8 Common Questions on Rice Sheath Blight Management

by Yeshi Wamishe, Extension Plant Pathologist - July 19, 2021

1. It has been difficult to control weeds in part of my rice field and sheath blight looks bad in these areas. If I use a fungicide early around the green ring, do I need to treat it again?

The sheath blight fungus has several weed hosts and, therefore, weedy susceptible rice may suffer severe sheath blight wherever weeds add to the canopy density. Fungicide application twice just for sheath blight is not profitable. It may be wise to partially treat the field. 

Usually delaying your fungicide application to pair it to the kernel and false smut fungicide timing can cut application costs. 

2. I have sheath blight more in the upper end of my row rice field than the lower end of the field. My variety is medium short and sheath blight is at the threshold. What do you recommend?

Check the growth stage and how far the disease has progressed to the height of the plant. Check how dense the crop is and the weather favorability for the pathogen. If the disease height is halfway or more than halfway to the height of the crop and the weather is favorable, you better suppress the disease from progressing. Optimum fungicide timing for sheath blight is at boot. However, if the disease is fast progressing, it can be applied as early as at panicle differentiation. Remember: the upper three leaves at any stage of the crop should not be threatened by the disease. 

3. I have an azoxystrobin fungicide with a different name (AFrame). Can I use it as Quadris?

There are several fungicides with different names containing azoxystrobin as their active ingredient (AI). Make sure to check the label and compare active ingredients. As long as the AIs are similar it should be okay. However, read labels carefully. Labels are the rule and make sure if the fungicide is labeled for rice in Arkansas. 

4. It is a large field but not uniformly planted due to mishaps with the planter. Some spots in the field have a thicker canopy than others. I see sheath blight moving actively in the thicker spots. Should I spray for sheath blight? 

In such a field it may be difficult to know the threshold level. Therefore, it is your judgment call. Historically, controlling sheath blight benefits in fields with high yield potential. This field may not be a good candidate for treatment to benefit from the fungicide application. 

5. If sheath blight appears at the green ring, moving fast, and reached threshold level, how many times do I need to spray for sheath blight in a season to minimize yield damage? 

If sheath blight is this aggressive at the green ring before canopy closure, it is very likely that the field has been excessively fertilized with Nitrogen fertilizers, or the crop may be suffering from severe potassium deficiency. These factors need to be addressed over time in the future. Although, spraying fungicides for she more than once for sheath blight alone is not economical, in this case, an application of Quadris would be warranted. However, you need to scout at booting to see if the disease gets active again. 

If the yield potential appears promising but sheath blight is active again try to pair your fungicide application with other diseases or apply a lower rate again. The question to be answered is why sheath blight is active so abnormally early in this field though.

6. If I plan for just a one-time application for sheath blight control on a susceptible variety, what is the optimum developmental stage I should spray the fungicide?

Typically, a fungicide for sheath blight would be applied from 5-20 days after mid-season (1/2 inch IE) to boot stage.

7. Remind me, what is the length of suppression of rice sheath blight using Azoxystrobin fungicides?

Previous research has shown that the highest dose at 12, 5 fl oz/acre rate suppresses the disease up to 4 weeks. If lowered to 8 or 10 fl oz/acre rate, its suppression would be lowered to two and three weeks, respectively. 

8. Is fungicide resistance a concern in Arkansas rice for sheath blight? 

It remains a concern not only for sheath blight but for other diseases as well. Most rice fungicides contain Azoxystrobin and this active ingredient is heavily used in rice and other crops. The insensitivity of the sheath blight fungus has been increasing in Louisiana rice fields. To reduce such a concern, we need to use fungicides for the targeted purpose only. Using a full rate and rotating fungicides of different modes of action are recommended.