When seedling disease attacks after emergence, symptoms are wilting plants. Dig to see if the wilting was caused by disease, frost or insect damage.
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."
- Mid-November 2013