The DMI (DeMethylation Inhibitors) or Sterol Biosynthesis Inhibiting (SBI’s) fungicides belong to FRAC code 3 which include the triazoles and imidazoles. Some of these fungicides are commonly known as Tilt (propiconazole), Rally (myclobutanil), Folicur (tebuconazole), and Procure (triflumizole) and are widely-used in agriculture.
DMI’s work by inhibiting the biosynthesis of ergosterol which is a major component of the plasma membrane of certain fungi and needed for fungal growth. Resistance by fungi to the DMI fungicides has been characterized and is generally known to be controlled by the accumulation of several independent mutations, or what is known as ‘continuous selection’ or ‘shifting’, in the fungus.
In any given field population, the sensitivity to the DMI fungicide by the fungus may range from extremely high (highly sensitive, and will be controlled by fungicide) to moderate (partially sensitive to the fungicide) or low (mostly resistant to fungicide).
This type of resistance is also known as quantitative resistance. With quantitative resistance, as stated above, there are different levels of resistance to the fungicide due to independent mutations, which, is unlike the target mutations that occur in qualitative resistance associated with the QoI (FRAC code 11) fungicides. Because different levels of resistance to the DMI fungicide may exist in the field, control of fungal population may widely vary based on the rate of the DMI fungicide being applied. It is suggested that using a higher rate of a DMI fungicide, may improve control when lower rates have failed.
For example, let’s say that a cucurbit powdery mildew population on pumpkin consists of 25% highly sensitive, 50% moderately sensitive, and a 25% low sensitive (resistant) population to a DMI fungicide. If fungicide is applied at the low rate, only 25% of the population (highly sensitive) may be controlled. Where, if the high rate was used, 75% (the 25% highly sensitive + 50% moderately sensitive) of population would have been controlled. The main point is that if low rates of DMI fungicides have been used and control seems to be weakening, increasing to a higher rate may improve control.
Unfortunately, it is difficult to determine what proportion of the powdery mildew population is sensitive or not sensitive by just looking at the field until you have begun a proper spray program. The best advice, if you are using low rates and think those rates are not working, is to increase to the high rate the next time the fungicide is sprayed, and if the high rate still doesn’t work it may be safe to assume the fungal population has grown mostly resistant. Importantly, if the high rate fails, whether you bumped up to a high rate or started with one, and control does not seem adequate, DO NOT continue to use the DMI fungicide.
Recognizing if and when fungicide chemistries are failing and when fungicide resistance is developing is critically important to producing successful crops and why scouting on a regular basis, at least before and after each fungicide application, is important. Regular scouting can help reduce unwarranted and ineffective fungicide applications and help reduce wasted costs.
Remember to always tank mix DMI fungicides with protectant (M) fungicides (i.e., chlorothalonil, manzate) to help reduce the chances for fungicide resistance developing. Always apply DMI fungicides according to label rates and resistant management recommendations and always be aware of the fungicide rate you are applying.