Keep your eyes open for these wet-season soybean diseases

Wet weather in much of the Soybean Belt adds insult to injury for growers, on top of battling market downturns and trade issues. With another wet planting season, here are some tips for common soybean diseases based upon the latest university research. More detailed disease information and scouting guides are available from the Crop Protection Network (https://cropprotectionnetwork.org).

Seedling Diseases

Seedling diseases rank among the top four soybean disease threats. They are difficult to diagnose and control and nearly impossible to predict when they will take a heavy toll, until it happens. Seedling health may be ignored because plant populations are acceptable, and stem and leaves do not express symptoms during early vegetative growth. University of Nebraska Plant Pathology Department Head Loren Giesler speaks for many pathologists when he cites seedling diseases as typically the most problematic soybean problems in wet seasons.

Scout right after soybeans’ first few true leaves emerge to spot early seedling blight symptoms. Send a sample to your state diagnostic lab to confirm disease or other abiotic issue, says Darcy Telenko, Purdue University Extension field crop pathologist. The water molds Phytophthora sojae and several species of Pythium fungi and true fungi such as Rhizoctonia solani and several species of Fusarium are important seedling pathogens that cause seedling disease and affect early crop stands. Infection may cause chronic symptoms or can remain latent until reproductive growth stages. Pathogenic seedling disease symptoms can be confused with symptoms caused by herbicides and physical stressors. For a list of common soybean seedling diseases, see Table 1.

Phytophthora Root and Stem Rot (Water Molds)

Cause: Phytophthora sojae

Favorable environments: Wet, heavy clay, poorly drained soils, compaction, soil temperatures above 60°F, and field history of the disease. It can affect nearly all phases of growth and damage seedlings and plants. Phytophthora infection causes root rot and stem rot, seedling decay, and damping off under warm, damp conditions.

“I like to use the name ‘water molds’ for both Phytophthora and Pythium because they only produce infective structures when soils are saturated,” says Anne Dorrance, The Ohio State University plant pathologist, professor, and associate dean for the Wooster campus. “Under those conditions, water molds germinate and form swimming spores that infect young seedlings. Phytophthora sojae is specifically attracted only to soybeans while another species, Phytophthora sansomeana, has a much broader host range.”

Alison Robertson, Iowa State University plant pathologist, adds: “This will likely lead to stunted plants, which are less vigorous and produce less pods and/or smaller seeds per pod and lower yields.”

Prevention: Phytophthora–resistant varieties. According to Dorrance, there are three forms of resistance: race-specific genes (Rps genes); Rps2, which confers incomplete and root-specific resistance; and partial resistance, which Dorrance says, “is not expressed until the first true leaves have developed, so emerging seedlings are still vulnerable to early infection.”

“Scouting helps, but knowing field history is really the best tool for preventing Phytophthora,” says CCA Lee Briese, Twin Valley Centrol Crop Consulting, Edgeley, ND.

Management: Seed treatments, resistant varieties. “We’ve had some weakening of Rps 1a, 1k, and 1c,” Dorrance says. “Partial resistance can be very high, but it involves many genes, so you can understand why it takes breeders some time to develop 100% resistant varieties when a company expands the range where it markets. But if you have a fully resistant variety, the farmer should never see stem rot develop.”

Effective seed treatments: Ethaboxam, oxathiapiprolin, metalaxyl, and mefenoxam. “Fortunately, Phytophthora sojae is very sensitive to metalaxyl at this time,” Dorrance says.

Effective fungicides: See Table 2.

You need to know: “Phytophthora has been in the U.S. for more than 70 years, and we’ve managed it with resistance,” Dorrance says. Phytophthora sojae overwinters in soil as oospores. Symptoms can be hard to distinguish from Pythium root rot and seedling blight and stem canker.

Pythium Species (or Water Mold)

Cause: Present in nearly all soybean fields, it causes seedling rot and/or damping off. “Spores carry over in soil, residues, plant tissues and roots,” says University of Missouri Extension Plant Pathologist Kaitlyn Bissonnette.

Favorable environments: Early planting in cool temperatures and wet soil. A broad range of Pythium species means the disease can be active in a very wide range of soil temperatures. Pythium can infect within 1.5 hours of planting. “Damage can be especially high during cool, wet springs,” says Iowa State’s Robertson.

Prevention: There is no genetic resistance to Pythium in commercial varieties. Plant into warm soils, improve drainage in severely affected field areas, and avoid compaction.

Effective seed treatments: Seed treatments are the main management tool. They should contain one or more of these active ingredients: mefenoxam, ethaboxam, and metalaxyl. “Soybean is very susceptible at the seedling stage to Pythium, so seed treatments are very effective,” says Ohio State’s Dorrance.

Effective fungicides: See Table 2. Sensitivity to fungicides varied among Pythium species. Research published in The American Phytopathological Society’s journal Plant Disease indicates that all Pythium species were more sensitive to metalaxyl, ethaboxam, Captan, and thiram than to the strobilurins, especially pyraclostrobin.

Rhizoctonia

Cause: Rhizoctonia solani fungus

Favorable environments: Moist and warm soils and late planting, according to Nebraska’s Giesler.

Prevention: Selected seed treatments. There are no genetically resistant varieties.

Effective seed treatments: “Seed treatments are key,” Giesler says.

Effective fungicides: See Table 2.

You need to know: There is some crossover with Rhizoctonia into corn, says University of Minnesota Extension Plant Pathologist Dean Malvick.

Sudden Death Syndrome (SDS)

Cause: Fusarium fungus overwintering in soil on soybean, corn and other crop residue. “With higher-than-normal rain the past few years, SDS has been more common in Minnesota,” Malvick says.

Favorable environments: Wet at planting and during reproductive stages.

Prevention: Planting the most resistant varieties is the foundation of an SDS management program. Also important are effective seed treatments and reducing soil compaction and crop density. Because SDS is newer in northern latitudes, resistance isn’t always available in related maturity groups, according to Malvick.

Effective seed treatments: “Your best bet is to use resistant varieties and seed treatments,” says Missouri’s Bissonnette.

ILEVO and Saltro are effective against SDS, Malvick says.

Effective fungicides: See Table 2.

Management: Begin scouting late July and early August for SDS leaf symptoms, advises Purdue Field Crop Pathologist Darcy Telenko. “SDS accounts for roughly 4 million bu yield loss of Indiana soybeans.”

You need to know: SDS leaf symptoms are easy to confuse with brown stem rot, white mold, late-season potassium deficiency, triazole injury, and stem canker. Corn and other legumes can host SDS.

White Mold (or Sclerotinia Stem Rot, Water Mold)

Cause: Sclerotinia sclerotiorum fungus

Favorable environments: Wet planting conditions, early-season moisture, early planting, temperatures below 82°F, high yield environments, and high fertility. “I see it where we have small fields surrounded by woods that cool off earlier,” says Ohio’s Dorrance.

Seed treatments: Seed treatments have not been shown to be effective on white mold.

Prevention: “Keep records of past disease levels and refer to them when planning on what to do,” advises Damon Smith, University of Wisconsin plant pathologist. “Pay attention to weather and anticipate favorable conditions during the R1–R3 growth stages.”

Resistant varieties are the best way to reduce white mold. Also important are a reduced plant population (140,000 seeds/ac, aiming for 100,000 emerged seedlings), wider rows, and foliar fungicides.

A 140,000/ac planting population plus an effective seed treatment (if warranted) typically translates to a high probability of return on investment, according to University of Wisconsin research by Extension Soybean Specialist Shawn Conley. This is based on 19 site-years (three years, 12-14 locations) of white mold/population, row-spacing, and fungicide research in Midwestern states from 2017 to 2019. Research from North Central Soybean Research Program on multi-pronged strategies for controlling Sclerotinia stem rot can be found at http://bit.ly/white_mold.

Effective fungicides: See The Crop Protection Network Foliar Fungicide chart at https://bit.ly/SB_Foliar_Fungicides and related Wisconsin 2019 trials at http://bit.ly/BadgerCropDoc. Effective fungicides and return on investment depend on the degree of infestation. In heavy white mold environments, Endura applied at 6 oz (two passes) at the R1 growth stage followed by Priaxor at R3 had the best control in Midwest and New Jersey studies. Aproach at 9 fl oz at R1 and again at R3 also did well. Environments with lower disease pressure can respond to other fungicides at a cost savings, Wisconsin’s Smith says.

Frogeye Leaf Spot

Cause: The fungus Cercospora sojina survives in infested soybean residue for at least two years. Other legumes, some weeds, and cover crops may also be hosts. Wind and splashing water may disperse spores.

Favorable environments: Extended warm, humid weather with frequent rain; non-resistant varieties, continuous soybean production, conservation tillage, and overhead irrigation.

Prevention: Choose the most resistant varieties for fields that have a chronic history of frogeye leaf spot. “The most resistant soybean varieties likely are using the Rc3 gene source of resistance although it’s probably not feasible to find out whether a variety specifically uses that source,” says University of Kentucky Extension Plant Pathologist Carl Bradley. If it is not a chronic problem in your fields, don’t sacrifice resistance to other diseases, such as soybean cyst nematode and Phytophthora. Tillage can lower inoculum levels by burying it; however, the benefits of leaving residue on the soil surface may outweigh any disease control benefits from tillage.

Management: “Frogeye leaf spot emerges later in season, so monitor for it and use effective fungicides,” says Ohio’s Dorrance.

Send disease samples to a diagnostic laboratory because it is easily mistaken for other diseases and disorders (such as herbicide injury).

Effective fungicides: See the Crop Protection Network’s foliar fungicide chart at https://bit.ly/SB_Foliar_Fungicides. Strains of the frogeye leaf spot fungus with resistance to QoI/strobilurin fungicides are widespread across the entire U.S. soybean production area. This means that you need to use products that contain active ingredients from different chemistry classes that are still effective against frogeye leaf spot. These include the DMI (triazole), MBC (benzimidazole), and SDHI classes of fungicides.

It is important to go back and check fields 2 to 4 weeks after a fungicide was applied to determine if leaves have been adequately protected against foliar diseases. Remember that no fungicide will ever provide 100% control on a susceptible variety. Although many factors influence fungicide efficacy (such as low-volume spraying, nozzle choice, carrier-water quality), inadequate control may indicate that the fungus is resistant to the fungicide.

You need to know: Observations of frogeye leaf spot in northern states have increased dramatically over the past few years. “I’m not sure what exactly’s driving this, but climate/environment certainly has to be a part of it,” says Kentucky’s Bradley. His lab has helped to document QoI (strobilurin)-resistant strains of frogeye leaf spot pathogen in Iowa, South Dakota, Nebraska, Michigan, and Minnesota. “Frogeye leaf spot is more widespread in Minnesota than before 2018, and we’ve also had a lot of summer wet weather that promotes that,” says Minnesota’s Malvick.

Anthracnose Stem Blight

Cause: The fungus Colletotrichum truncatum and others.

Favorable environments: Rainy, humid periods in July and August.

Prevention: Tillage and crop rotation to non-legumes. Treating infected seeds with a fungicide may be beneficial. Fungicidal sprays may be helpful after flowering begins. Soybean varieties differ in their resistance to anthracnose.

Management: Foliar fungicides during the early to mid-reproductive growth stages can reduce disease.

Effective fungicides: See Crop Protection Network’s foliar fungicide chart at https://bit.ly/SB_Foliar_Fungicides.

You need to know: Anthracnose is a stem disease that is often not seen until plants reach maturity. Symptoms can be mistaken for pod and stem blight. The disease typically has minimal effects on yield, but it can reduce yields, stands, and seed quality.

Target Spot

Cause: Corynespora cassiicola fungus.

Favorable environments: Extended wetness from southern Missouri southward. Initial leaf infections start with five to seven days of high relative humidity (above 80 to 85%) or moisture from light rain or heavy dew/irrigation and warm temperatures. The disease overwinters in soybean and cotton residue and is typically found on Deep South healthy soybeans or cotton with high fertility and dense canopy.

Prevention: Rotate crops. Fungicide timing is the key to suppress disease development but is inconsistent in yield protection, says Travis Faske, University of Arkansas plant pathology associate professor.

Effective fungicides: See the Crop Protection Network’s foliar fungicide chart at https://bit.ly/SB_Foliar_Fungicides.

Southern Stem Canker

Cause: Diaporthe phaseolorum.

Favorable environments: Wet weather in the Deep South.

Prevention: Where there is a field history, select resistant varieties.

Effective seed treatments: None.

Effective fungicides: None.

You need to know: Not the same species as Northern stem canker. Southern stem canker fungus can survive in infected stem debris for several years, and like other soilborne disease, once it is present, it cannot be eradicated. If you have a susceptible soybean variety, Southern stem canker can wipe out entire fields, according to Arkansas’ Faske. “The fungus infects an individual plant early and then has a long latent period without symptoms until the reproductive stage when plants begin to die.”

Pod and Stem Blight, Stem Canker, and Phomopsis Seed Decay

Cause: The Diaporthe/Phomopsis complex of fungi. These fungi, which also include Pythium and Phytophthora, cause more losses than any other soybean disease or disease complex, with the possible exception of the soybean crown and root rot complex. Symptoms can include main stem wilting and widespread plant death in field zones. These fungi can also damage seed quality during wet and delayed harvest seasons and can be transmitted by infected seed, says Wisconsin’s Smith.

Favorable environments: Wet, rainy conditions from early to midseason. Stem blight is more common in wet and delayed harvests, non-rotated soybeans, and no-till systems. Weeds such as velvetleaf and pigweed are also Diaporthe/Phomopsis hosts.

Prevention: Plant genetically resistant, high quality, certified and cleaned seed germinating more than 80 to 85% in a warm germination test or more than 70% in a cold germination test. Cold tests are preferable because they may closely simulate unfavorable field conditions after planting. Seed treatments and crop rotation can also aid in prevention.

Management: Researchers determined that chemical products tested in this research do not seem to offer a positive return on investment, when Diaporthe-induced diseases are the main goal of control, according to the North Central Soybean Research Program. Scout for leaf blight after rain or hailstorms. See also Table 2. Your state university plant diagnostic clinic (http://bit.ly/state_clinics) can avoid misdiagnosis. Fields with lots of stem canker may be detected from flowering to well into pod set and development from late July on. At this stage, canker expansion and plant death may be rapid.

Effective fungicides: Fungicide applications for pod and stem blight between R3 (beginning pod) and R5 (beginning seed) may reduce stem and seed infection especially. Although fungicide applications may reduce disease and improve seed quality, yield may not be affected.

You need to know: Stem canker can cut yields by up to 50% on susceptible cultivars under favorable conditions. Stem canker field symptoms can resemble early crop maturity.