There is no genetic resistance for the wind-borne fungus known as Asian soybean rust. Corn rust, stripe rust and leaf rust in wheat are similar to Asian rust: all reproduce quickly and can move long distances rapidly.
“If the environment is favorable, these fungi are very successful,” said Rick Cartwright, Arkansas Extension plant pathologist, who spoke at the White County production meeting in Searcy, Ark.
“An example is in Brazil. If uncontrolled, they can experience 80 percent yield losses with Asian rust. We've compared Arkansas rainfall patterns, temperatures and so forth with Brazil. Arkansas is much more erratic, so we don't think the rust will be quite as bad. But there will be areas and certain years when it could be.”
Asian soybean rust was first identified around 1900 in Japan. It's since made its way around the world, following the adoption of soybeans. North America is the last row crop continent to host it.
“Many of us feel it was introduced into our country by Hurricane Ivan. The spores were probably dispersed across a much larger area than we officially detected it. When we found it, it was late in the year and not many susceptible plants were still in the field.”
The rust needs a living plant to survive, but it has many hosts. Besides soybeans, it has been found on at least 90 other bean species.
It also needs a frost-free zone to survive over time and between crops. In balmy Brazil, the fungus tends to survive year-round.
“In the United States, we anticipate it will freeze out of Arkansas down towards the Gulf or Caribbean. It will then move back into our fields. The suspected movement of Asian rust shouldn't be foreign to us. We've been dealing with corn and wheat rusts. They're killed back to the Gulf and move back annually. They're pretty successful, but we've been dealing with them so long we have tools in place — especially genetic tools — that help regulate their behavior.”
As a plant pathologist, “it bothers me that we have no genetic resistance for Asian rust. Since we don't, our regulators will have to be the environment or our management.”
One thing about soybean rust that differs from corn and wheat rusts is length of crop susceptibility. With corn and wheat rusts, there is typically a short period of risk in a given area. That may be less than a month for stripe rust. Corn rust may have a window of six weeks to damage the crop.
“With soybeans, though, we'll be planting between March and July. That means green, flowering, susceptible plants for many months.”
Based on the experience of other countries, Asian rust will be controlled with fungicides.
“There's a lot of talk about other things that may help, but they won't get you out of the fungicide game. (The benefit of) early planting or late planting is unproven in respect to this disease. It may help in certain regions just a bit, but it won't get you away from fungicides.”
A fundamental problem with fungicides — “the ones available are good products” — is Asian rust can last a long time and soybean plants have long periods of susceptibility: six to eight weeks, at least.
“Fungicides usually last a couple of weeks — maybe three weeks if you're lucky. Please don't underestimate the rust diseases. They can move very fast and are hard to look for.
“Timing and correct identification are important. Asian rust can be deceptive and look like another disease. However, the spores of Asian rust look like nothing else: if it's producing spores, we can recognize it quickly.”
Rust diseases must be detected early, and they aren't visible from the road.
“We saw this in corn rust, last year. If you back-track the corn rust epidemic, it was pretty active in mid-June. We just weren't aware of it. By mid-July, I started getting calls about orange fields. If we're not scouting carefully, rust can hide pretty easily.”
Fungicides to use on Asian rust fall under one of two chemistries: strobilurin (like Quadris and Headline) and triazoles (like Tilt, Laredo, Folicur and others). Many have already been approved under Section 18s or have full labels.
“It's possible we'll have to make two applications. That's the norm in Brazil. But as you move into more erratic environments like Argentina and Paraguay, farmers can get by with one application. I hope we'll be in the one-spray-and-then-watch-closely category. That's not a given, though — we're preparing for the worst-case scenario.
“I cannot overstate the importance of proper application and timing of the first fungicide. They are absolutely critical. Disease epidemics are much easier to halt early in the game. Once a disease is established, in many cases you've already lost. This is especially true with rust.”
And fungicides, even though somewhat systemic, aren't like herbicides which may kill a whole plant with a large drop.
“Fungicides just don't work that way. When spraying, you must have many small droplets to get good coverage. Airplanes applying fungicides in South America do a good job, except where they miss. If the plane misses, rust will take hold and feed spores to the rest of the crop. Be aware of that if you're growing beans under power lines or somewhere a pilot can't get to. If left alone, those areas can turn into rust nurseries.”
Cartwright advises either mixing the two types of fungicides or rotating them. The fungus, at least in theory, can adapt quickly to a particular chemistry.
In preparing for early detection of Asian rust this year, there will be no substitute for eyeballs.
“We're training 700 to 1,000 first detectors — farmers, agents, consultants, and farm supply providers — to help monitor soybeans around the state. We also want home gardeners to be aware of this. Black-eyed peas, lima beans and green beans are all susceptible to this rust.”
e-mail: [email protected]