Since early September, Asian soybean rust has been very active in Arkansas. The weather — a long string of rainy, cloudy days — has been “perfect” for disease to develop, says Rick Cartwright, Extension plant pathologist with the University of Arkansas.
“It’s developing extremely rapidly where it’s present — and it is present in almost all of Arkansas, now. The only exception may be the southwest corner of the state. It’s been found on the western border, the Arkansas River Valley and all the way up the Missouri line.”
(For the latest on rust finds in southeast Missouri, see Soybean rust in southeast Missouri.)
Have there been any changes in fungicide recommendations?
“We’re still saying if you’ve got late beans in growth stage 5.5, or younger, then spray them with a triazole fungicide. Once the beans are past growth stage 6 — where the beans are touching in the pods — we give up spraying, pretty much, under the assumption the crop will outrun any damage.
“We still have a small percentage of the soybean acreage in the 5- to 5.5.-range. That’s being treated as the weather permits.”
Since early September — “really the last two weeks” — rust has “taken off. It’s been eye-opening how fast it can move and develop.”
Cartwright, speaking on Sept. 23, provides an example. “Last Friday, I got several calls from northeast Arkansas saying, ‘I’m not sure this rust is going to get here very quickly. I’m not sure it’s all that big of a deal.’ On Monday, the same guys called me and said, ‘Forget what I said on Friday. This thing is moving through here like a freight train.’
“That tells you, in three days, we’re finding that rust can visually change the appearance of bean fields. It’s that fast.”
Does Cartwright suspect some fields have lost yield to the rust?
“I know in southeast Arkansas there have been some yield losses. But mainly it’s in fields where the farmers decided they couldn’t afford to spray. Cliff Coker (Arkansas Extension plant pathologist) says some of those fields were defoliated, for the most part, in about 10 days. Those fields certainly lost yield.
“Now, in the northeast part of the state, if vulnerable beans are sprayed (the week of Sept. 21) they’ll be okay. The aerial applicators are very good at their jobs — when the weather breaks, they’re getting over late fields very fast.”
Once it picked up the pace, has rust reacted any differently here than what is observed in Brazil?
In the last three weeks “it’s acted the same as it does in Brazil. It may have moved even faster, in some cases.”
The latest weather conditions have “apparently been optimum for it. Let me tell you: if rust had started in Arkansas a couple of weeks earlier, I believe we’d have been in bad trouble. Luckily, it came in later because of prevailing wind patterns earlier in the summer. But with this late crop we have, if the epidemic had begun two weeks earlier this summer, I’m not sure we could have kept up.”
Fortunately, the growing season’s weather has been an outlier. “But I’ve found this rust to be about the fastest thing — plant disease-wise — you’ll ever see under these conditions.”
If the current conditions are good for soybean rust, what about other soybean diseases?
“There’s a lot of frogeye in some areas. We’re not terribly concerned about it.
“The biggest problem is simply the beans staying in the field and being slow to develop. This cloudy, rainy weather is hindering them from finishing out. We’re seeing pods splitting, germinating in the pods.”
The forecast appears favorable. “But if we don’t get a period of dry, sunny weather we’ll have a mess at harvest. Hopefully things will dry down in the next few days.”
There is some positive news, says Cartwright. According to east Arkansas farmers he’s spoken with, “there are some good, cheap triazole fungicides available. It’s interesting that with the boom in rust, these products — in some cases, as low as $3 or $4 per acre for the chemical — have shown up. That’s saving farmers some money.”
For more on soybean rust, see DFP: Soybean rust.
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