By Yeshi Wamishe, Extension Rice Pathologist
When recommended fungicides fail to suppress or protect a crop from certain diseases, the following questions may be asked:
- Is the fungicide the correct or recommended type?
- Is the timing of application right?
- Is the rate correct?
- Was the frequency of application as recommended?
- Was it tank- mixed? If so, were the chemistries compatible?
- Were you sure if the field has been truly sprayed?
- Was the fungicide applied by air?
- Have you grown this variety and used this fungicide before?
- Is it a new race or variant of the pathogen?
- Did you use 10 or at least 5 gallons per acre rate of water for coverage?
Often the answers to the first 8 questions are ‘yes’, for #9 ‘maybe’ because is not easy to know. If the answer for #10 is ‘no’, this alone can be the cause for not getting the expected results.
Fungicides for blast and sheath blight of rice are QoI (strobilurin) fungicides listed on Table 1. It is generally very important for the fungicides to touch the surfaces of the crop tissues to enhance mobility. Read more on the advantage of using adequate amount of water volume when using of QoI fungicides on StrobilurinFungicides under the sub- title Mobility and under the subtitle Volume at sprayers water volume.
Moreover QoI fungicides work better as preventative than curative for blast protection. For sheath blight, these fungicides suppress the disease but they do not cure the tissues that have already been damaged.
Leaf blast is usually managed by maintaining water depth to at least 4 inches. We ran simple tests in the greenhouse to determine the efficacy of QoI fungicides when applied 48 hours before and after inoculation i.e., as preventative or curative. In agreement with literature and our recommendation, blast was suppressed significantly when applied before inoculation (Figure 1, 2, and 3). Read more on StrobilurinFungicides in the last paragraph under sub-title Mobility. Fungicide resistance is not new. Resistance to the QoI (strobilurin) fungicides is already known in several pathogens of different crops including on sheath blight in rice. However, there is no hard evidence of resistance to rice diseases in Arkansas. To read more and be mindful of fungicide resistance read StrobilurinFungicides under the sub-titles Resistance and Guidelines to Reduce Resistance.