Winter is a good time to reflect on what happened during the last cropping season and to think about how to improve pest control next year. For homeowners, poor pest control can be attributed to lack of efficacy of available products. Also, improper timing of sprays, poor application technique, or high pH of spray water results in ineffective pest control. Another increasingly important factor in poor pesticide performance is the development of pest resistance when the same product is used repeatedly during the growing season and/or in successive years. Diseases, insects, mites, and weeds can potentially become resistant to a single or multiple pesticides.
Pests of any kind differ genetically among the population. When a pest varies on a plant or in a field, it is called a biotype. The genetic difference within a pest species allows some biotypes to survive a pesticide application. As the surviving pest reproduces, its offspring also carry the pesticide-resistant gene(s), allowing these biotypes to increase within the population, resulting in pest resistance with repeated use of the same product or one with the same mode of action.
Pesticides are grouped by their mode of action. Thus, products in the same group kill the target pest in the same way. When a pest becomes resistant to two pesticides in the same group, it is called cross-resistance. In contrast, multiple resistance occurs when a pest develops resistance to pesticides in two or more different groups.
Fungicides are classified by their mode of action, using a Fungicide Resistance Action Committee (FRAC) code. For example, captan has multi-site contact activity against pathogens and has a FRAC code M4. This group of fungicides is considered a low risk group for developing resistant fungi. In contrast, thiophanate-methyl (Topsin-M®) has a high risk for developing fungal resistance and has a FRAC code 1. Three commonly-used classes of fungicides at high risk for developing pathogen resistance on fruit crops are the sterol-inhibitors (Indar, Inspire Super, Procure, and Rally products, FRAC code 3), the strobilurins (Cabrio, Flint, Pristine, Sovran products, FRAC code 11), and the succinate dehydrogenase inhibitors (Luna Tranquility, Luna Sensation products, FRAC codes 7 and 9). A complete listing of FRAC codes can be found at: http://ipm.ifas.ufl.edu/resources/success_stories/T&PGuide/pdfs/Appendices/Appendix6-FRAC.pdf. Resistance has been reported for apple scab and powdery mildew, using sterol-inhibitor and strobilurin fungicides in the Midwest. Streptomycin resistance occurs in Missouri when it is overused to control fire blight on apple trees (Figure 1). Botrytis bunch rot and downy mildew resistance has also been reported when the same groups of fungicides are overused.
Insecticide Resistance Action Committee (IRAC) codes are used to identify the insecticide groups with similar modes of action and their potential for becoming resistant to arthropods. Several states in the Midwest have reported codling moth resistance to organophosphate insecticides (Imidan, Diazinon, IRAC code 1B). IRAC codes can be found at: http://www.irac-online.org/documents/moa-classification/?ext=pdf. Insects become resistant to a pesticide by loss of sensitivity to poisoning, changes in their anatomy, behavior, or more rapid metabolism, resulting in insecticide detoxification.
Weeds can also become resistant to herbicides, especially since there are few products registered for use in fruit plantings. More than 250 herbicide-resistant weed species have been identified globally. In Missouri, 14 weed species have been reported in agronomic field crops, but none have been studied in fruit plantings. Worldwide, weed biotypes have developed resistance to 23 of the 26 known herbicide sites of action. A listing of Herbicide Resistance Action Committee (HRAC) groups can be found at: http://weedscience.com/Documents/ShowDocuments.aspx?DocumentID=1193. In orchards where multiple annual applications of glyphosate are used to control weeds in tree rows, there is a potential for developing herbicide-resistant weed species.
To manage pesticide resistance, plant disease-resistant apple cultivars when possible. 'Liberty' and 'Enterprise' apples have good resistance to fire blight, apple scab, powdery mildew, and cedar apple rust. Use recommended cultural practices, such as removing infected prunings from plantings, as well as dropped fruit from the orchard floor to limit disease pressure. Codling moth mating disruption, using pheromones, insect trapping, or the introduction of natural insect predators are alternative management strategies. When using pesticides, scout for pests to determine when pesticides are needed. Try to use a low dosage of the pesticide rate listed on the label whenever possible. Do not use more than two sequential applications of a pesticide with the same FRAC, IRAC, or HRAC code. Rotate the use products with different pesticide groups during the growing season and from year to year. Also, using mixtures of products that combine two or more codes may also delay the development of resistant pests.
REVISED: February 21, 2017