Rots, Blights, and Leaf Diseases of Sorgo

E. V. Abbott, P. E. Bouchereau.

Sorgo is the term applied to the juicy, sweet-stemmed members of the genus Sorghum that are grown for sirup production. Sorgo is grown for sirup on approximately 90,000 acres in the Great Plains and the South. More than 80 percent of the acreage is in 11 Southern States.

Sorgo might some day be a commercial source of sugar, but it is not that now because we do not have varieties with the necessary productiveness, disease resistance, and adaptability to the areas where they would be grown as a sugar crop.

The sweet sorgos are subject to the same diseases as the forage and grain sorghums, but the relative importance of some of the diseases varies.

The head smuts, for example, which are important in the grain sorghums, do less damage to the sorgos. Some leaf and stalk rot diseases are much worse in the sorgos, particularly those grown in the more humid regions. The longer growing period required for sorgo grown for sugar production leads to greater difficulty with stalk rots.

Three groups of diseases affect sorgo: Seed rots and seedling blights, leaf diseases, and stalk rots.

The seed and seedling diseases maybe caused by several different fungi. Their severity depends on weather and soil conditions. They are more severe when cool, wet weather follows planting. They often reduce stands, but satisfactory stands can usually be obtained, even though the diseases are not completely controlled, by treating the seed with fungicides before planting. The hazard of loss is reduced by delaying planting in the spring until favorable temperatures for germination are in prospect. These diseases are not considered a major obstacle in the development of sorgo varieties for sugar production.

Of greater importance are the major leaf diseases, anthracnose (caused by Colletotrichum graminicola), zonate leaf spot (Gloeocercospora sorghi), and rust (Puccini .a purpurea). They are worst in the humid sections near the Gulf.

Anthracnose causes small, roundish, discolored spots on the leaves. The young lesions are reddish orange or dark purple and later become grayish or dark straw-colored. In moist weather their surfaces are covered with the pink masses of fruiting bodies of the fungus. Long, rather oval lesions also occur commonly on the leaf midribs. Destruction of leaf area may seriously injure the plant, particularly by delaying maturity.

Zonate leaf spot, so called because of the large spots with wide bands of reddish purple alternating with straw-colored tissue, may defoliate some plants before they mature. Some of the more vigorous and otherwise desirable varieties now available are susceptible to it. They may mature sufficiently for satisfactory sirup production, but the destruction of leaf area prevents the degree of maturity necessary for sugar production in humid sections. The disease limits the production of sugar sorgos in the sugar-producing areas of Southern Louisiana.

Rust on sorgo, resembles the leaf rusts of small grains. Its raised pustules are covered with a thin membrane, which eventually breaks and permits the escape of the powdery, reddish-brown spores of the fungus. Sometimes only a few scattered rust Pustules are present on a leaf, but in warm, humid weather they may become so numerous on the leaves of the susceptible varieties as to cause premature death and drying of the leaves.

The three leaf diseases occur also on Johnsongrass. Where the weed is prevalent, as it is in the sugar-producing area of southern Louisiana, it provides a ready source of initiating epidemics of disease in commercial plantings of sorgo. The disease organisms presumably live over the winter on Johnson-grass. Zonate leaf spot and rust generally decline somewhat in severity northward from the Gulf of Mexico.

Other leaf diseases that may be important on some sorgo varieties in some years are leaf blight (caused by Helminthosporium turcicum), gray leaf spot (Cercospora sorghi), rough spot (Ascochyta sorghi), and bacterial stripe (Pseudomonas andropogoni).

The major disease problem to be met in developing varieties of sorgo suitable for sugar production is red rot of the stalk. The causal organism, the same fungus that causes leaf anthracnose, is closely related to Colletotrichum falcatum, which causes a similar disease of sugarcane. Strains of the sorgo red rot fungus occur on several other grasses, including Johnsongrass.

Red rot of the stalk is characterized by reddish, purplish, or orange discoloration of the pith (depending on the variety). Lighter bars extend crosswise through the darker background. The general pattern of the disease is like that produced by red rot of sugarcane. The fungus may break through the rind, producing elongate, sunken lesions, which are frequently covered with the fungus fruiting bodies. Besides lowering the quality of juice for sirup or sugar, the disease, by causing the stems to break, greatly reduces tonnage. Its critical importance in considering varieties for sugar production arises from the fact that the disease gets worse as plants mature.

Ordinarily the stalk-rotting phase of red rot is not of major importance in sorgo grown for sirup. But sorgo grown for sugar needs a greater degree of maturity and hence a longer growing period; red rot might therefore rot completely the stalks of some varieties. Breeding varieties that resist red rot and have other essential qualities is the main problem to be met in the development of sorgo for sugar production.

Neither the leaf diseases nor red rot is yet amenable to economic control through the use of fungicidal dusts or sprays. Practical control depends on varietal resistance. In the genetic material now available to the sorgo breeder, resistance to all of the important diseases fortunately is present in one or more varieties.

The task of combining through breeding this resistance with the other qualities that would be required of a commercially adaptable sugar-producing sorgo is being carried forward by sorgo breeders in the Department.

While encouraging progress has been made, hybrid varieties satisfactorily meeting all of the requirements have not yet been produced.

E. V. ABBOTT is a pathologist in the Bureau of Plant Industry, Soils, and Agricultural Engineering.

P. E. BOUCHEREAU is a pathologist of the division of sugar plant investigations of the Bureau, and has headquarters at Beltsville, Md.

Sugar beet curly top.