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Will excess leftover debris lead to more disease, pest pressure?

Last fall, two hurricanes hit Louisiana's coast and then headed north and east. In a harvesting frenzy, farmers scrambled to get combines and harvest equipment through their fields before the hurricanes hit land. Unfortunately, wet weather arrived before all harvest and land prep could be completed.

Now, with extra debris and crop trash in the fields, many are wondering if pests and disease will have a stronger foothold at the outset of this growing season.

“In general, I think last year's hurricanes could contribute to the pathogen populations that will be in our fields this spring. The potential for increased pathogen problems is probably pretty uniform across Louisiana. The hurricanes came up from the south, hit the coast and then moved north,” says Boyd Padgett, Extension/research plant pathologist at the LSU AgCenter's Macon Ridge Research Station near Winnsboro, La.

“Research shows that increased organic matter in no-till or reduced-till cotton can support higher pathogen populations. That's only potential, though. Whether those pathogens show up in a big way will be directly related to the weather at planting. If we have conditions conducive for cottonseed germination, those pathogens probably won't be much of a problem.”

Soybeans could be a different matter, though, says Padgett. “There are diseases that overwinter on soybean stubble. In south Louisiana, they were able to get the Group 4 soybeans out. But some of the Group 5s were left in the field because farmers couldn't combine them. That worries me.”

The amount of soybean residue still in Arkansas fields concerns Cliff Coker because decomposition is being slowed.

“If you don't get into a field after harvest and turn the soil over or get good soil/plant material contact, decomposition takes longer,” says Coker, an Arkansas Extension plant pathologist. “What often happens is soybean diseases like anthracnose, pod and stem blight, and stem canker hang around in the plant debris and become more of a threat and problem than they otherwise would. By not having good soil contact with the debris, the diseases are given a break from decomposition that normally helps with control.”

The debris problem is being seen all around Arkansas — although more in the southern half of the state.

What remedies does Coker suggest?

“I tell growers who have this field situation that they'll need to scout very carefully this season. They need to pay special attention when selecting soybean varieties to plant this coming year. If a field has had a history of stem canker in the past, make sure you get a resistant — or moderately resistant — soybean variety to plant. This is especially true in fields where debris was left on a field's surface all winter.”

Coker isn't nearly as concerned with Arkansas cotton debris. “I think most of the cotton residue was chopped up and taken care of last fall,” he says.

Roger Leonard says Louisiana's real debris problem isn't with hurricane events or increasing pest populations. The problem also isn't a function of having so many soybeans left unharvested across the state. “Our problem is as farmers go to more conservation tillage systems, they're reducing the amount of physical movement of soil. They do this in using stale seed bed systems or in simply leaving crop refuse — corn and grain sorghum stalks aren't even bush-hogged — and thus increasing the overwintering populations of southwestern corn borers and the sugarcane borers,” says Leonard, an LSU AgCenter research entomologist.

Such reduced-till practices have allowed pests to more easily survive winter and produce larger populations the following year. If you start off with high populations in the spring, you'll end up with higher populations later in the season when crops are very susceptible.

“So that's what we're worried about — not last fall's weather patterns. We've spent a lot of time last fall speaking with growers about performing some tillage or cultural practices to reduce stubble. They need a mowing operation, a mowing and re-bedding or something else. It's better to go with a form of stale seedbed rather than true no-till, particularly in corn and grain sorghum in both northern and central Louisiana,” says Leonard.

At this point, there's not a lot that can be done with cotton trash. Growers certainly need to get their burndown out as soon as possible — at least four or five weeks prior to seeding the crop, says Leonard. That helps to starve all the pests feeding on native vegetation.

Padgett estimates that reduced till cotton is planted by between 70 percent and 80 percent of Louisiana growers. That leads him to believe that any additional cotton disease pressure due to the hurricanes will be minimal.

“Growers are leaving cotton debris in their fields regardless,” says Padgett. “And if they wait until soil temperatures warm up before planting, they probably won't have any more disease pressures than normal.”

On soybeans, though, pod and stem diseases are much more worrisome. “Stem canker will show up if you leave debris and plant a susceptible variety. Any field with a touch of stem canker last year will be primed for the disease this year,” says Padgett.

“Pod and stem blight are ubiquitous — you get some of that in nearly every field. At least that's been my observation in Louisiana. If growers have excess debris in the fields, such diseases could contribute to yield loss and troubles. And these diseases really won't manifest themselves until later in the year. Growers should keep an eye out.”


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