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Southern rust in Arkansas corn

Fungicides were the focus when Jason Kelley spoke at the Terral field day in Greenville, Miss., July 11. The topic proved well-chosen when, two days later, the Arkansas Extension corn specialist announced southern rust had been found in north-central Arkansas.

“Weather conditions currently are very conducive for development of this aggressive disease,” wrote Kelley in an advisory. “Many fields are at or very near black layer — physiological maturity — in south Arkansas if they were not replanted after the Easter freeze. Corn at black layer or corn that will be at black layer in the next few days are not the fields that we are as concerned about.”

Rick Cartwright, Arkansas Extension plant pathologist, believes the disease is “moving around the state — at least a bit. We’re not sure how widespread it is, but as for applying a fungicide, I’m telling growers ‘better safe than sorry.’ Late-planted/replanted corn may not be out of danger yet.”

How much of the crop is vulnerable?

“My guess is about half the crop, maybe a little less, was either replanted or planted late. Any of that corn is eligible for a treatment but probably 20 percent to 30 percent is all that’s really in danger. Much of the Arkansas corn crop has already been treated with a fungicide and won’t be again.”

The disease will affect corn only until the grain is filled. Once starch isn’t being put in the grain, “it doesn’t matter too much what happens with southern rust. But up until the plant quits grain-fill, the leaves must be protected.”

This corn crop is very valuable, adds Cartwright. “It’s worth protecting. Because of the recent rainy weather, a lot of spraying has already occurred. But farmers should pay close attention to disease possibilities (through the end of July) anyway.”

At this point, Kelley’s best advice is “to scout closely in these fields to see what disease is there, if any. Fields … sprayed with a fungicide earlier in the season should be protected. But these fields still need to be carefully monitored.”

In 2006, Louisiana, Mississippi and Arkansas had a little over 800,000 acres of corn. This year, corn acreage in the three states is at 2.3 million.

“That type of increase — all three states at a three-fold bump — is incredible,” said Kelley. “And on a national basis, Arkansas, Louisiana and Mississippi are the top three states as far as increased corn acreage.”

To protect that acreage, a lot of fungicides have gone out. Kelley pointed at rebate programs as further enticement to use fungicides.

“It depends on where you’re located in the region, but a lot of companies had a rebate or yield guarantee program. You might put their product on at silking, or tassel, and keep track of untreated strips until harvest. If you don’t get a big enough yield increase to pay for (the application), the companies will make it right.

“So, we’ve had a lot of corn sprayed because of that. Come harvest, I bet a bunch of growers will be keeping track of those untreated strips.”

In 2004, there was a lot of rain in May. “I remember driving through Mississippi, not too far from here, in early June and looking at corn that wasn’t nearly this green. It had a lot of dead leaves and lesions on the leaves, a lot of northern corn leaf blight.”

At the same time, a lot of southern rust came into Arkansas. In many instances, “we felt the corn was far enough along to where it wouldn’t have a great impact on yield. Well, we were probably wrong.”

At harvest, producers found a lot of cornfields with southern rust had shut down prematurely. “The kernels weren’t completely formed, weren’t to black layer yet. But the plants had to fill the kernel some way and it robbed the stalk. That led to a lot of weak stalks that went down. We ended up with corn that should’ve made 200 bushels and ended up 130 to 150 bushels. A lot of that was because we couldn’t pick the corn up off the ground.”

After that, fungicide studies moved up the priority list. In 2005-06, “we put out 15 to 20 trials. However, it was hot and dry and there wasn’t a lot of disease. In that sort of environment, we saw little benefit from the fungicides. Depending on the fungicide, we got anywhere from nothing to a 4-bushel advantage. That isn’t a lot. When putting the pencil to it, the best-case scenario was breaking even.”

This year, many believe the crop is so large it may have to stay in the field for a while. “It may take a long time to harvest and growers may have to wait in line to dump it. We hear a lot about plant health and standability. A fungicide will help with those, especially if the corn has to stand in the field.”

There’s a lot of late-planted corn in Arkansas. Nearly a third of the state’s crop was replanted because of the Easter freeze. “We’d heard, ‘(A freeze) won’t hurt corn at all.’ Well, I guarantee it will. I saw instances where, out of a 40-acre field, there may have been a dozen surviving plants.”

Kelley and colleagues have also been queried on fungicides plus insecticides. “In some instances, there have been attempts to control corn borers on a non-Bt hybrid. On some late-planted corn, the timing probably matched up — fungicides at tassel/early silk while hitting the corn borers with insecticides. For early-planted corn, the timing probably wasn’t as close.”

Overall, Kelley believes the Mid-South has an “exceptionally good” crop. That’s especially true where the crop was irrigated. “But some dryland fields also (look good). Despite many dry areas, there are some good pockets of dryland corn. Taken all together, that’s encouraging.”


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