Soybean Disease Atlas

2^nd Edition

Southern Soybean Disease Workers

FUNGAL LEAF SPOTS

Fig. 13

Brown Spot

(Septoria glycines)

Brown spot is a common disease which occurs frequently on young plants. If warm moist weather conditions continue, infection, and defoliation progress from lower to upper leaves. Usually the hot dry weather of midseason arrests disease development, but the disease may become active again near maturity. If conditions favorable for disease development continue into the season or reoccur before maturity, the resulting defoliation can cause serious yield reductions. The pathogen overwinters on infected leaf and stem

debris, and the disease is most severe when soybeans are grown continuously in the same field.

Cotyledons, primary leaves, and lower trifoliolate leaves typically show brown spot symptoms. The brown to red lesions vary in size from pinpoint to 4mm wide, but may coalesce and form larger irregular shaped spots (Fig. 13). Severely affected leaves turn yellow and drop.

Although there are differences in susceptibility of cultivars, no highly resistant cultivars are available. Control is best achieved by rotation and deep plowing to reduce disease incidence.

Fig. 14

Downy Mildew

(Peronospora manshurica)

Downy mildew, one of the most common foliar diseases of soybean, occurs worldwide, but is seldom a serious problem. This disease reduces seed quality and seed size. if extensive defoliation occurs, yields can also be reduced.

Disease development is favored by high humidity and cool temperatures. Symptoms are found on young plants, but the disease does not become widespread in a field until the late vegetative or early reproductive stages. The increased resistance of older leaves and higher temperatures at mid-season stops disease development before extensive damage occurs.

Lesions start as small light green to yellow indefinite spots on the upper leaf surface (Fig. 14). As spots enlarge they become yellow-brown to dark brown. A grayish downy tuft of sporangiophores develops on the lower leaf surface during periods of moist weather. Severely infected leaves drop prematurely.

Cultivars with some resistance are available and should be used. However, many races of the pathogen have been identified, and cultivars which are resistant to all known races have not yet been developed.

Fig. 15

Powdery Mildew

(Microsphaera manshurica)

Powdery mildew occurs sporadically on soybeans. Yield reductions are possible if foliage is affected during pod fill or extensive defoliation occurs. Plants are susceptible at any growth stage, but symptoms are rarely seen in the field until the mid to late reproductive stages. Disease incidence is greater in seasons with cooler than normal temperatures.

The first symptom is usually a circular area of white to light-gray, superficial powdery growth on the upper leaf surface. Under favorable conditions, cool humid nights and mild dry days, these areas expand rapidly, coalesce and cover the entire leaf surface (Fig. 15). All green plant parts may be covered by the powdery growth. Heavily infected leaves turn yellow, then brown, and drop prematurely.

Powdery mildew is controlled with resistant cultivars. Most determinate soybean cultivars are highly resistant to powdery mildew, while many indeterminate cultivars are susceptible.

Fig. 16

Aerial Blight

(Rhizoctonia solani)

Aerial blight is found in several states, but is primarily of economic importance in Louisiana where extensive yield losses have been reported. The fungus survives in the soil and on plant debris, as well as on certain weed hosts. Infection usually begins at flowering during prolonged periods of high humidity and warm temperatures.

Symptoms include leaf spots, leaf blighting, or defoliation. Leaf spots are irregular water-soaked spots that develop reddish-brown margins during drier conditions (Fig. 16). Tissue in the center of lesions falls out giving the plant a ragged appearance. Brown lesions may also form on the petioles, stems, and pods. Severe infection causes pod blight.

Control strategies include the use of good management practices (seed bed preparation and weed control) and applying foliar fungicides. Some cultivars are more tolerant and should be planted in fields with a history of aerial blight.

Fig. 17

Frogeye Leaf Spot

(Cercospora sojina)

This destructive disease occurs regularly in areas with high humidity. In other areas, the occurrence of frogeye is sporadic, but is more severe in seasons with frequent rainfall. When a high percentage of leaves are heavily infected, serious yield loss is common. The use of resistant cultivars has reduced losses.

Young expanding leaves are extremely susceptible, while fully expanded leaves are more resistant to invasion. However, because visible lesions develop nearly two weeks after invasion, lesions are never seen on young expanding leaves. Leaves that expand during periods of weather unfavorable for invasion remain relatively disease free and result in "layers" of healthy leaves between "layers" of heavily diseased leaves.

The foliar lesions measure up to 5mm in diameter but may coalesce to form larger spots. Young, fully developed lesions have a gray to brown center with 'a distinct purplish to reddish-brown margin (Fig. 17). As lesions age, the center becomes tan to nearly white and the margin darkens. Lesions also occur on stems and pods.

Crop rotation and plowing under crop residues will reduce disease incidence. Fungicides applied at the late flowering and beginning seed growth stages will protect against frogeye infection. Several races of the pathogen have been identified, but cultivars with resistance to all known races are available.

Fig. 18

Cercospora leaf blight

(Cercospora kikuchii)

Cercospora leaf blight is a late season disease that is often mistaken for early senescence. Symptoms are first observed at the beginning of seed set. Extensive losses from the disease have been reported in some years. While this fungus also attacks pods and causes purple seed stain, there is no consistent relationship between the occurrence of leaf spot and purple seed stain.

Symptoms are first observed at the beginning of seed set. Small, reddish purple, angular to irregular lesions occur on the upper leaves which become leathery and dark purplish-red with bronze highlights (Fig. 18). This discoloration may extend to the upper stems, petioles, and pods. Heavily infected leaves rapidly turn yellow and fall, mimicking senescence. Lower leaves remain green.

Although some cultivars possess resistance, control is primarily achieved by application of foliar fungicides starting at early pod set.

Fig. 19

Alternaria leaf spot

(Alternaiia spp.)

Alternaria leaf spot has been reported from all soybean growing areas. This fungus is often a secondary invader following mechanical injury, insect damage, or another disease. Altemaria leaf spot is a late season disease that developsonmaturingleavesor on younger leaves if injured. If uninjured, most improved determinate cultivars appear to be resistant until leaves start to senesce. Since lesions develop after the onset of yellowing, yield losses are considered minimal. The causal agent appears to be seedborne.

Lesions are nearly circular unless they are restricted by a major vein or coalesce with another lesion (Fig. 19). Single lesions range up to 25mm in diameter, have brown concentric rings, and a well defined margin.

Control measures are not considered necessary.

Fig. 20

Target Spot

(Corynespora cassiicola)

Target spot occurs regularly in low incidence in many areas. The disease usually occurs too late in the season to cause economic damage . However, the disease is potentially serious on late-maturing cultivars or on susceptible cultivars in seasons of high rainfall. Leaf symptoms are seen prior to maturity during periods of high humidity or when free moisture is present.

Lesions on leaves are reddish-brown, circular, and variable in size up to 15mm in diameter (Fig. 20). Spots may also be found on petioles, stems, and pods. The larger lesions often show distinct concentric zones of dead tissue and may have a narrow, indistinct yellow-green halo. Lesions which form along veins may be narrow and elongated. Severe infections may cause premature leaf drop.

Most well adapted, high yielding, determinate cultivars have tolerance to target spot.