an effective fungicide is a cheap form of crop insurance. Treatment of sufficient seed for most farms is economical with regard to chemicals and labor, because only 2 to 5 pounds is usually required to plant an acre. This quantity can be treated at a cost of a cent or less for material.

Fungi, which may rot the seed or kill the seedling, can be combated to a great extent by treating the seed with an effective fungicide, of which there are several on the market. The fungicides also control the two kernel smuts and prevent the spread of head smut to the soil of other fields by means of spores on the seed. Although seed treatments cannot be depended upon to prevent bacterial and fungus leaf diseases, they may prevent their spread to new areas.

Methods and materials for treating seed are discussed on page 134.

R. W. LEUKEL is a plant pathologist in the division of cereal crops and diseases at the Plant Industry Station, Beltsville, Md. He received his training at the University of Wisconsin and is a native of that State. He has been engaged in the study of the cause and control of cereal diseases since 1919 and is the author of more than 50 articles on that subject.

JOHN H. MARTIN is senior agronomist in charge of sorghum investigations in the division of cereal crops and diseases at Beltsville. He has written numerous bulletins, circulars, and other publications on cereal culture and is senior author of a widely used textbook, Principles of Field Crop Production. He, R. W. Leukel, and C. L. Lefebvre are authors of Sorghum Diseases and Their Control, Bulletin 1959 of the Department of Agriculture.