Soybean Disease Atlas

2^nd Edition

Southern Soybean Disease Workers

ROOT AND LOWER STEM DISEASES

Fig. 3

Phytophthora Root and Stem Rot

(Phytophthora megasperma var. sojae)

This soil borne disease is largely confined to poorly drained clay soils, but may occur on other soils. Severe plant losses and yield reductions are common in susceptible cultivars. The fungus survives on crop debris in the soil, but can also survive in the soil for long periods without the presence of soybeans. Disease outbreaks are associated with periods of high soil moisture and rainfall. Infection and damage occur at any stage of growth.

Symptoms include stand reduction caused by seed rots or preemergence damping off. Postemergent symptoms include water soaked lesions on the stem and roots, yellowing of leaves, wilting, and seedling death. Older plants are killed more slowly. Leaves on older plants become chlorotic between the veins followed by a general wilting and death (Fig. 3). The withered leaves generally remain attached. Lower stems have dark brown lesions that often extend beyond the first trifoliolate leaves.

The disease is best managed by planting resistant cultivars. Numerous races of the pathogen have been identified, but cultivars with resistance to all races are available. Low, poorly drained fields with a history of Phytophthora should be avoided.

Fig. 4

Fig. 5

Sudden Death Syndrome

Sudden death syndrome (SDS) is a mid-season, soilborne soybean disease which usually occurs in well-managed fields with high yield potential. The cause of SDS is unknown, but a Fusarium species has been implicated. Sudden death syndrome appears to be favored by the presence of soybean cyst nematodes, irrigation, and cool, wet weather. Losses can be high due to a reduction in pod number and seed size.

Symptoms first appear at flowering as yellow spots between the veins. The spots expand into yellow streaks which eventually become necrotic, leaving only the major veins green (Fig. 4). Severely diseased leaflets fall leaving the petioles attached to the plant. The root systems are reduced with a light brown vascular discoloration. This discoloration may extend up the stem for several nodes, but the pith remains white (Fig. 5).

Foliar symptoms of SDS are similar to stem canker. Sudden death syndrome can be distinguished from stem canker by the lack of a canker, the vascular discoloration, the white pith, and the dropping of severely infected leaves.

Since the cause of SDS is uncertain, control measures are not available. However, some cultivars may be resistant to the disease.

Fig. 6

Fig. 7

Red Crown Rot

(Cylindrocladium crotalariae)

Red crown rot, also known as black root rot, causes root and stem rot in soybeans and other legumes in several southern states. Losses up to 50% have been reported in some locations. The fungus survives in the soil and on crop debris as microsclerotia. Symptoms usually do not appear until plants are at early-pod.

The earliest symptom is yellowing of leaves of individual plants or patches of plants within a field. Leaves of infected plants develop yellow spots between the veins, which eventually turn light brown and may shatter (Fig. 6). Many of the leaves drop prematurely. The fungus spreads from the roots, causing a grayish-brown discoloration of the internal stem. The outer surface of infected stems turns red and is killed. Reddish-orange structures (pycnidia) develop on the stem 1 to 3 inches above the soil surface (Fig. 7). These fruiting structures are smaller and darker red than the fruiting structures of Southern Stem Blight.

Control-measures include planting resistant cultivars and delaying planting to escape disease. Crop rotation will reduce overwintering inoculum.

Fig. 8

Southern Stem Blight

(Sclerotium rolfsii)

Southern stem blight occurs on a wide range of host plants, including soybeans. Soybean losses due to this disease vary considerably, with damage occurring as scattered localized areas of dead plants. The fungus survives in the soil and on plant debris and is most active during the summer, with symptoms usually developing during late reproductive stages of plant growth.

The typical sign is a white cottony mold on the main stem at the soil surface. Small brown fruiting bodies (sclerotia) are associated with the mold (Fig. 8). Infected plants are often killed.

To reduce losses, deep plowing to bury sclerotia, and rotation with nonhost crops is suggested. Resistant cultivars are available, and should be planted in fields with a history of southem stem blight.

Fig. 9

Charcoal Rot

(Macrophomina phaseolina)

Charcoal rot occurs worldwide and is common in the southern United States. Yield losses are difficult to measure, as there are no known resistant cultivars or fungicidal controls. The disease is most severe when plants are under stress from moisture or nutrient deficiencies, soil compaction, nematodes, or other pathogens. Overwintering inoculum survives as sclerotia on plant debris or in soil. Symptoms appear in hot, dry weather usually after flowering.

Charcoal rot is primarily a root and basal stem disease, but may be seen on above ground parts of infected plants. Diseased tissue in the taproot and lower stem develops a grayish discoloration. Eventually the lower stem is girdled, causing wilting and death. Infected soybean plants have many tiny, black specks (sclerotia) on the roots and lower stem just beneath the epidermis or bark (Fig. 9). The sclerotia resemble a sprinkling of powdered charcoal, hence the name charcoal rot.

Since the disease is often associated with weakened plants, maintaining healthy, vigorous plants will reduce losses. Maintaining vigorous plants requires proper fertilization, weed management, and irrigation, where possible.