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Fungicide-resistant Rhizoctonia solani found in Louisiana

Fungicide-resistant Rhizoctonia solani found in Louisiana

Soil-borne Rhizoctonia solani fungus found to be resistant to strobilurin fungicides. Fungus causes sheath blight in rice and aerial blight in soybeans. So far, only in a small area of southern Louisiana.  

A suspected mutation of the Rhizoctonia solani fungus has been found to be resistant to strobilurin fungicides. So far, the soil-borne fungus – which causes sheath blight in rice and aerial blight in soybeans – is only in a small area of southern Louisiana

Even so, “this is a significant discovery,” said David Lanclos, Louisiana agronomic service representative for Syngenta, on Friday morning. “The situation developed last year -- primarily in Acadia Parish in the Mowata area, between Crowley and Eunice -- when we began noticing some product issues in the field. We were called and began trying to figure it out.”

Lanclos and colleagues solicited input from “very experienced plant pathologists (at Syngenta’s research facility) in Vero Beach, Florida.”

Following a series of major tests, the pathologists “came up with (fungicide) tolerance levels for Rhizoctonia solanithat are too high for the traditional strobilurin products to work anymore.”

The week of Jan. 2, “we sent information to the LSU AgCenter about it – they’ve been in the communication loop the whole time. Now, we must educate farmers about the discovery.”

Among Lanclos’ other comments:

Who first discovered it?

“Originally, one of our salesmen, Josh Zaunbrecher, who works in southern Louisiana, found it.”

What was the timeline on this?

“We were called out near the end of last year’s rice-growing season, just as the water was being pulled off.

“We went into the impacted area and pulled samples and got the results back after the first of the year. Preliminary results weren’t good but we’ve now done (intense testing) that confirmed the tolerance.”

The pathologists tested “for resistance to azoxystrobin (Quadris) and propiconazole (Tilt). The azoxystrobin results were mixed but not the propiconazole. In all cases tested in Vero Beach, all of the samples came back susceptible to propiconazole, which is great news. Tilt will have to be a component to a successful fungicide program in 2012 for southwest Louisiana.

“The strobilurin class of fungicide chemistry -- including Quadris, Stratego, and Headline – are not going to be effective against Rhizoctonia solani in that area. Fungicides in the same class of chemistry share the same mode of action against pathogens and therefore they are assumed to be cross-resistant.”

What are the fears about the resistance moving?

“That’s a very good question. It will be an educational experience for us all.

“The disease is rather unique in the way it’s transferred from field to field. It isn’t like a traditional rust disease with air-borne spores.

“It’s mainly moved through soil and crop debris. It can be moved by people’s feet walking in contaminated soil. It can move through equipment, combines, anything along those lines.

“Can we control it? At this point, the fungicides that we have available will have diminished or no control on it specifically.  However, they can and will continue to have control or efficacy against lots of other diseases. The way that we are going to manage the disease is through BMP’s (Best Management Practices) and, hopefully, in the future with (new) fungicides.”

On suggested BMPs…

“First, we need to make sure everyone understands what this disease is and how it spreads.

“Equipment is the main spreader of the fungus. Based on that fact, we suggest equipment be pressure-washed, combines be blown out, things along those lines.

“Going into the 2012 planting season, what can we use as crop strategies to rotate away from Rhizoc? The first and, likely best, strategy is crawfish production (followed by) sorghum, corn or pasture. I fully realize that these rotation strategies don’t occur overnight. It will be a long-term rotation strategy with other crops that will have to be used to combat this.

Hybrid selection extremely important

“Hybrid selection will be extremely important. We’ll need to plant the hybrids and varieties with the most tolerance to Rhizoctonia solani.

“Seed treatments will be a major player, as well. Whether it’s rice, corn or soybeans, you want every advantage in your favor to get a crop out of the ground and growing faster.

“Don’t under- or over-fertilize -- doing that can actually” promote the fungus.

“As for plant populations, we suggest folks plant on the low side. That’s true whether it’s rice, soybeans, corn, grain sorghum, whatever.

“But we really must focus on cleaning equipment. We must start cleaning debris from equipment when it’s moved from field to field or farm to farm.”

On further research…

“Syngenta will continue testing in 2012 to extend our understanding and knowledge base of actually where and how widespread (the disease) is.

“Right now, the map is based only on the samples we were able to pull late-season last year. We pulled approximately 40 samples and were able to plate 400 isolates from those. Some samples simply didn’t have Rhizoc on them.

“We will continue investing in research and development. We’ll be (in the affected area) in 2012 testing a lot of our old chemistries as well as our new ones to see if there’s anything that may have been missed with fungicide opportunities. We want to provide the farmers with help with this disease.”

Did the discovery of this answer questions about problems that might have occurred in previous seasons?

“We did have some eyebrows raised in a couple of fields in 2010.

“We looked at a couple of (odd situations) but those could be explained away through misapplications, equipment issues, too much rainfall immediately after a (fungicide) application and things along those lines.

“This really developed last year in five or six fields.”

What about flooding?

“That’s one of the BMPs. We’ll promote some rather non-conventional BMPs and flooding will be one of them.

Rhizoc does have the potential to move in water – but that isn’t likely to be the main avenue it will take. Plant and crop debris along with soil are the major concerns.

“For those who continue to grow rice in that area, we’ll suggest burning fields. How much that will help is yet to be determined but you’ll remove some organic matter and destroy crop debris.

“We’ll promote increased disking at the end of the growing season.

“We’ll also promote the elimination of a ratoon crop. That won’t be popular, but the practice will escalate the disease incidence long-term.

“We’ll suggest pulling levees and holding water over the fall and winter and fallow ground.

“However, I can’t stress cleaning equipment enough.”

On strobilurins and soybeans…

“This will not just impact rice.

“Rhizoctonia solanibasically develops two diseases: sheath blight in rice and aerial blight in soybeans. Aerial blight is the Number Two disease in southwest Louisiana soybeans -- second only to cercospera.

“With the (fungus) being cross-resistant to the strobilurin class, other competitor products won’t have any control. Producers will be looking for alternatives.”

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