Bob Mulrooney, Extension Plant Pathologist; bobmul@udel.edu
The following article by Andy Wyenandt, Ph.D., Specialist in Vegetable Pathology at Rutgers University is particularly timely since many vegetable growers are spraying fungicides to prevent many foliar diseases at this time of year especially on vine crops where DMI or SI fungicides are used very frequently.
The DMI (DeMethylation Inhibitors) or Sterol biosynthesis Inhibiting (SI’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). SIs 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 SI 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. Such that, in any given field population the sensitivity to the SI 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 SI fungicide may exist in the field, the fungal population may behave differently to different rates of the SI fungicide being applied. If that is the case, it is suggested that using a higher rate of a SI 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 SI 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 population may have been controlled.
The main point is that if low rates of SI 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 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 SI fungicides with protectant (M) fungicides (i.e., chlorothalonil) to help reduce the chances for fungicide resistance developing. Always apply SI fungicides according to label rates and resistant management recommendations and always be aware of the fungicide rates you are applying.