Late Season Soybean Diseases By Laura Sweets

This has been an unusual year for soybean diseases. Septoria brown spot was very prevalent early in the season and is still causing yellowing or browning of leaves and premature leaf drop in some fields. Frogeye leaf spot has not been widespread this year. Cercospora kikuchii and Colletotrichum spp. have shown up in some fields causing leaf spot, yellowing and shoot dieback. Downy mildew has been evident in the upper canopy of some fields. Sudden death syndrome has not been nearly as widespread or severe as it was during the 2004 season. And losses from soybean cyst nematode are always likely in Missouri soybean fields.

Although yield loss from moisture stress may be greater than yield losses from late season soybean diseases in much of the state this season, yield losses from these various late season diseases can still occur. Yield losses from late season soybean diseases will vary depending on when symptoms began to occur, number of plants infected, severity of disease in infected plants and weather conditions from now to harvest.

Septoria brown spot	Initial symptoms of frogeye leaf spot

Septoria Brown Spot
Septoria brown spot causes small brown spots on the unifoliolate and lower trifoliolate leaves. The individual spots may run together forming irregularly shaped brown blotches on the leaves. Infected leaves may yellow and drop prematurely. Brown spot usually starts on the lower portion of the plant. Under favorable weather conditions (warm, wet weather), the disease may move up through the plant. Brown spot was evident in many Missouri soybean fields earlier this season. With the hot, dry conditions for much of the growing season, Septoria brown spot has not been active through the entire season. However, in fields which haven’t canopied fully, the symptoms from earlier in the season may still be evident. Distinct small brown spots may be found on leaves in the midcanopy of many plants. Lower leaves may show more extensive spotting or larger blotches or may have yellowed and even dropped prematurely. Symptoms can move up through the canopy to the upper leaves of soybean plants. Upper leaves may also show spotting and yellowing.

The fungus which causes this disease, Septoria glycines, survives in infested residues left on the soil surface. Fields with continuous soybean production are more likely to show damage. Planting disease-free, good quality seed of resistant varieties, rotating crops with at least one year between soybean crops and maintaining good plant vigor should reduce losses from Septoria brown spot.

Frogeye Leaf Spot
Frogeye leaf spot is occurring in some fields in the state but has not been particularly widespread or severe this year. Frogeye leaf spot begins as small, reddish brown circular lesions on soybean leaves. The individual lesions do not increase greatly in size but as they mature the center of the lesion turns light tan to gray. The mature lesions have a light center with a dark purple or brown border. On severely infected leaves the lesions may merge killing larger areas of leaf tissue. These areas may drop out giving the leaves a very tattered or lacy appearance. From a distance fields with high levels of frogeye leaf spot may have a dry, brown cast.

Initial symptoms of Cercospora blight.

The fungus which causes this disease, Cercospora sojina, can survive in infested residues left on the soil surface. Fields with continuous soybean production are more likely to show damage. The disease is favored by warm, wet weather, high humidity and conditions which increase moisture levels in the canopy. Shaded fields, fields with heavy tree growth along the edges and river bottom fields have been more likely to have severe frogeye leaf spot this season. Planting disease-free, good quality seed of resistant varieties, rotating crops with at least one year between soybean crops and maintaining good plant vigor should reduce losses from frogeye leaf spot.

Cercospora Leaf Spot and Purple Seed Stain
Cercospora kikuchii can infect soybean seeds, pods, stems and leaves but is most commonly found on the seed. However, this year we are seeing some cases of leaf spot or leaf blight caused by this fungus. Infection is primarily occurring on the uppermost leaves and begins as reddish purple to reddish brown, angular to somewhat circular lesions on the soybean leaves. These lesions may coalesce to kill larger areas of leaf tissue. The uppermost trifoliolate leaf and petiole may be blighted and brown. One striking symptom of this disease may be the premature yellowing and then blighting of the youngest, upper leaves over large areas of affected fields. In most fields, the symptoms have not progressed down the plants more than one to two nodes. Pods at the uppermost node may develop round, reddish purple to reddish brown lesions. This pathogen may also infect seed causing purple seed stain. Infected seed show a conspicuous discoloration ranging in color from pink to pale purple to dark purple. The discoloration may range from small specks to large blotches which cover the entire surface of the seed coat. Temperatures of 82- 86°F with extended periods of high humidity favor disease development.

At this point in the season control of Cercospora leaf spot and purple seed stain is not feasible. It is important to remember that since this fungus can infect the seed, seed from heavily infected fields should not be used for seed. If infected seed must be planted, seed lots should be thoroughly cleaned and an appropriate seed treatment fungicide used. Rotating soybean with crops other than legumes will also help reduce Cercospora leaf spot and blight in future soybean crops.

Colletotrichum species
Colletotrichum truncatum and several other Colletotrichum species cause anthracnose of soybean. Typically, anthracnose is a late season stem and pod disease of soybean. Symptoms occur on stems, pods and petioles as irregularly shaped, light to dark brown spots, streaks or lesions. Eventually black fungal structures may be evident in these lesions. Anthracnose may also cause a tipblight. The tipblight phase of anthracnose causes a yellowing or browning of the uppermost leaves and pods. The blighted tips may dry up and die prematurely. Anthracnose is favored by warm, wet weather, and the tipblight phase of anthracnose is most likely to occur after a rainy period.

Purple seed stain.

Again, at this point in the season control of anthracnose is not feasible. This fungus may also infect seed so seed from heavily infected fields should not be used for seed. If infected seed must be planted, seed lots should be thoroughly cleaned and an appropriate seed treatment fungicide used. Rotating crops with at least one year out of soybean will also help reduce anthracnose.

Sudden Death Syndrome
Symptoms of sudden death syndrome (SDS) may appear several weeks before flowering but are more pronounced after flowering. Foliage symptoms begin as scattered yellow blotches in the interveinal leaf tissue. These yellow blotches may increase in size and merger to affect larger areas of leaf tissue. Yellow areas may turn brown but veins remain green giving the leaves a striking appearance. Infected plants may wilt and die prematurely. Severely affected leaflets may drop off the plant leaving the petiole attached or may curl upward and remain attached to the plant. Root systems may show deterioration and discoloration of lateral roots and taproot. When split open, internal tissues of the taproot and stem may show a light gray to light brown discoloration.

Management options for SDS are somewhat limited but should include planting varieties which have performed well where SDS has been a problem, improving drainage in poorly drained fields, avoiding compaction, staggering planting dates, delaying planting until soils are warm and dry, avoiding continuous crop soybean, maintaining good crop vigor, avoiding crop stress including stress from soybean cyst nematode and harvesting fields with SDS in a timely fashion.

Laura Sweets, Plant Pathology
573-884-7307