Louisiana Rice: More Sheath Blight – Potential Reasons

I am hearing more reports of sheath blight in rice fields than I would like. There are several reasons this could be. The most likely is the extended rain pattern we had this year. I do not remember a year this wet with as many rainfall events.

That increases disease severity over a longer period of time and increases fungicide weathering. Typically, rice fungicides last 14 to 28 days under regular conditions. Normally, that takes us through the critical grain filling period by holding down sheath blight below the upper third of the canopy.

Late season development after grain filling usually does not affect yields and quality as much as early season disease. Hopefully, this is the case and damage will not be too great.

A second possible reason is fungicide resistance in the sheath blight pathogen population. We know that strobilurin resistance is prevalent in many fields and is moving into new fields every year. This requires us to switch to an alternate mode of action – the SDHI fungicides Sercadis and Elegia.

These two fungicides are good against sheath blight but tend to wear off faster than azoxystrobin (Quadris and Quilt). Also, you may be using a strobilurin fungicide and not know you have resistance in that field.

Resistance to the SDHI fungicides may also occur, based on recent field experience, and that could be causing some of this sheath blight development. Hopefully, we will have Amistar Top next year, which has a component with a different mode of action and has activity against the wild type, strobilurin, and SDHI resistant strains.

A third possible problem is application technology. With all of the rain we had this year, there were limited windows to correctly apply fungicide to our fields. From fungicide timing trials conducted at the Rice Research Station, the time of day did not affect fungicide performance. Applications in these comparisons were made at 8 a.m., 10 a.m. and noon, and then 2 p.m., 4 p.m. and 6 p.m.

However, rain just before and right after application did reduce fungicide performance. Fungicides need 2 to 4 hours of drying to become resistant to weathering. Also, large water droplets on the plant after a rain allowed the fungicide to roll off the leaves and into the flood water.

In several fields, sheath blight was rampant in areas near power lines, buildings and tree lines where planes had to pull up to avoid obstructions. With all of the difficulties, I think our aerial applicators do a great job of taking care of our fields.

A fourth possible factor is varietal susceptibility. Unfortunately, a large portion of our current varieties are very susceptible to sheath blight. We have been able to grow these varieties because of the availability of effective fungicides and tolerance to sheath blight in newer varieties.

Tolerance is expressed as the ability of a crop to yield even in the presence of disease. Older very susceptible varieties like Lemont and LaBelle could lose 25% to 50% of yield to sheath blight. But newer varieties with the same rating only lose 14% to 17% of yield under severe disease pressure.

That is still too much and we need to use fungicides, but the risk is not as great as it once was. I have also heard from several consultants and farmers that CL153 appears to have more sheath blight than other similar susceptible varieties. In my inoculated sheath blight disease nurseries, CL153 rated 7.3 and CL111 rated 7.5 on a zero to 9 severity scale, and those ratings are not significantly different.

However, in one of my off-station trials in a farmer’s field of CL153, I noticed more initial sheath blight lesions than I had seen before. I also noticed that the CL153 had tillered very well and had a thick canopy early in the season. This, I believe, allowed the sheath blight to infect earlier and spread more effectively in the lower canopy.

Thirty some years ago, I was with Bob Verret, the Acadia Parish agent at that time, and we were visiting with an older rice farmer who we had just checked his field for disease. He said that he always made more rice in the years when he pumped more. That statement stuck with me, since almost all of our diseases need moisture to develop and every dry year we had problems with disease developing in our nurseries and trials.

Although diseases are more severe during years with numerous rains, rains can also cause problems with pollination, seed filling, shattering, nutrient loss, etc.

Let’s hope for a better year next year with fewer problems.

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