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Asian soybean rust percolates in Texas

New cases of Asian soybean rust have popped up in Louisiana (the latest in Grant and Acadia parishes), southern Oklahoma and southwest Arkansas. But the fungal disease seems to have taken a shine to Texas.

“We're running out of soybean-growing counties in Texas to turn red on the map,” says Tom Isakeit, Texas A&M plant pathologist. “The High Plains is the only area where we haven't found it yet. There's a sentinel plot there and we'll be scouting the area regularly.”

Since mid-July, the worst of the Texas ASR has been around Victoria. In some fields, the fungal disease has progressed, with lesions now occurring in the upper canopy along with defoliation.

“I believe that area had a combination of factors making it vulnerable: proximity to spore sources and lots of rain over a long period.

“Further north, there's a lot less ASR and it's hard to find. Robertson County (in east-central Texas) is a good example. The first searches showed no ASR.”

On a recent trip through the county, however, “I found a trace amount. That's not uncommon throughout the northern part of the state. Initially, when I found the ASR in Robertson County, I was very concerned we'd see an ASR explosion in that production area. I advised that fungicide applications be made if plants were R-5, or younger.”

Isakeit is now backing off that recommendation. “After going back a week later, I did not see a great progression of the disease. Plus, the rain hasn't fallen with as much frequency lately.”

Because the Texas soybean crop and geography are so varied, “it's hard to make concrete, blanket fungicide recommendations. But the general rule is if the crop is late R-5 or R-6 with no rust or a very low incidence only in the lower canopy, it doesn't need a fungicide. Growers are very reluctant to spray.

“I'll check fields and say, ‘Your fields are fine if you're only two weeks from R-6. I don't think this will blow up on you.’”

Before they spray a fungicide, “I try to get growers to consider their soybeans' yield potential. If they're going to get 30 bushels or so, a fungicide application very late — R-5 and up — probably won't be cost-effective. That's assuming they won't have a catastrophic yield loss because of ASR. And, at that late stage, I don't think most fields would badly defoliate.

“Honestly, it's not one size fits all. I explain my opinion and prognosis and leave the spraying decision to the farmers.”

Isakeit will likely be looking at alternate hosts during the off season. One is the Texas bluebonnet, the widespread, official state flower.

“It's a lupine species with relatives that are susceptible to ASR. We need to know if the bluebonnet is too.

“I scouted it on a limited basis in the early season and found nothing. I'm considering greenhouse studies and inoculations to see if it's a host.”

Another question asked mostly by south Texas farmers who want to grow two crops of soybeans annually: how long do spores survive in soybean debris?

“Although ASR is an obligate parasite (it needs a living host to grow and carry out its lifecycle), spores are viable for a period away from the host. That could be a few days to a few weeks. There doesn't seem to be a lot of information from U.S. researchers on that.”

Theoretically, farmers planting second-crop soybeans may be near a harvested field that was infected with ASR. Could any of the fresh debris still harbor and vector ASR? “It may be unlikely, but we need definitive answers.”

Interestingly, reports continue to surface that jicama, grown year-round in Mexico, is an ASR host. Also known as yam bean, ASR has been found on the crop in the Mexican state of Vera Cruz.

“Jicama is a legume I've not seen grown in the United States. The part that's eaten looks like a turnip. It grows in the tropical region of Mexico, so it isn't on the border. It's a couple of states deep.”

Hearsay reports claim widespread ASR in Mexico's jicama crop. Confirmation has come from only Vera Cruz, though.

Regardless, Isakeit says the discovery may answer the question about where Texas ASR originates. “For wheat rust, we often talk about the rust pathway from south Texas to the Great Plains.

“I can see the same type of thing working for ASR. The right weather and enough dispersed jicama in Mexico it could allow ASR to generate spores that would move into south Texas. That's just a hypothesis — but one worth testing.”

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