In this, the final article in our series on making the most of your fungicide dollars, I am considering fungicide resistance management. There are some key topics that should be addressed, and I am covering them by answering questions. Obviously, if your fungicide fails due to resistance, your dollars are wasted.
Which pathogens become resistant the quickest?
The pathogens that become resistant to fungicides most quickly are those that reproduce quickly and in high numbers. These include powdery mildew fungi (e.g. Oidium), downy mildew fungi (e.g. Peronospora), Botrytis, Pythium, Phytophthora and bacteria (e.g. Pseudomonas, Xanthomonas, Erwinia, etc.). It is interesting that although bacteria develop resistance to copper quite readily, fungi don’t seem to. The FRAC (Fungicide Resistance Action Committee) actually has copper listed as low risk of resistance development.
Which fungicides are most likely to have resistance concerns?
In this case, the more narrow the mode of action the more likely the target fungi can develop resistance to that fungicide. Fungicides like chlorothalonil and mancozeb actually have multiple sites where they attack the fungus, making them less likely to have resistance problems.
In contrast, some of the newest fungicides have very narrow, specific modes of action, making resistance development more likely. That has led to very strict instructions on their labels concerning the number of times they may be used on a crop and/or if they must be tank mixed.
How can you reduce the potential for resistance development?
The most important method is to do everything in your power to reduce disease, including using alternative methods–practice integrated pest management (IPM). If you do nothing but spray fungicides, you are placing too much strain on them and they are more likely to fail. Even the best fungicides work poorly when cultural and environmental controls of IPM are ignored. Don’t wait, be preventative when necessary.
Downy mildew and some bacterial diseases are not easy to control if preventative applications are not made. Use products according to their labels–the fungicide manufacturers really do know more about how to use their products than anyone else does. Learn more about important diseases of your crops so you can find their Achilles heel.
Botrytis can be controlled without any fungicides if humidity and temperature are firmly under your control. Some diseases must be present on the seed, cutting or liner because they do not blow in with the wind. Finally, rotate or tank mix products.
Is tank mixing or rotation best?
Most studies show both methods work for resistance management, so the preference is yours. If you make a tank mix with two products for the same disease, then you are doing so to manage resistance. If, however, you combine products with different spectrums, you are simply using a shotgun approach. This gives you some assurance you will control an undiagnosed situation, or one that is caused by more than a single pathogen.
I find many growers like to use tank mixes. They think they can reduce costs by using lower rates of the fungicides if they mix them. Sometimes this is true, but not always. You can reduce the rate of fungicides in a tank mix when you are treating preventatively, disease pressure is low, both products work on the target fungus and you are sure you do not have resistance to either product.
You should not go below labeled rates in most circumstances. Do not reduce rates when you do not know the cause of the disease, more than one disease is present, the fungicides have a narrow or similar mode of action or the product is new to you. Pre-mixes can make the entire question moot.