The three legs of the Disease Triangle, the host (your crop), the pathogen and the environment, must all interact at the right time to create a disease problem.
You can minimize disease risks by understanding the threat, your hybrids and the environment
Surprises are great for birthday parties, but not with corn disease. If you do your homework, you won’t have many surprises when you scout your fields midseason, says Farm Journal Field Agronomist Ken Ferrie.
The way to avoid yield-slashing surprises is to manage the disease threat, Ferrie explains. That requires understanding the Disease Triangle. The three legs of the triangle are the host (your crop), the pathogen and the environment. "All three factors interact, and they must all come together at the right time to create a disease problem," Ferrie says.
"If one factor is missing, disease won’t be an issue," he explains. "For example, in 2011 many areas had severe infestations of Goss’s wilt because they had warm, wet weather, high humidity and hail or wind damage to the leaves, which opened avenues for infection. In 2012, everybody worried about that disease, but it turned out to be rare. The inoculant was there (in old crop residue), and the host crop was present, but the weather was hot and humidity was low. The environment made all the difference."
A critical factor. Environmental conditions affect whether diseases survive, how they are dispersed (blown in or carried in by insects) and if they are able to colonize on plants. While most fungi can infect plants, bacteria and viruses need a way inside—typically insects or damage such as hail injury.
Of the three factors in the Disease Triangle, environment might be the most critical, Ferrie notes. Besides determining whether a pathogen builds to damaging levels, it also affects your disease control planning process, which begins long before seed hits soil.
In 2013, failing to consider environmental conditions resulted in a few unpleasant surprises. "Some growers had gotten used to scouting for gray leaf spot, which had been a problem during several warm growing seasons with high humidity," Ferrie says. "As a result, they failed to identify eyespot and northern corn leaf blight, which resulted from cool, wet conditions in the spring."
To manage disease, you must understand the vulnerabilities of your hybrids. "Suppose a hybrid is a really good yielder but has low resistance to Goss’s wilt," Ferrie says. "If you’re in a continuous corn rotation, don’t plant that hybrid in a field where you had Goss’s wilt last year, even if you do tillage. If you’re in a corn/soybean rotation and you strip-till or no-till, don’t plant it where you had Goss’s wilt two years ago. The disease inoculum will still be present in the old corn residue. Plant that hybrid somewhere else, where Goss’s wilt has not been a problem."
Size up hybrids. Test plots don’t always reveal a hybrid’s weakness. "If a hybrid won five county plots, that tells you the environment was perfect for it that season," Ferrie says. "It does not tell you how it will perform under different growing conditions. In that situation, you need to search for the hybrid’s weakness."
When evaluating hybrids for disease resistance, make sure you understand each company’s disease rating scale. "They are not all the same," Ferrie says. "For one company, a rating of 1 may signify a high level of disease resistance, and a score of 9 may mean very little resistance. But another company’s scale may be just the opposite."
Even if two companies use the same scale—1 being least resistant, 9 being most resistant, for example—hybrids with the same numerical rating may not have the same level of resistance.
"The ratings are based on how new hybrids compare to a check hybrid," Ferrie says. "If two companies use different check hybrids, a rating of 5 from both companies does not signify an identical level of resistance."
This showed up in one farmer’s field in 2013, Ferrie says. The grower had made a special effort to plant hybrids with resistance to Goss’s wilt and chose two hybrids from different companies. Both had a resistance rating of 7 on a scale of 8 being most resistant.
But in mid-August, one hybrid had Goss’s wilt in some fields, while the other did not. "That doesn’t mean the grower’s disease management wasn’t successful," Ferrie says. "Even with the disease present, he still had 200-bu. corn. If he hadn’t planted disease-resistant hybrids, Goss’s wilt might have devastated his fields. But it does show that, even though the companies used the same rating scale, their ratings were based on different standards, from comparisons to different hybrids."
The easiest step in managing disease is to understand your hybrids. "The more disease resistance built into the hybrid, the better," Ferrie says. "But that doesn’t relieve you from scouting and applying fungicides if they are needed. There are instances where a susceptible hybrid sprayed with a fungicide outyields a resistant hybrid."
How to scout. Selecting next year’s hybrids begins with scouting this year’s crop. "Scout for disease in midsummer," Ferrie advises. "If you wait until harvest, you’ll see the damage but you won’t be able to identify the disease.
"Carry a scouting manual so you can confirm what you find. Record the predominant diseases in each field; you’ll be dealing with them as long as crop residue remains in the field."
By scouting, you are searching for each hybrid’s weakness. "If you understand the role of environmental conditions, you’ll know what leaf diseases to expect before you enter the field, so there won’t be surprises," Ferrie says.
"For example, expect to find gray leaf spot if you have had consistently high humidity. If spring weather was cool and wet, anticipate northern corn leaf blight and common rust."
Year-round prevention. The second step in season-long disease management occurs as you select hybrids for next year. Before you open a seed catalog, consider your tillage system and crop rotation. Having crop residue in your field increases disease risk.
With that risk in mind, consider yield potential and disease rating for various hybrids. "If you no-till continuous corn, you have a greater disease risk. So you’ll want to pick a hybrid with more resistance," Ferrie says.
"Base fungicide application decisions on the weaknesses you identified in your hybrids and the weather during the growing season," Ferrie says. "Warm, wet weather increases the risk of diseases such as gray leaf spot."
Weather is the only thing you can’t change as you evaluate disease risk. But if you understand the Disease Triangle, weather should be the only big surprise you’ll encounter.
You can e-mail Darrell Smith at firstname.lastname@example.org.
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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.
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- November 2013