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Climate favors grain diseases in Arkansas River Valley

Because of Arkansas' disease-favoring climate, there was plenty for Extension plant pathologist Rick Cartwright to speak about at the Yell County corn and wheat production meeting on Dec. 14.

“One thing that gets us from time to time — especially in dryland production in hot, dry years and in late-planted corn — is aflatoxin,” said Cartwright at the meeting held in the Yell County Wildlife Federation Building outside Dardanelle, Ark. “The major problem with aflatoxin in corn production is it's very erratic. It's hard to measure in a large load of corn “because it certainly isn't uniformly distributed as you harvest. So there are many issues surrounding aflatoxin that hurt farmers. We have no good way to deal with that because of the way the regulations are set up.”

To prevent aflatoxin, dryland producers need to maximize the chance for rain. They can do so by planting early.

“Removing stresses helps control aflatoxin, too. That's why Bt corns seem to have less trouble with aflatoxin.”

Then, harvest at the right time. “Lots of times, dryland systems need to wait for the corn to dry down because there's no place to go with it. You'll just have to do the best you can.”

Because aflatoxin can spread in the bin, there's potential for problems if corn is harvested too wet and binned up. “You've got to be careful with the storage part.”

And if an elevator pulls a hot sample, don't give up. “I'd have it retested. The testing is imperfect.”

In Arkansas' River Valley area, most corn producers have seen their crops afflicted with viruses. Over the years, hybrid companies have come up with varieties better able to resist the Southern corn virus complex. As a result, for those who've selected a good hybrid, the complex has become less and less of a problem.

“When I first went to university back in the age of the dinosaurs (late 1960s/early 1970s), as my son reminds me, this was a huge problem. I'm sure we're all happy that's changed somewhat but it can still get after a crop.”

In 2004, Southern rust hit Arkansas corn hard. “It's a fast-moving disease that can shoot through a crop quickly. But it's an occasional problem — probably once every decade or two. Nevertheless, when it does happen it can go to town and cause problems.”

But it's such a rare event, “it's kind of hard for us to develop routine guidelines. Still, we're looking at fungicides and trying to figure out what the trigger points are for using them. We need to know because, in the past, we haven't had much luck with regular fungicide use on regular corn hybrids. Many of the hybrids have outstanding yield potential but don't seem to react well to fungicides.”

In 2005, (Arkansas Extension corn and wheat specialist) Jason Kelley and Cartwright — “along with some other colleagues” — put out many corn fungicide tests. The researchers were trying to “catch whatever was out there and see what the benefits were to the different fungicides registered for corn.”

Corn fungicides are often registered for one reason: to protect breeding programs for hybrid seed production, not field corn production. Nevertheless, they're registered for field corn. The hybrids “don't respond to fungicides well and, therefore, aren't really economical to use.

“A fungicide commonly marketed in the state is Quilt, a combination of Quadris and Tilt. It's a good fungicide and is registered anywhere between 10.5 to 14 ounces on corn.

“Kelley and his crew put Quilt out on a couple of hybrids and locations. We've tried to find any effect on yields. In 2005, unlike 2004 when there was quite a bit of disease pressure — especially from Southern rust — it was dry. A fungus doesn't like dry conditions and makes it hard for a disease to develop. So it's hard to glean much from this year's tests.”

In general, in scanning the test results, there isn't a measurable yield difference, said Cartwright.

“We compared Quilt to some of the other products. Even though we had quite a bit of variation, there weren't any consistent effects in 2005. That tells me these hybrids are good and the weather conditions weren't favorable for significant disease. These fungicides weren't terribly useful in these particular tests.”

The tests need to be repeated for several years to get a better idea of what fungicides are capable of “so we can develop recommendations for different systems and, especially, different weather conditions.”

In any given year, if a producer follows corn with corn, “you're employing a system that's very favorable for certain diseases. Nematodes and foliar leaf diseases are hot for that system. If you're going with corn after corn, fungicides may be more important for you than for someone consistently rotating crops.”

Although they show up occasionally, leaf blights are another problem largely held in check with new varieties. “Nowadays, hybrids have pretty good resistance to the leaf blights.”

The stalk rots “are probably a bigger problem than we let on in many years. These are related to fertility of the soil, stress and late-season leaf health. This is also an area we're interested in with fungicides.”

For whatever reason, aphid populations in the state were very low this fall. As a result, Cartwright doesn't anticipate any barley yellow dwarf problems next spring.

“We certainly hope that's the case. Barley yellow dwarf is a virus transmitted by aphids in the fall, primarily. The kind of year we're having isn't very favorable for aphids and isn't favorable for transmission early in the fall.”

Over the last few years, stripe rust has been a significant problem for Arkansas wheat producers. While there is resistance to stripe rust in some varieties, the fungus continues to change in response to what is planted.

“It's an ongoing battle. Frankly, this rust whips us in certain areas.”

Even though a producer may plant resistant varieties, he “must monitor those early in the spring — March and April. This is a cool season rust and it's better to scout for it when the flat leaf is starting to come out. Err on the early side — that's when it will blow up on us.”

Fortunately, there are good fungicides for control of stripe rust. Without them, high-quality wheat can be badly damaged. In tests, an untreated check with potential 100-bushel wheat ended up yielding 60 bushels, said Cartwright.

“It'll eat your pocketbook up.”

But if stripe rust isn't on the scene, it's hard to show much of a benefit for fungicides in wheat. Find your weak fields with high yield potential and protect those, suggested the plant pathologist. “Even on wheat there's a guessing game — you can't just go out and spray everything with a fungicide and expect it'll be profitable every time.

“Thank goodness we have fungicides, though. Once you see stripe rust it's hard to stop. Once it's in a field, you need to use a triazole — a Tilt-based product — and not Quadris alone. Quadris by itself is a good preventative but it isn't very good at stopping stripe rust once it's in the crop. Also when you spray, expect stripe rust to keep going for a while. It will continue moving for two, three, even four days before it'll stop.”

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