Andy Wyenandt, Assistant Extension Specialist in Vegetable Pathology, Rutgers University; wyenandt@aesop.rutgers.edu
The DMI (DeMethylation Inhibitors) or Sterolbiosynthesis Inhibiting (SI) fungicides belong to FRAC code 3 which include the triazoles and imidazoles. Some of these fungicides are commonly known as Folicur (tebuconazole), Tilt (propiconazole), Rally (myclobutanil) and Procure (triflumizole).
SIs work by inhibiting the biosynthesis of ergosterol, which is a major component of the plasma membrane of certain fungi and is needed for fungal growth. Resistance by fungi to the DMI fungicides (FRAC code 3) 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. Hence, in any given field population the sensitivity to the DMI fungicide by the fungus may range from extremely high (highly sensitive, i.e. will be controlled by fungicide) to moderate (partially sensitive) or low (mostly resistant to fungicide). This type of resistance is also known as quantitative resistance. With quantitative resistance 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 fungicides (FRAC code 11).
Because different levels of resistance to the FRAC code 3 fungicides may exist in the field, the fungal population may behave differently to different rates of the SI fungicide being applied. Therefore, it is suggested that using a higher rate of a FRAC code 3 fungicide, may improve control when lower rates have failed. For example, let’s say that a powdery mildew population on pumpkin has 25% high, 50% moderate, and 25% low sensitivity to a DMI fungicide. If fungicide is applied at the low rate, only 25% of the population (highly sensitive) may be controlled. Whereas, if the high rate was used, 75% of the population may have been controlled. The main point here is that if low rates of FRAC code 3 fungicides have been used and control seems to be weakening, bumping 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 looking at the field until you have begun spraying. The best advice, if you are using low rates and think those rates are not working like you feel they should, the rate should be bumped up to the high rate the next time the fungicide is sprayed, and if the high rate 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 fungicide. Recognizing if and when fungicide chemistries are failing and when fungicide resistance is developing is critical 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 FRAC code 3 fungicides with protectant (M) fungicides (i.e. chlorothalonil, mancozeb) to help reduce the chances for fungicide resistance developing. Always apply FRAC code 3 fungicides according to label rates and resistantce management recommendations and always be aware of the fungicide rates you are applying.