Dealing with Early Season Diseases
As farmers look forward to another spring, it’s important to remember soybean seedling diseases have the potential to cause losses in Midwest soybean fields
David Wright, Iowa Soybean Association (ISA) director of contract research, says, “Frequently, seedling health is ignored because plant populations are acceptable and stem and leaves do not show symptoms during early vegetative growth. Monitoring the health and vigor of soybean stand in the seedling phase can be beneficial in making adjustments to crop management in subsequent years and may explain symptoms or less-than-anticipated yield later in the season.”
Most Common Pathogens of Soybean Seedling Diseases
“Each of these pathogens favors slightly different environmental conditions at and soon after planting,” says Iowa State University Extension Field Crops Pathologist Alison Robertson. “For example, very wet conditions are required for infection by Phytophthora sojae and Pythium spp. Soil temperatures also are important. Pythium prefers cool soils (lower than 60°F) so in early planted soybeans, with lower soil temperatures and saturated conditions, the risk of Pythium seed decay and damping off is high. When soil temperatures are above 60°F and conditions are saturated, the risk is higher for Phytophthora root rot.”
Southern Illinois University Plant Pathologist Jason Bond says farmers may need to deal with both Pythium and Phytophthora. “You can have both pathogens in the same field,” he says. “Both lead to seed rot or a decay of seedling tissue prior to emerging from the soil or shortly after, known as ‘damping off.’”
Bond explains, “Both Pythium and Phytophthora produce primitive spores that ‘swim’ through the wet ground and infect the plant. If conditions are right for rapid emergence and development, this may be avoided.”
Seedling diseases in cool, wet soils may also be caused by up to 10 species of Fusarium, including the species that causes sudden death syndrome (SDS). Fusarium species are less aggressive than Pythium and Phytophthora, which kill the plant to consume the dead material and can be found on roots, cotyledons and hypocotyls.
“Basically, Fusarium causes root rot symptoms,” Robertson says. “Symptoms are pre- and post-emergent damping off. Seedlings are stunted and weak, with brown, weak roots and poor
Rhizoctonia seedling blight is another seedling disease caused by a soilborne fungal pathogen. It is more likely to be found in warmer soil (above 75°F) and drier conditions, where soil has better aeration, as in sandier ground.
“Rhizoctonia can often be found in both regular season and doublecrop situations and can be made worse when there is injury from insects, nematodes, or herbicides,” Bond says. “Because of the heat in Illinois last year, Rhizoctonia root rot was bad, especially in double-cropping situations.”
Symptoms of Rhizoctonia root rot generally show up on seedlings as dry, dark reddish-brown lesions just above the soil line. Symptoms also include an uneven stand from pre-emergence damping off. Or plants die within a week after emerging, often just disappearing.
“Seedling loss from Rhizoctonia root rot is less common than from Pythium seed decay and damping off and Phytophthora root rot, but when present, stand loss can be severe,” Wright says.
Practices to Help Reduce Loss
Bond notes, “Once the ground is infested with any of these pathogens, they are always in the ground, though they will be minor in years when conditions are good for establishment of young plants.”
Wright says, “The first 10 to 14 days following soybean planting is the window of opportunity for Pythium and other soilborne fungi to infect the plant. Therefore, farmers should implement practices that promote quick germination and seedling emergence to prevent infection. Plant high quality seed into a good seedbed and consider that germination will be quickest when soil temperatures are above 65°F.”
Wright also encourages planting where drainage is adequate, or improving drainage is possible.
Robertson says it’s important to remember diseases are managed field by field. “Know the history of each field. If a field has a history of disease, know what the disease is so you can manage accordingly. If it is Phytophthora root rot, for example, resistant varieties are available. While resistance has not been developed to the other diseases, some varieties are less susceptible than others, so you can make selections accordingly.”
Planting dates can also be adjusted. “If the problem has been Pythium, you may want to plant those fields later, when soil has warmed up,” Robertson says.
Bond says treating seed can help, though it is not complete protection. “Most treatments cannot provide long-lasting systemic protection, but they can help, especially for early planting or for fields that stand in water.”
He notes that deciding to use a seed treatment used to be simpler because farmers had just one or two choices, but now it is more complicated, with many insecticides, fungicides and nematicides available.
“However, as the industry has evolved and there are more options, the differences are better defined so they are more easily understood be farmers,” Bond says.
Robertson says, “While fungicide seed treatments can be effective in preventing or reducing damage from seedborne pathogens, not all seed treatment fungicides are equally effective against all fungal pathogens. Choice of a fungicide seed treatment will usually depend on knowledge of what disease problems are prevalent in a particular field. Products that contain the active ingredients metalaxyl or mefenaxom, e.g. Allegiance and Apron XL, are excellent against Pythium and Phytophthora. Other active ingredients found in various products, e.g. axoxystrobin, captan, carboxin, fludioxonil, PCNB, thiram and tiabendazaole, are effective against Fusarium and Rhizoctonia. Combination seed treatments are available and can be used when this information is not available.”
While Bond doesn’t recommend certain brands, he says, “Products from major companies have excellent activity. Some have advantages, but most have excellent activity against seedling
Actions to Take if Seedling Diseases are Present
Wright says, “If dead plants are scattered throughout the field and no significant stand reduction is apparent, then no action is needed if the disease is Pythium or Rhizoctonia
When making a decision about replanting, Wright and Bond both say it is important to work with a seed agronomist. “Because soybeans can compensate for poor stand, a certified crop advisor can help determine whether replanting is necessary,” Wright says.
If the decision is made to replant, a fungicide seed treatment can be used to avoid further damping off, especially for Rhizoctonia and Phytophthora.
“Replanting without treatment within 10 to 15 days may offer risks,” Bond says. “Usually, by the time you replant, it is 15 to 20 days later, and the window will have shifted in favor of the plant, due to more favorable conditions. If there is a lot of rain, you may be repeating the same scenario, so it would be good to use a treatment.”
Because it is important to “manage field by field,” Wright urges farmers to take good field notes and use preventive measures such as seed treatment or resistant varieties to reduce the likelihood of a disease problem in the next soybean crop.