Reports on Plant Diseases

Figure 1. Upper and lower leaves with symptoms of Septoria glycines.

BROWN SPOT

Symptoms

Brown spot, or Septoria brown spot, is caused by the fungus Septoria glycines (teleomorph Mycosphaerella uspenskajae). The disease has become more important in Illinois during recent years. It appears early in the growing season as small spots that are somewhat angular to irregular and light to dark brown. The spots develop on both surfaces of the lower leaves (Figure 1). Some heavily infected leaves quickly turn yellow and drop off. During warm wet periods the fungus moves progressively up the plant; and, if severe, may cause extensive defoliation and major yield reduction. Late in the growing season, infected leaves become a rusty-brown before dropping off prematurely. Brown lesions of an irregular size and shape with indefinite margins develop on the main stem, branches, petioles, and pods of the maturing plants. Symptoms on these organs are not sufficiently distinct from those of other diseases to be diagnostic.

Brown spot is most severe when soybeans are grown continuously in the same field. Areas with poor drainage favor the spread of the Septoria fungus. Yield reductions of 8 to 15 percent or more are common.

Disease Cycle

The Septoria fungus overwinters in diseased crop debris as minute brown fruiting bodies (pycnidia) and mycelium in infected leaf and stem debris and in diseased seeds. During wet weather, the microscopic spores (conidia) produced within the pycnidia on living or dead plant parts are discharged. The needle-like spores are splashed by rain or blown by the wind onto healthy leaves where infection takes place. Lesions that develop on infected cotyledons and unifoliate leaves provide spores for later infections. Infection and disease development are favored by warm, moist weather. The spread of the disease is checked by hot, dry weather. The fungus enters the soybean plant through stomata and grows between the cells, killing those next to the fungal hyphae. Pods are penetrated through stomata or by growing through placental and funicular tissue, later invading the seed coat.

Resistant cultivars are not available. Some differences in susceptibility between cultivars has been observed. Determinant cultivars tend to suffer more from this disease than do indeterminant cultivars.

Symptoms

This disease is caused by the fungus Peronospora manshurica (synonym: P. sojae). Downy mildew frequently is prevalent over most of Illinois. The first symptoms are the appearance of indefinite, yellowish green areas on the upper leaf surface. These areas later enlarge and turn a pale to bright yellow to a grayish brown or dark brown and are surrounded by yellowish green margins (Figure 2).

Downy mildew can be readily distinguished from other soybean diseases by tufts of a grayish to pale purplish mold on the lower leaf surface in humid weather (Figure 3A). Severely infected leaves turn yellow, then brown, curl at the edges, and drop early. The causal fungus also grows within the pods and may produce a whitish, crusty growth of mycelium and oospores on the seed. Oospore-encrusted seeds often appear dull white and have cracks in the seed coat (Figure 3B). The disease causes some defoliation, lowers seed quality, reduces seed size, and may cause yield reductions of up to 8 percent.

Disease Cycle

The Peronospora fungus overwinters as thick-walled oospores in infected leaves and on seed. Seedling hypocotyls are also infected by soilborne oospores. Planting infected seeds can produce a few systemically infected seedlings under cool conditions. Such plants remain small–with mottled, gray-green leaves that curl downward. Microscopic conidia (or sporangiospores) produced in the downy growth on newly infected leaves during moist weather are disseminated by air currents. Spores that land on leaves may germinate within 12 hours and penetrate the leaf through stomata or directly by forming appressoria and penetration pegs.

Older leaves are resistant to infection. The number of lesions increases and the size decreases as the leaves age. Downy mildew is favored by high humidity and a temperature of 68 to 72 F (20 to 22 C). The production of spores occurs between 50 and 77 F (10 to 25 C) with no sporulation above 86 F (30 C) or below 50 F (10 C). Over 30 physiological races of the causal fungus have been identified in the United States on the basis of disease reactions, using a set of differential cultivars. This greatly complicates the development of resistant varieties.

Figure 4. Frogeye leaf spot caused by Cercospora sojina. A. Lesions on upper and lower leaf surfaces; B. Lesions on pods (courtesy J.T. Yorinori).

Cercospora blight and leaf spot, as well as purple seed stain, which occur worldwide, are caused by the fungus Cercospora kikuchii. The disease is first observed on the young upper leaves at the beginning of and throughout seed set. The sun-exposed leaves have a light purple discoloration which may darken and extend over the entire upper surface of affected leaves, giving them a dark, reddish purple to bronzed, leathery appearance. Angular, to irregular, reddish purple lesions later appear on both leaf surfaces. The lesions, which vary in size from pinpoint spots to irregular patches, may merge to form large dead areas. Numerous infections result in rapid yellowing and browning of leaf tissues followed by defoliation, starting with the upper leaves. Green leaves usually remain below the defoliated area.

Lesions on petioles and stems are reddish purple and slightly sunken. The lesions may merge and girdle affected stems and petioles. The petioles remain attached to the branches but the leaflets commonly drop prematurely. Reddish purple lesions, which later turn purplish black, form on the pods. A general, dark reddish purple discoloration also occurs on the upper stems, petioles, and pods exposed to the sun. Some foliar infections are latent and express no symptoms.

The disease is most conspicuous on the seed. A pink or lavender to a dark purple discoloration in the seed coat easily identifies the disease (Figure 5). The discolored areas range from specks to large, irregular blotches, which may completely cover the surface of the seed. Cotyledons are not usually discolored. Seeds that are invaded by the fungus may not show symptoms.

Severely affected seed is often cracked, rough, and dull. Such seed commonly has a reduced germination percentage. The discoloration of the seed may lower the grade and is objectionable to the producer of pure seed. The purple color disappears after heating and is thus not detrimental to soybean processors. U.S. grading standards, however, do not allow more than 5 percent purple-stained seeds in No. 1 yellow soybeans.

Infected cotyledons are often shrunken, turn dark purple, and drop early. Cotyledon infection may spread to the stem, forming dead areas that may girdle the young stem, killing the plant. Other seedlings are stunted but survive. During seedling emergence, if the weather is warm and humid, a velvety, grayish white mass of conidiophores and conidia may appear on the cotyledons and stems.

Symptoms

Alternaria leaf spot, caused by species of Alternaria, appears as dark brown lesions, often with concentric rings, 1/4 to 1 inch in diameter (Figure 6). The spots usually occur on leaves and pods nearing maturity, but occasionally appear on seedlings and on young plants. The lesions enlarge and may merge to produce larger dead areas on the leaves. Infected leaves later dry up and fall early. Dark brown to black pod and seed infections are usually associated with senescence, frost injury, insect feeding, or wounding. Infected seed are commonly smaller than normal, shriveled and discolored dark brown to black.

Disease Cycle

The Alternaria fungi are seed-borne. Seed invasions occur most commonly through breaks in the pod walls. Infection increases as harvesting is delayed. The disease is favored by warm, moist weather late in the growing season.

Figure 7. Soybean anthracnose on stems (courtesy T.M. Sjulin).

Anthracnose is usually caused by two fungi–Colletotrichum truncatum (synonyms: C. dematium var. truncatum, C. dematium var. truncata, and C. glycines) and C. destructivum (teleomorph Glomerella glycines)–that produce similar symptoms and may affect soybeans at any stage of growth. Both fungi have wide host ranges. Symptoms typically appear in the early reproductive stages on stems, pods, and petioles as irregularly shaped brown areas that may resemble pod and stem blight.

Stems, pods, and leaves may also be infected but show no symptoms. Anthracnose fungi primarily attack older, mature plants, usually in the late reproductive stages. Indefinite, enlarging areas that are dark brown or reddish brown may cover the surface of infected stems and pods. Later, these areas may become black-speckled with fungus fruiting bodies (acervuli), which form black spines (setae) that can be seen with the unaided eye (Figure 7). The lower, shaded leaves and branches may die and fall prematurely. Young pods may be attacked and killed. Infected seed within diseased pods may be shriveled, dark brown, and moldy or fail to form. Less severely infected seed may show no external sign of infection. Germinating seeds may be killed before or after emergence. Dark brown, sunken cankers develop on the cotyledons of emerging seedlings. The anthracnose fungi may grow from infected cotyledons into young stems where small deep-seated cankers may form that kill young plants.

Disease Cycle

Both of the principal anthracnose fungi overseason as mycelium in diseased crop residue or in infected seeds. Planting infected seed may cause the seedlings to die (damping-off) before or after they emerge. Seedlings may also be infected, but may show no symptoms until the plants begin to mature. Secondary stem and pod infections mostly occur in the reproductive stages from bloom to pod fill during warm, moist weather (below 95 F, 35 C) when free moisture is present for 12 hours or longer.

Symptoms

Phyllosticta leaf spot or leaf blight, caused by Phyllosticta sojiecola (synonym: P. glycines), teleomorph Pleosphaerulina sojicola, is a minor disease in Illinois, rarely spreading beyond the first few trifoliate leaves. Where the disease is severe, young plants may be partially defoliated, but they recover rapidly. Areas that are pale green, round-to-oval, or V-shaped appear at the tips or margins of the leaves of young plants. These areas become tan to dull gray in 1 to 3 days with a narrow, dark brown or purplish border. Numerous small, black specks (fungus fruiting bodies, or pycnidia) form in older lesions (Figure 8). The fungus may grow from the leaf blades into the petioles and then to the stipules and stem tissues at the leaf scar. Superficial, light gray, tan or brownish lesions with a narrow, brown or purplish border may form on the petioles, stems, and pods. The pod lesions are roundish with a reddish margin. Seeds located beneath the pod lesions may become infected.

Figure 8. Phyllosticta leaf spots (U.S. Department of Ag photo).

Figure 9. Powdery mildew.

Symptoms

Powdery mildew, caused by the fungus Microsphaera diffusa (synonyms: Erysiphe polygoni and E. glycines) is a minor disease of soybeans. White, powdery patches form on both leaf surfaces and on the stems and pods about midseason (Figure 9). These areas may enlarge to cover much of the aboveground plant parts. On very susceptible cultivars, severely affected leaves may turn yellow, wither, and drop prematurely. Heavily infected pods usually contain shriveled, deformed, undeveloped, and flattened seeds.

Disease Cycle

The fungus overwinters in speck-sized, black fruiting bodies (cleistothecia) within the mildew colonies late in the fall. Ascospores produced within the cleistothecia are wind-borne in the spring and early summer, producing primary infections. Secondary infections result from other spores (conidia) produced on infected leaves and other plant parts.

Powdery mildew is favored by warm, dry days and cool, damp nights. Cool weather of 65 to 76 F (18 to 25 C) favors disease development, while temperatures above 86 F (30 C) check the growth and reproduction of the fungus. The Microsphaera fungus infects a wide range of crop and weed plants.

Symptoms

Target spot is caused by the cosmopolitan fungus Corynespora cassiicola (synonyms: Cercospora melonis, C. vignicola, Helminthosporium vignae, and H. vignicola) which infects a wide range of economic plants (Table 1). The disease affects the leaves, stems, pods, seeds, hypocotyls and roots. The reddish brown leaf lesions are round to irregular varying from specks to mature spots a centimeter or more in diameter. A dull green or yellowish green halo commonly surrounds the lesions, which often become concentrically ringed at maturity–hence the name target spot (Figure 10). Severely affected leaves fall prematurely. Dark brown specks to elongated, spindle-shaped lesions form on the petioles and stems. Pod lesions are round and small but may enlarge and merge to cover the entire pod during wet or very humid periods. The fungus may sometimes grow through the pod wall and form small, blackish brown lesions on the seeds.

Round to oval, dark reddish brown lesions form on the hypocotyl and roots sometimes girdling the lateral roots. The lesions turn a dark violet brown when the fungus sporulates, producing masses of conidiophores and conidia. As the soybean plant ages the lesions elongate and merge, frequently girdling the taproot and stem. Entire roots may be discolored. The fungus that infects the hypocotyl, roots, and stems is probably a different species from the one that infects the leaves, pods, and seeds.

Disease Cycle

The Corynespora fungus overseasons in infected soybean debris and seeds and can survive in fallow soil for more than two years. The fungus can colonize a wide range of plant residues in soil as well as the cysts of the soybean cyst nematode.

Leaf infections occur when free moisture is present on the leaves and the relative humidity is 80 percent or above. Dry weather inhibits infection and colonization in both leaves and roots. Stems and roots first become infected in the seedling stage. Soil temperatures of 59 to 64 F (15 to 18 C) are optimal for infection and disease development. Large lesions form on the primary roots at 59 F and growth of the secondary roots is retarded. At 68 F (10 C), symptoms are less severe and seedling produce almost normal root systems. No symptoms develop at soil temperatures above 68 F.

Figure 10. Target spot (U.S. Department of Agriculture photo).