Fungicides: Tools for vegetable disease management

Fungicides are tools that, when combined with other tactics, can minimize vegetable diseases.

Preventative management tactics are the best approach to manage diseases, and those tactics fall within different principles of plant disease management. (To learn more, read “Disease management tactics for vegetables” from Michigan State University Extension.) Plant protection with fungicides is one of the most practiced tactics for disease management.

Chemical protection

Fungicides and bactericides can kill the pathogen or inhibit the infection process. Preventative fungicide applications protect the plant from initial infection while curative applications help slow down the disease development, but are unable to “cure” the plant. When selecting products to spray, one must considered the following factors.

Crop variety. Planting resistant varieties can reduce the number of pesticide applications, although the use of host resistance may be compromised if there are multiple pathogens present. On the contrary, planting susceptible varieties may warrant more frequent fungicide application for continuous tissue protection.

Pathogen detection and forecast. Spray decisions and intervals must be adjusted based on the pathogen detection in the field or in neighboring states. This is especially important for airborne pathogens like Phytophthora infestans (tomato late blight and potato late blight) or Pseudoperonospora cubensis (Cucurbit downy mildew). Consult the Michigan Late Blight Risk Monitoring forecast, the USAblight national forecast and the Cucurbit Downy Mildew National Forecast during the season to guide your fungicide decisions.

Type of fungicide. Fungicide mobility varies. For example, contact fungicides do not move into plant tissues and remain at the site of application unless moved by excessive precipitation or irrigation events. Mobile fungicides can penetrate plant tissues, moving within the plant locally (locally systemic or translaminar) or systemically (true systemic-amphimobile and xylem mobile-acropetal). Some examples can be found in Table 1 of the APS topics in plant pathology, “What are Fungicides?

Weather. Precipitation, wind, amount of UV radiation (sunny versus cloudy days) and humidity need to be considered when planning a pesticide application for disease control. If intense or prolonged precipitation is forecast, select mobile fungicides. Contact fungicides are more prone to be washed off by precipitation than systemic ones. Light winds of 2-6 mph can assist with the movement of the spray within the crop, while stronger winds greater than 10 mph may dissipate the product, missing the intended area and increasing the risk of drift. Drift can also occur during temperature inversions.

Determine the rate, timing and correct application. Proper equipment calibration, nozzle selection, boom pressure and water volume are critical to deliver the intended rate of fungicide to the plant tissue. Always read and follow the product label and recommended rates for optimum results. Avoid conditions that may result in increased risk of phytotoxicity. Similar consideration were discussed in Annemiek Schilder’s article, “How to get the most out of your fungicide sprays,” and are important for vegetable crops as well.

Choose the right sequence of products for your spray program. Rotation or alternation of fungicide with different Fungicide Resistance Action Committee (FRAC) groups is critical to delay the development of resistance in pathogen populations. FRAC groups can be found on fungicide labels and were developed to help you identify products that attack fungi or watermolds in different ways. For example, the QoI fungicides (quinone outside inhibitors), also known as strobilurins, disrupt fungal respiration and are grouped in FRAC 11. This is only one example of the various FRAC groups that represent a single mode of action and, therefore, have a high risk of resistance development in pathogen populations.

For current labeled products by crop and disease, consult the MSU Extension bulletin E312, “2015 Insect, Disease and Nematode Control for Commercial Vegetables,” available as hard copy or downloadable PDF at the MSU Extension Bookstore. To learn how to use the MSU Extension bookstore, see “MSU Extension publications that help minimize yield loss from diseases and other pests.”

Learn more about integrated pest management (IPM) at the Online IPM Academy website. Among the webinars MSU Extension is offering, the webinar tilted “Tactics for vegetable disease management” is now available.

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