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This has been an unusual year for soybean diseases. Although some of the different late season diseases are showing up, no one disease appears to be prevalent across the state. Symptoms of environmental stress are more pronounced than disease symptoms in many fields. And in some cases, environmental stress has led to colonization of these weakened plants by root and stem fungi. Stunted, poor vigor plants may have cankers or lesions close to the soil line. Macrophomina phaseolina (the charcoal rot pathogen), Rhizoctonia solani, Phomopsis species and Fusarium species can be recovered from these damaged stem areas.

Sudden death syndrome has developed in some fields, particularly fields planted early prior to the extended period of cool, wet weather in late spring. It has also become more prevalent in the northern part of the state where cooler temperatures and rain came several weeks ago. Septoria brown spot has been causing yellowing or browning of leaves and premature leaf drop in some fields. Cercospora kikuchii and Colletotrichum spp. have been causing leaf spot, yellowing and shoot dieback. Charcoal rot is evident in fields that suffered moisture stress earlier in the season or in areas suffering from drought conditions this fall. Losses from soybean cyst nematode may be greater when plants are stressed. This would be a good year to sample fields for SCN.

Yield losses from these various late season 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. In some cases although yellowing of the upper nodes may be quite widespread and spectacular in a field, damage is limited to the uppermost leaves and pods so yield loss should be minimal. In other cases, especially with sudden death syndrome, the entire plant may have been killed prematurely. If large areas of a field are thus affected, yield losses will be greater. Although it is too late in the season to do much to control these diseases this year, management strategies to prevent or minimize these diseases next season are also given below.

Sudden death syndrome
Symptoms of sudden death syndrome (SDS), caused by a strain of Fusarium solani, 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.

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. Hot, drier weather during July slowed down disease development. But late season rains can trigger a reoccurrence of Septoria brown spot. Symptoms move up through the canopy of soybean plants. Lower leaves may show heavy spotting, yellowing and dropping prematurely. Upper leaves may also show spotting and yellowing. Some fields which have a yellow cast from the road may be showing symptoms of Septoria brown spot rather than SDS.

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

Initial symptoms of frogeye leaf spot.

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 and fields with heavy tree growth along the edges and river bottoms 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 degrees 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.

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.

Charcoal rot
Charcoal rot may cause a seedling infection in soybean but is more common later in the season. Infected plants are less vigorous and may have smaller leaves. Leaves may turn yellow and wilt. The taproot and lower stem may develop a silvery gray to light gray discoloration of the epidermis. Fine black specks or microsclerotia may be evident in tissues below the epidermis and eventually in epidermal tissues. If the lower stem and taproot are split open, a reddish brown to blackish discoloration may be seen in vascular tissues of the taproot and stem.

The fungus which causes charcoal rot, Macrophomina phaseolina, is a common soil fungus in Missouri. Corn and grain sorghum may also be hosts of the charcoal rot fungus. Charcoal rot is favored by hot, dry weather so symptoms usually appear at temperatures between 82-95 degrees.

Management of charcoal rot of soybeans should include rotating to cereals, cotton or other nonhosts for one or two years. Maintaining good crop vigor through the season will help reduce losses from charcoal rot.

Laura Sweets
573-884-7307