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Disease management in Louisiana soybeans

Words matter, so when people speak of “controlling” agricultural diseases, Clayton Hollier is bothered.

“Listen, I wish we had disease control, but we don’t,” said the LSU AgCenter plant pathologist at the Louisiana Soybean Association annual meeting in Alexandria on Jan. 17. “What we have is disease management. Saying ‘control’ implies we’ve dealt with it and it’s gone. Unfortunately, that isn’t reality.”

Fertility management, stress management, cultural practices — “all those sorts of things, all taken together” — are part of a disease management program.

“When we select varieties we do so for all sorts of factors. Obviously, one is yield. But we want to grow the right maturity group in the right place. Edges of fields might be avoided because of tree lines. Power lines can hurt application attempts. All of these things must be considered when managing within a system.”

The same should be true of disease management. “We should look at things as a whole instead of just doing a ‘spray and pray’ approach.

Everyone is familiar with aerial blight, “something I cringe at when someone talks about row spacing. Row-spacing is very important but when there are narrow spacings not on beds, diseases like aerial blight — caused by a soil-borne fungus — love the humidity within the canopy and will flourish.”

In the 1970s and 1980s, when soybeans were frequently grown in narrow systems, crops had many problems with aerial blight.

“There were fungicides to deal with it. But the cultural practices of narrow rows hurt us more than anything else.”

More recently, Asian soybean rust (ASR) has been the focus of much angst.

“It hasn’t yet been the problem we expected it to be. However, the practices many growers have employed these last couple of years are part of the reason why we haven’t had the rust problem we might have.”

Every year, Hollier and fellow LSU AgCenter plant pathologist Boyd Padgett compile yield loss estimates due to various pathogens in the state. Those are sent to North Carolina State University where a similar list is compiled for the whole country. For the past two years, the pair found yield loss in Louisiana due to ASR has been only 1 percent, or less.

“Folks might ask what’s the big deal? But it’s 1 percent because growers managed it well. It’s 1 percent because you’ve listened and heeded the announcements made based on observations in the field and scouting sentinel plots. It’s 1 percent because you sprayed early enough to manage all diseases as a whole and ASR was part of that.”

Despite ASR stealing headlines, cercospora blight remains the number one disease in Louisiana. It’s often a subtle malady that causes the reddish-purpling color in leaves. “Some say, ‘Well, that isn’t bad.’ But I’ve seen fields taken out by that same disease.

“The point being, we’ve emphasized ASR so much in the last few years that many people have forgotten other diseases. Please, never do that.”

Many Louisiana producers — especially from Alexandria south — applied fungicides for years prior to the arrival of ASR. There were other diseases to manage because of the yield loss associated with them.

“Well, ASR came along and works a bit differently than other diseases. But there are similarities, including that the pathogen is attacking the leaves. So, let’s look at ASR as simply a part of the whole spectrum of diseases. If we do, we’ll do a much better job of helping our crops.”

Several charts compiled by Hollier and Padgett — “for Extension agents, consultants, growers and applicators” — were presented. The first included summaries of products and their effects on various diseases.

“We’re dealing with an abundance of fungicides that have abundant benefits against several diseases. Many carry full labels, not Section 18s or 24Cs.

“An important point: this chart is a guide and not labels. Go back to the actual labels and read them specifically.

“What you’ll find in the bottom section are newer products that were labeled during last year’s growing season. Many of these weren’t available to use against ASR and other diseases. We hope they will be in 2008.”

Regarding the list, “It’s nice to have all these tools in our chest. Many of these came to us due to ASR. Prior to that, chemistry was limited (as were) modes of action.”

So far, the earliest ASR found in Louisiana was in R-3 soybeans. If ASR doesn’t show up in Rapides/St. Landry parishes but is in on the coast in St. Mary Parish kudzu, “I wouldn’t panic. The disease just hasn’t built up that rapidly in the past two years.”

How much does temperature influence ASR? “It plays a big role. As it gets very hot — say, 90 degrees plus — temperatures help suppress ASR. Also, closed canopies are to the benefit of some diseases.”

When fields get rain, two things happen. First, spores are washed out of the air. That may not be an issue since they won’t travel long distances.

“The part that concerns me, though, is the moisture would be maintained in the canopy. In that situation, (treatment) would be a judgment call.

“And that isn’t parish-by-parish but almost a field-by-field decision. If you have 20 fields, you may need to do 20 different things. That’s unfortunate but true.”

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