Soybean Disease Atlas

2nd Edition

Southern Soybean Disease Workers

BACTERIAL LEAF BLIGHTS


Fig. 21

Bacterial Blight

(Pseudomonas syringae pv. Glycinea)

Bacterial blight is the most common bacterial disease of soybeans and occurs in all areas where soybeans are grown. Although the disease is of limited importance in most production areas, it can cause significant yield reductions on susceptible cultivars under heavy disease pressure.

The bacteria overwinter on soybean seeds and in infected plant residue. Bacterial blight, which is favored by cool rainy weather, is most conspicuous prior to mid-season. Dry hot weather stops disease development. Initial infections normally occur during seedling emergence with secondary disease outbreaks often following windy rainstorms or crop cultivation while the foliage is wet.

Bacterial blight is primarily a leaf disease, although stems, petioles, and pods may be affected. Leaf symptoms begin as small, angular, water-soaked spots that turn yellow and then dark reddish-brown to black with age (Fig. 21). Spots are surrounded by a water-soaked margin bordered by a yellow halo. Spots often run together to form irregular brown areas, portions of which may drop out, resulting in a ragged appearance. Management involves planting resistant cultivars and planting pathogen free seed. Cultivation of fields when foliage is wet should be avoided to prevent spreading the pathogen.


Fig. 22

Bacterial Pustule

(Xanthomonas campestris pv. glycines)

Bacterial Pustule, although common in most soybean production areas, is of minor importance because of the high level Of resistance to the pathogen in many commercially available cultivars. The bacterium overwinters in infected crop residue, in seeds and on the roots of wheat and some weeds. Unlike bacterial blight, f) bacterial pustule infections are not reduced by high air temperatures. Consequently, new infections occur whenever wet conditions prevail.

Bacterial pustule is characterized by small, yellow-green spots with reddish-brown centers (Fig. 22). Later in the center of the spots a small, raised pustule develops which is most noticeable on the lower leaf surface. Pustule formation and the absence of a water soaked appearance in the early stages of lesion development distinguish bacterial pestle from bacterial blight. Diseased leaves develop a tattered appearance as dead tissue is torn away because of weathering. Severe infection often causes some defoliation of young leaves which are more susceptible than older leaves.

Controls are the same as those suggested for bacterial blight. Resistant cultivars exist and should be planted.


Fig. 23

Wildfire

(Pseudomonas tabaci)

Wildfire bacteria overwinter in infected crop residue and seed. The ability of the bacteria to overwinter is reduced in areas with considerable seasonal temperature fluctuations. The pathogen enters plants primarily through the lesions of bacterial pustule. As a result, wildfire is seldom a problem unless pustule is widespread.

The symptoms of wildfire are light brown spots of variable size and shape, which are surrounded by a broad yellow halo (Fig. 23). Smaller dark brown to black lesions sometimes form without the typical halo. During wet weather, the lesions expand to form large dead areas that eventually tear away resulting in a tattered appearance.

Controls are the same as those suggested for bacterial blight and bacterial pustule.