There are a number of factors that go into scheduling foliar fungicide applications. For example, choosing a product with your crop and suspected disease on the label is a must. You will want to select a product with good efficacy. It is important to choose a product with a Pre-Harvest Interval (PHI) and Re-Entry Interval (REI) interval that fits your harvest schedule and fieldwork. It is also important to select foliar fungicides in such a way as to avoid fungicide resistance.
Fungicide resistance is when a fungicide does not work as well as it had in the past because the fungi which cause disease have adapted to the fungicide. Fungicide resistance is also known as fungicide insensitivity because the target disease-causing fungi are not sensitive to fungicide any longer.
If a foliar fungicide is used repeatedly to control a specific disease, it is possible that the fungi which cause the disease would adapt by mutation so that the fungicide would not inhibit the fungi anymore.
Fungicides all have a FRAC code (FRAC=Fungicide Resistance Action Committee). The FRAC code indicates the mode of action-how the fungicide works to inhibit the fungi. Most systemic fungicides, products that move within a plant, have a FRAC number. Most systemic fungicides are relatively likely to become fungicide resistant since they only have a single mode of action, represented by the single number. For example, Quadris® has the FRAC code 11. To avoid causing fungicide resistance, after the application of Quadris® to manage early blight of tomato, one would choose a fungicide with a FRAC code other than 11. Since Cabrio® also has a FRAC code of 11, one would avoid choosing this product because Cabrio® has the same mode of action as Quadris®. One could choose Inspire Super® which has two active ingredients and FRAC codes of 3 and 9. After the application of Inspire Super®, one would want to avoid products with FRAC codes of 3 and/or 9. It would be OK to return to Quardis® after the application of Inspire Super®.
Some fungicides do not move in the plant. They work instead by covering the outside of the plant with fungicide. These products work by inhibiting the fungi they contact on the surface of the plant. Thus, these products are called contact products as opposed to systemic products that move within a plant.
Many contact products have FRAC codes of M since these fungicides have multiple sites of action. Contact products with FRAC codes of M are very unlikely to cause fungicide resistance since they have multiple sites of action. For example, chlorothalonil products are listed as a FRAC M05. Fungicides with chlorothalonil as an active ingredient have been available since the 1960s and no evidence of fungicide resistance linked to this fungicide have been observed. Therefore, chlorothalonil products such as Agronil®, Bravo®, Echo® or Equus® can be used repeatedly without alternation. Or chlorothalonil products can be used in alternation with systemic products.
Another contact fungicide that may be used without alternation are those with the active ingredient mancozeb such as Dithane®, Mancozeb® and Penncozeb®. These products have the FRAC code M03 and may be used repeatedly.
Products with copper compounds as an active ingredient such as copper hydroxide or copper sulfate usually have FRAC codes of M01. However, strains of the bacterium that cause bacterial spot of tomato in Indiana may have resistance to copper. In general, copper products are usually better for bacterial diseases than fungal diseases.
Most fungicide labels have information about fungicide resistance. Some labels will specify that the product may be used 2 or 3 times before alternating with another FRAC code. My recommendation is to alternate to a different FRAC code after each application, but of course you may follow the instructions on the label.
Please feel free to contact me about selecting fungicides to avoid fungicide resistance.