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Fungicide resistance: an ongoing concern
Author: Yeshi Wamishe, Extension Rice Plant Pathologist


By Yeshi Wamishe and Travis Faske, Extension Plant Pathologists


During the 2011 cropping season strobilurin-resistant isolates of Rhizoctonia solon, the causal agent of sheath blight, were confirmed in a few rice fields in Louisiana. This is the same pathogen that causes aerial blight of soybean.

This was an “alert” call for Arkansas rice and soybean producers to be vigilant in their fungicide use.  Now, only four years later, fungal isolates were identified that are insensitive to fluxapyroxad (Sercadis), an SDHI fungicide, in spots within the rice field that did not suppress sheath blight in Louisiana.

These new strains were confirmed to have cross-resistance with flutolanil (Elegia) another SDHI fungicide.  Fluxapyroxad is one of the active ingredients in Priaxor, which is used on soybean in Arkansas.

Currently, no isolate of Rhizoctonia solani have been reported to be resistant to either fungicide in Arkansas. Growers and consultants should be aware of these findings and continue to utilize practices that minimize the selection of fungicide-resistant pathogens that affect rice or soybean.

To read more on the situation in Louisiana go here.

Fungicide resistance is the reduction or loss in fungicide efficacy to a fungal pathogen. It is generally accepted that fungicide resistance occurs when reduced rates of a fungicide are used or when fungicides are applied in the absence of disease. Thus, the fungal population is consciously exposed to a fungicide and isolates that may be less sensitive are better suited to survive and infect the host crop.

This population increases over time due to repeated use of the same fungicide chemistry (see figure below) until a field failure is observed, which is what has happened with the sheath blight pathogen for two fungicide chemistries in Louisiana.

Managing fungicide resistance is critical to disease control as soybean producers have experienced with strobilurin-resistant frogeye leaf spot. In some situations, another fungicide chemistry (i.e FRAC code) can be used, but dependency and pressure on a single chemistry is risky and often short lived, which was the case in Louisiana.

Currently, Arkansas producers are relying on one chemistry to control frogeye leaf spot but still have options to control sheath blight of rice. Managing fungicide usage to prevent fungicide resistance is much easier than trying to manage resistance after it has developed.

Fungicides with a single target site (i.e strobilurin) are at a greater risk to develop resistance than the others, but prolonged use of a single mode of action with any fungicide chemistry increases the risk of fungicide resistance. Therefore, we need to implement resistance management strategies starting when the fungicides become available for commercial use.


Utilize host plant resistance

Rice and soybean cultivars with resistance to sheath blight or frogeye leaf spot are the most economical and practical methods to manage fungal diseases.

Follow the Label

Follow directions on the label for specific resistance management guidelines on tank mixing, rate, and total amount to be applied per season.

Apply in the presence of the Disease if the disease does not require protective fungicide

Utilize fungicides to control disease that has been observed in the field.

Do Not Overspray

Do not use fungicides as a rescue treatment. Applying high rates after the disease reaches a high disease severity increases the chances for the development of resistant populations of plant-pathogenic fungi.

Rotate Different Fungicide Modes of Action, If Available

Fungicides with the same mode of action or FRAC group (i.e. FRAC 11) target the same genes in the fungus, thus it is highly probable that pathogen populations that are resistant to one mode of action will be resistant to other within that same classification.  For example rotating from Quadris to Headline is NOT rotating chemistries as they are both strobilurin fungicides (i.e. FRAC Code 11).  Such a situation is referred as cross resistance.

Utilize Crop rotation

Because Rhizoctonia solani can reproduce to high population densities on soybean and rice, rotating with other less susceptible crops like corn or grain sorghum can be beneficial to delay the initial severity of disease for the subsequent crop.

Additional Tips that may help manage Fungicide Resistance

  1.  Utilize good agronomic practices for crop development
  2. Keep updated on the current resistance issues
  3. Scout and report situations where a fungicide failure was observed that is possibly linked to fungicide resistance

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