Although 2003 was a great year for Arkansas soybeans, that doesn't mean disease gave up stalking the crop. Cliff Coker, Arkansas Extension plant pathologist, still had plenty on his plate during the growing season. At the Arkansas Soybean Research Conference held in Brinkley, Ark., Coker offered a summary of the disease problems encountered by growers. Among the diseases covered:
Aerial blight. Coker says this was spotty around the Arkansas Delta and was seen primarily in the northeast section of Arkansas.
“Even though it's only a couple hundred miles apart, the northeast's environment is a just a little different than that in the Delta,” he says. “That's demonstrated well in Clay County, where a couple of rivers seem to hold the morning dew and moisture levels higher. That's conducive for aerial blight development. The same disease organism causes sheath blight in rice, and that's why any farmer in a bean-rice rotation needs to watch out for aerial-sheath blight.”
Frogeye leaf spot. A little more widespread this year than last, Arkansas had quite a few fields sprayed for this disease.
“Historically, we've seen frogeye leaf spot in the Arkansas River Valley, and this year was no different. But for the last couple of years, frogeye has also been widespread throughout the eastern side of the state. We treat both aerial blight and frogeye with fungicides. They occur at different times of the year but may overlap in the same field.
Syngenta (the company that supplies Quadris, the fungicide used to spray the two diseases) estimates that, based on sales, between 500,000 and 600,000 acres of Arkansas soybeans were sprayed this year.”
Stem canker and root knot nematode (RKN). These two diseases were “easily” found in 2003, says Coker.
“We didn't hear a lot about them because they were scattered. In most soybean fields, I could find at least a plant or two with stem canker.
“Stem canker is a soilborne disease that we mustn't forget about — farmers should keep planting resistant or moderately-resistant varieties. In speaking to other soybean pathologists in the state, we were surprised we didn't have more of a problem because of the rainfall pattern. We had a sort of Midwest rainfall pattern, which is very conducive to stem canker development. We attribute that not happening to growers planting a large percentage of acreage with resistant varieties. Farmers know how bad stem canker can hurt a crop and have responded well.”
Coker says RKN is usually found in sandier soils and “isn't backing off any. We don't have very many resistant varieties, so it's a slowly encroaching problem.”
Soybean cyst nematode (SCN). This disease, once again, was “big-time,” says Coker.
“We had the first reports of this around Prairie County in the Grand Prairie area of the state. There were a number of fields hurt severely early on. Some fields were replanted. Again, though, an adequate rainfall pattern sort of ‘masked’ the problem.”
Timely rainfalls allowed soybeans to get the moisture and nutrients they needed. Arkansas still had a lot of fields with SCN, but it wasn't as obvious or devastating as it was in 2001. Don't be fooled, though, says Coker.
“I'll tell you this: we're set up for a bad problem with SCN. We don't have enough resistant varieties to the prevalent races. Most beans have resistance to races 3 and 14, but those aren't the races that are the problem currently. If we don't get good rainfall, SCN will show up big.”
Charcoal rot. Again, ample and timely rainfall in 2003 helped growers out not only with SCN, but also with charcoal rot. Every year, charcoal rot takes more yield from Arkansas' soybean crop than any other disease, says Coker.
“It's soilborne and is stress-related. This disease is very sneaky — it will express itself late in the season after the lower stem has been colonized by the disease, resulting in the plant shutting down early. It's a tough disease to pick out while the plants are growing.”
Cercospora leaf blight. This disease was active in the state again and, among other things, caused a purple stain on seed. Foliar symptoms include a “leathering” of the leaves.
“This year, we saw quite a bit of the foliar symptoms but very little seed damage. I can't explain why that was the case.”
Green bean syndrome. Related to stink bug damage, this disease was very spotty in 2003.
Seed rotting fungi (anthracnose, stem blight, etc.).
“We had good harvest weather and beans weren't left out in wet fields too long. That meant seed rot wasn't much of a problem this year.”
Target spot. A relatively new disease to Arkansas, it was found in a few northeast fields.
“Previously, the only time I'd seen it was during 2001. Usually, the disease is a late-season foliar disease, and we don't know how much yield impact it has.”
Asian soybean rust. There has been a lot of justifiable news coverage on the threat posed by this disease, says Coker.
“If — or when — it does hit here, it will take the front page on every newspaper. It's that serious. And it's already shown up in Brazilian fields again. It hasn't been found in the United States, and we don't want it. It's a serious disease, and if it ever arrives, we'll be in trouble.
“Apparently, it hit South America (arriving from Africa) around 2000 and has since seriously impacted their soybean crops. If it gets into the southern United States in May, it will probably spread to Canada by the end of the growing season. Unfortunately, most researchers and pathologists believe it's a matter of ‘when’ not ‘if’ it will arrive here.”
Soybean rust must have a living host to spread, must have green tissue to be active on. That means it won't come in on seed or debris like stems, says Coker.
“The most likely scenario for it coming in is on a legume crop shipped in or on the air currents. In my opinion, it will probably come up through Mexico like wheat rust now does.”
How prepared are we to quickly identify Asian rust if/when it arrives?
“County Extension agents have a really good idea what it looks like,” says Coker. “We've had in-service trainings over the last few months and plan more in 2004. We're also providing this information at grower meetings. Different labs around the state and country are ready to check any suspect samples. Within 12 hours after receiving a sample, a lab should have an answer.”
If Asian rust is confirmed in the United States, there is a plan — still being tweaked, says Coker — to combat the disease.
“The first thing we'll try is to identify it quickly and then isolate it to a particular region first. To be honest, I'm concerned about Asian soybean rust, but it isn't my greatest fear for Arkansas soybeans.”
Coker's greatest fears concern threats already here. SCN and stem canker are the two diseases he's most concerned about in the near future.
“We haven't done an efficient job of sampling soybean fields around the state to know exactly what nematode races are out there. Every year we have a very small — much too small — sampling to go on.”
And stem canker has gone nowhere. “We have it and still remember how it impacted our yields in the late 1980s. If we make a mistake and plant susceptible varieties in a year with the wrong kind of weather, stem canker will roll through our crop again.”
Coker says growers wanting to check on diseases and varieties resistant to them can check the Arkansas Extension Soybean Update put out annually. The 2004 version should be out in the next few weeks. Producers can also check the Arkansas Extension Web site for information: www.uaex.edu.
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