Bacteria, Fungi, and Viruses on Soybeans

Howard W. Johnson, Donald W. Chamberlain.

Twenty-five parasitic diseases are common on soybeans in the United States. They are a constant menace to the crop. Three are caused by bacteria, nineteen by fungi, and three by viruses.

THE BACTERIAL diseases are bacterial blight (caused by Pseudomonas glycinea); bacterial pustule (Xanthomonas phaseoli); and wildfire (Pseudomonas tabaci).

The fungus diseases and the fungi that cause them are alternaria leaf spot (species of Alternaria), brown stem rot (Cephalospotium gregatum), frogeye (Cercospora soling), purple seed stain (Cercospora kikuchii), two kinds of anthracnose (Colletotrichum truncatum and Glomerella glycines), target spot (Corvnespora cassiicola), stem canker (Diaporthe phaseolorum var. batatatis), pod and stem blight (Diaporthe phaseolorum var. sojae), fusarium blight or wilt (Fusarium oxysporum f. tracheiphihim), charcoal rot (Macrophomina phaseoli), yeast spot (Nematospora coryli), downy mildew (Peronospora manshurica), phyllosticta leaf spot (Phyllosticta sojicola), pythium root rot (Pythium ultimum), rhizoctonia root rot and blight (Rhizoctonia solani), stem rot (Sclerotinia sclerotiorum), sclerotial blight (Sclerotium rolfsii), and brown spot (Septoria glycines).

Upper left: Purple stain caused by a fungus on seeds of soybean. Upper right: The purple stain disease on soybean seedlings that developed from infected seed. Lower left: A healthy soybean leaf (top) and leaves beneath infected with virus. Lower right: Fruiting bodies of southern blight fungus attached to a dead stem of soybean.

The virus diseases are soybean mosaic (caused by Soja virus 1), yellow bean mosaic (caused by Phaseolus virus 2), and bud blight (caused by the tobacco ring spot virus).

BACTERIAL BLIGHT is one of our most widespread diseases of soybeans. It can affect the stems and pods, but is most common and conspicuous on the leaves. The first symptoms on the leaves are small, angular, yellow spots, frequently water-soaked at the center and sometimes surrounded by a small, yellowish-green halo. Later the spots become brown or black as the tissues die, with marginal water soaking around the dead central part and yellowing of the surrounding area. Many small infections sometimes run together, causing large dead areas on the leaf. Under certain conditions the infection travels along the tissues next to the veins. At times this may result in badly shredded leaves, especially during periods of wind and rain. Heavy infection may cause defoliation. Bacterial blight is likely to be most serious during periods of cool weather and frequent rain. The bacteria are seed-borne and can survive also on dead leaves from one growing season to the next. Susceptibility varies with the varieties. Flambeau and Hawkeye, for example, appear to be less susceptible than most northern varieties, but are not immune.

Bacterial pustule is found to some extent over most of our soybean areas. In the Northern States its prevalence and severity seem to vary considerably with the season, but in the South it is more uniformly severe. Although the pods may become infected, it is most common on the leaves. The symptoms are much like those of bacterial blight. At first they appear as small, yellow-green spots with reddish-brown centers, more conspicuous on the upper surface of the leaf. A small, raised pustule usually develops at the center of the lesion, especially on the lower leaf surface. This is the stage at which the disease is most readily distinguished from bacterial blight. The pustule and the absence of water soaking serve to distinguish bacterial pustule from blight. The latter shows water soaking at the center or at the margin of the dead area in the early stages of infection. In bacterial pustule, small infections may run together and cause large, irregular brown areas surrounded by a yellow margin. Parts of the brown dead area may break out, giving the leaf a ragged appearance. The bacteria overwinter in diseased leaves and also are seed-borne. Although most commercial varieties are susceptible to bacterial pustule, the soybean variety CNS is highly resistant. Ogden also has some resistance.

Wildfire was first recognized as a disease of soybeans in 1943. It was reported in most of the North and South Central States in 1944. Generally it is more serious in the Southern States than in the Midwest. The symptoms are distinctive. Light-brown necrotic spots of variable size, surrounded by broad yellow halos, appear on the leaves. At times the necrotic spot is dark brown or black with an indistinct halo. In damp weather the lesions enlarge and run together, forming large dead areas on the leaf. Such severe infection may cause considerable loss of leaves. Several investigators have noted that wildfire infection is most prevalent when bacterial pustule is common. The presence of pustules at the center of wildfire lesions has led to the belief that wildfire infection takes place most readily when the leaves are already infected with pustule. Experimental evidence obtained at the North Carolina and Illinois Agricultural Experiment Stations indicates that most wildfire infection takes place through the bacterial pustule lesions and that bacterial blight lesions may also serve the same purpose. Attempts to infect soybeans with the wildfire disease in the absence of other bacterial diseases have been largely unsuccessful. Further, the variety CNS, which is highly resistant to bacterial pustule, has remained generally free of wildfire infection in the field. Tests at the North Carolina station show that the organism is seed-borne and that it can live for 3 to 4 Months in infected leaves on the ground. In buried leaves, the organism lived less than 4 months. Seed stored for 18 months produced plants free of wildfire. It appears that fall plowing might give some control of infection that is carried over in diseased leaves.

OF THE DISEASES CAUSED BY FUNGI, we discuss only the more important and representative ones.