Once inoculum invades a field, blight is always a threat, but management minimizes losses
While we have to cope with winter first, it’s hard not to get antsy for spring. Just like warm breezes, green grass and baby animals, you can count on seedling blight being a threat if you’ve ever had it in your field. That’s because Pythium, Fusarium, Diplodia, Rhizoctonia and Penicillium root rots are soil-borne diseases. They survive on crop residue—not just corn but also soybean residue. Except for Pythium, they also are seed-borne, providing a second avenue into fields that might not previously have had the disease.
You can manage seedling disease if armed with knowledge, says Farm Journal Field Agronomist Ken Ferrie. You need to because seedling blights can easily reduce a corn stand by 20%.
As with any pest, whether seedling diseases develop depends on the pest triangle—the presence of a pest, a susceptible host and environmental conditions. "All three must come together for the disease threat to materialize," Ferrie says.
"Some years, depending on the weather, seedling diseases won’t be much of an issue," Ferrie adds. "But if you take them for granted, when the three factors come together, they will jump up and bite you."
That happened to some farmers this past spring. As always, disease inoculum was present on crop residue—one leg of the pest triangle. Then farmers planted corn seed—a susceptible host and the second leg of the triangle. Finally, environmental conditions—the third leg of the triangle—were ideal for seedling disease.
"Because of the 2012 drought, there was more undecomposed crop residue than usual in fields," Ferrie says. "Then the spring of 2013 turned cool and wet."
Because disease organisms live on old crop residue, seedling blight is a threat in conventional tillage, as well as no-till. Set your planter to avoid placing the seed in contact with residue.
"We expect more seedling disease in no-till because there is more crop residue," Ferrie continues. "In 2013, we saw more seedling disease than ever in conventionally tilled corn fields. With less residue decomposing during the fall, winter and early spring, tillage operations buried it right at the level of the seed, below the reach of row cleaners. As I scouted fields after planting and dug around plants, I often found residue tucked right around the seed—fine material we don’t usually have to deal with. As a result, we found as much blight in conventional tillage as in no-till this year."
Three-pronged attack. Seedling diseases are primarily caused by fungi, Ferrie explains. "They can attack the seed, the seed root (a root established from the seed, as opposed to crown roots that develop later) or the mesocotyl (which grows upward from the seed to the soil surface). So they do their damage early in the season."
Fighting seedling diseases requires protecting the seed after planting. "The seed provides the food supply for the young seedling until it reaches the V4 or V5 growth stage," Ferrie says. "This supply of food pushes the mesocotyl to the surface. Seedling diseases can destroy the food reserve in the seed, or they can destroy the mesocotyl, so the tiny plant can’t transport food."
Your first defense measure is to know where seedling disease is already present. That requires scouting right after emergence, prior to the V4 or V5 growth stage.
"One symptom of seedling disease is missing plants, killed before they emerged," Ferrie says. "Dig down in the furrow and find the cause—seedling disease or skips caused by your planter. If it’s seedling disease, the seed will be soft and decaying, usually with mold around it. If you find no seed, the planter was skipping."
If seedling disease attacks after emergence, the symptom will be wilting plants. Digging will reveal whether the wilting was caused by disease, which killed the seed or the mesocotyl before the crown roots emerged and took over the feeding of the plant, or something else interfered, such as frost or insect damage.
If you don’t scout until late summer, you won’t be able to identify the causes of missing plants, Ferrie emphasizes.
Even worse than missing plants are plants that survive seedling disease only to become stunted. "When a plant is missing, you have a hole in the stand," Ferrie says. "Because it lets sunlight penetrate the canopy, adjacent plants may flex their ears, and you will gain some yield from those plants. But if a stunted corn plant doesn’t die, it functions as a weed."
The ideal environment for seedling blight is wet soil with soil temperatures in the 50°F to 55°F range (except for Rhizoctonia, which prefers slightly warmer soil). Residue in contact with seed increases the risk. That can result from no-tilling without row cleaners, which might hairpin residue in the furrow, or by shallowly incorporating residue at the seed depth.
"It’s not conditions on the day of planting that determine the amount of seedling disease," Ferrie says. "It’s the conditions from planting until the plant can sustain itself with the crown roots, which is usually around the V4 or V5 stage. If you plant in ideal conditions and the weather turns cold and wet, you’ll be fighting seedling blight."
Survival tips. Here are several steps to help seeds fight off disease:
- Select vigorous, high-quality seed. "The longer it takes from planting to emergence and for the root system to develop, the greater the risk from seedling blight," Ferrie says. "Quick emergence and early vigor help a seed survive adverse conditions. I witnessed a comparison in which a farmer put two hybrids in his planter: with the less vigorous hybrid, 15% to 20% of the seeds failed to survive. With the more vigorous one, there were no failures."
High quality means good cold germination scores and very little pericarp damage. (Damage to the pericarp, or seed coat, allows disease to enter.) "The seed tag usually only gives the warm germination score," Ferrie says. "But cold and saturated cold germination scores and pericarp damage make the difference in disease resistance.
"Some seed suppliers will provide cold germination scores if you ask for them. They probably won’t know the amount of pericarp damage; to get that, you may have to send a sample to a seed laboratory," he adds.
Eliminating standing water reduces the risk of seedling blight. Improve drainage and remove compaction in the spring.
- Eliminate compaction in your fields. "Horizontal layers put in by spring horizontal tillage will back upwater and create a saturated environment for the seed to sit in," Ferrie says. "Most seedling disease organisms are water molds, so they become very active if soil is saturated for 24 hours."
- Building good surface structure will prevent soil from sealing after a rain, making it easier for plants to emerge.
- Avoid working soil wet and creating a rough seedbed. Cloddy conditions cause seed/soil contact problems, resulting in poor germination.
- Be patient with planting. "Avoid planting into cold, wet soil," Ferrie advises. "If you know a field has a seedling blight problem, plant that field later, when the soil is warmer."
- Don’t depend entirely on the fungicide applied by your seed company. "Most fungicides only provide 10 days of protection," Ferrie says. "If cold, wet weather delays emergence, that’s not long enough. In 2013’s cool, wet spring, some corn planted on April 9 didn’t come up until May 15. So try to wait for the soil to warm up."
- Do everything you can to reduce stress on the seed because stress slows growth. "Adding an insecticide at planting may help," Ferrie says. "Avoid fertilizer burn and herbicide injury."
- Apply starter fertilizer. "We tend to have fewer blight problems when we apply starter because it pushes the corn along," Ferrie says.
- Set your planter for soil conditions present in each field. "Furrow management is a big factor in seedling disease," Ferrie says. "Too much down pressure on the planting unit smears the sidewalls. That causes poor seed-to-soil contact, and the furrow functions like a rain gutter. This happens in conventional tillage as well as no-till."
- Don’t plant any deeper than you have to, especially in fields with a history of seedling blight. "Don’t plant so shallow that you get rootless corn syndrome," Ferrie says. "But the deeper you plant, the more disease issues you will have. Early in the planting season, increasing planting depth from 1¼" to 2¼" can delay emergence by three weeks. The deeper you plant, the colder the soil is, creating better conditions for blight and worse conditions for crop growth. You must put the seed in moisture, but over the years, I’ve seen more corn lost from planting too deep than from planting too shallow, because of seedling blight."
Remember, Ferrie concludes, once an area has seedling blight issues, they won’t go away. "From that point on, you must manage seedling disease," he says. "If you don’t, you’ll get bitten every few years, when conditions are right for disease organisms."
Learn and Profit from Corn Navigator
The Corn Navigator series will focus on various sources of crop stress and how to manage the situation in order to drive corn yields and profits higher.
You can e-mail Darrell Smith at email@example.com.