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2022 Midwest Vegetable Guide

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Page 47 of 291

Disease Management Strategies 48 Midwest Veg Guide 2022 Chemical Control and Resistance Management with Fungicides and Bactericides Disease control products (fungicides, bactericides, and nematicides) are pesticides. Be sure to read the general section about Pesticide Application and Safety for information about safety, equipment, calibration, formulations, storage, and other important topics. For a disease control material to be effective, you must apply an appropriate material at the right time, in the right concentration, and in the right way. Fungicides can be broadly classified as either contact or systemic. Within those groups, however, there are many active ingredients and multiple modes-of-action. Some fungicides are allowed in organic vegetable production, and some are designated as reduced-risk, including certain biopesticides/biological controls. Both contact and systemic fungicides are most effective if you apply them before disease develops. Many diseases are very difficult or impossible to control with chemicals once a severe epidemic is underway. Throughout the crop-specific sections of this book there are details about when you should apply fungicides in order to be most effective (and information about when they may be ineffective). Pathogens usually require a specific temperature and moisture range in order to cause diseases. For some diseases, knowing those specific requirements can help you time fungicide applications to coincide with disease risk. In some cases, the guidelines are informal, and may simply make you more aware that a wet season may require more applications and a dry season may require fewer. In other cases, the pathogen life cycle is understood well enough that you can use a formal disease forecasting system. Here are some resources for tracking weather and predictive models related to diseases: • • • Contact fungicides (also called protectant fungicides) provide a "coat" of protection on the plant's surface when applied properly. These fungicides are designed to kill fungi on- contact on the surface of plants — hence, the name. Most contact fungicides have multiple modes of action, so fungal pathogens are unlikely to develop resistance to all of these different modes of action at the same time. For this reason, alternating contact fungicides is unnecessary. Good coverage is essential to maximize the efficacy of contact materials. Systemic fungicides (sometimes called eradicant or curative fungicides) don't merely coat the surface, they also enter the plant. Once inside the plant, some stay relatively localized. Others move across to the opposite leaf surface, some move upward in the plants, and a few move downward into the roots. They can sometimes eradicate or cure a portion of existing infections. Most systemic fungicides have a single mode of action, so the risk of pathogens developing resistance to these products is greater. The fungicides target a very specific function of the pathogen's cells, and sometimes the fungus develops methods to evade the activity. Powdery mildews and downy mildews are particularly prone to resistance development, but there are other examples. The crop sections in this guide flag diseases that are at risk for fungicide resistance. Always read and follow label directions that list how to alternate systemic fungicides and/or combine and rotate with contact fungicides to minimize the resistance development. The Fungicide Table lists several fungicides and their modes of action to help in resistance management. Each fungicide label is marked with a FRAC (Fungicide Resistance Action Committee) code to designate a mode of action group and help growers design a rotation plan. Try to avoid using products with the same FRAC codes repeatedly to conserve their efficacy against diseases. Bactericides (copper and antibiotic compounds) can help reduce the risk of early-season bacterial disease epidemics, but are most effective when used with other control methods. Copper compounds also are mediocre fungicides and are handled similar to protectant fungicides. Antibiotics serve a similar purpose in certain crops.

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