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Stripe rust moving through wheat fields

During this, the second week of April, Rick Cartwright has been out walking wheat fields. He's found plenty to worry about.

“I got a little comfortable (the first week of April) thinking the stripe rust wouldn't be as serious as was first suspected. But after seeing fields this week, I'm shocked at how quickly the disease has developed in just three days,” says Cartwright, Arkansas Extension plant pathologist.

“I was walking in fields yesterday and 50 feet in, my pants and shoes were orange from the fungus spores,” he notes. “The disease is active and all over the place. We've put out an alert that, at least for the next few days, it's critical for growers — especially those below I-40 — to get out and check their wheat. East-central and southeast Arkansas wheat fields are getting a whole lot of fungicide treatments right now.”

Typically, stripe rust moves from south to north. If weather conditions (cool nights and warm days, which are great for stripe rust) continue, the disease will keep marching north, says Cartwright. Next week, farmers north of I-40 need to start checking their fields carefully, he says.

“Without a hot night or two to slow this down, we could be in even more trouble.”

April is the “disease season” for wheat. About this time in 2000, Arkansas experienced a big stripe rust epidemic.

“At that time, it became obvious to those of us that work in wheat that fungicides are very important,” says Cartwright. “By using them, we averted a major loss in about 300,000 acres of wheat.

“That epidemic, we felt, was somewhat unusual. Normally, stripe rust epidemics of that magnitude are rare — maybe one every 10 or 15 years. We documented some field losses of 50 percent in 2000. Fields that were treated cut 70 bushels per acre dry. Untreated fields were cutting in the low 30-bushel range.”

In such situations, growers are incredibly fortunate to have some effective, registered fungicides, says Cartwright. With this particular disease, fungicides are about the only option growers have.

On wheat, Arkansas has at least four fungicides registered for use through early heading. It's important not only that these fungicides are available, but also that they're registered so they can be used in the most effective manner, says Cartwright.

“Years ago, we had several fungicides that were registered in a way that we couldn't use them after growth stage 8 (normally, far too early to spray). Now, fortunately, we have a much wider window for spraying.”

Arkansas already has a lot of damaged wheat from flooding and heavy rains. With prices what they are, damaged wheat fields probably won't get the extra expense of a fungicide. But the highly productive wheat fields — those with 70 bushel-plus potential — will be scouted carefully and, if needed, a fungicide will be applied, says Cartwright.

Rice is the other big crop for foliar fungicides in Arkansas. The state treats some 300,000 acres of rice (about 20 percent of total rice acreage) per year with fungicides.

“It turns out for most of the crops, we haven't been able to create a resistant variety that means freedom of all major diseases. For example, a variety of rice may be resistant to blast but it's still susceptible to sheath blight. Until we get a super-resistant variety, fungicides will have a place.”

Sheath blight is the target of most rice fungicide treatments as there isn't good resistance in southern rice varieties to the disease, says Cartwright.

“There is some tolerance, but no good resistance. To combat that disease, we use a combination of cultural practices and fungicides. Most of the treatments go on semi-dwarf, long grain varieties.

“Scouting is key. We've done a lot of study on this and we can predict with good accuracy which of the affected fields will benefit from a fungicide. In general, if a farmer has everything else in order, the fungicides will make money for him.”

Rice and wheat are low value compared with other crops often treated with fungicides like strawberries or grapes, says Cartwright.

That means there isn't as much economic wiggle room with wheat or rice in prescribing or using fungicides. Dealing properly with each field is where scouting, education and experience pay off.

“It's common sense, but by making the proper decision on whether or not to spray a field, you can make money one of two ways. First, if a grower treats only the fields that need treating, the fields will benefit and a return will be seen from higher yields. But a grower will also save by not treating the fields that aren't in bad enough shape. You've got to know when to pull the trigger.”

The other rice disease often targeted by fungicides is blast — a disease seen sporadically in Arkansas. But with many blast susceptible varieties being planted currently, Cartwright has his antennae up.

“Blast is a disease I worry about because, while there are several choices for sheath blight, Quadris is the only effective fungicide registered for blast. To feel comfortable, we need a few more fungicides with different modes of action on blast. Having only one is a little risky.”

This year, with cooler and wetter conditions at planting time, fungicides will be used more and are more likely to pay off. The last couple of years, with warm and dry weather at planting, growers weren't as pressed to put fungicides out, says Bill Robertson, Arkansas Extension cotton specialist.

“Because it's been so wet, we're currently behind in getting our soils worked up. That leads me to believe growers will look to more no-till scenarios. I'm concerned those who aren't familiar with no-till systems don't realize how much slower the ground warms up.

“If they put cotton in cooler soils, I think fungicides are more important than ever. But in the end, fungicides are just one piece of the puzzle. They won't make a cotton plant 10 feet tall and bulletproof,” says Robertson.

Growers need to base many of their fungicide decisions on the history of seedling disease within a given field. “They need to look at soil temperatures when planting and pay attention to the five-day forecast. As we get deeper into planting season, generally the soils warm up and we can get away from fungicides.”

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