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When is the Best Time to Apply Fungicides for Foliar Diseases?

Updated August 2020.

Here is a printer-friendly .pdf version of this factsheet.

Apply in the following conditions:

  1. Light (3 to 5 mph) or no wind to achieve best coverage and minimize drift potential. Light wind is best when there is a temperature inversion (colder air is near the ground and warmer air is above, which is characterized by stagnant air and is likely to occur under high pressure conditions). Spraying in light wind during a temperature inversion reduces the possibility of fine spray droplets getting trapped in the inversion, rising, and drifting away. Midday is the best time to spray during a temperature inversion as the conditions are less pronounced at that time. For more information, see the North Dakota State University Extension Service’s Temperature Inversions presentation.
  2. Dry leaves, or when only light dew is present (this is okay when gallonage is not high). Dew can enable runoff.
  3. Leaves expected to stay dry (with either no rain or heavy dew) through the rainfast period. Most products will need at least 4 hours. The required time is often specified on the label.
  4. With systemics or penetrants, apply when the conditions will promote slow drying to maximize uptake, such as calm, cloudy days and late-afternoon to dusk. Stomates are more likely to be open under these conditions, which should enhance uptake.
  5. Before (as opposed to after) conditions are forecast to favor disease development. Most fungal and bacterial pathogens require leaves to be wet for several hours for infection to occur. Some fungi can infect when relative humidity is at or above 90%. Powdery mildew fungi are an exception and can infect in dry conditions. Rain is actually unfavorable for these pathogens.
  6. Inactive bees. While most fungicides are not toxic to bees, minimizing direct exposure and disturbance is desirable. Neem oil is an exception and is toxic to bees.

Additional practices to maximize control:

  1. Ensure the sprayer in use is providing good coverage. In spring, check its calibration and replace worn nozzles. All nozzles should deliver the same amount with less than 10% variation. Consider new nozzle types that might improve coverage, such as twin-jets. Use a gallonage that covers leaves well without runoff. Use adequate pressure, at least 60 psi, to obtain small droplets as they provide better coverage than larger droplets. Avoid extremely high pressure, as tiny droplets are prone to drift. Use water-sensitive paper to check crop coverage. Obtaining spray deposition on the underside of leaves is especially important with diseases like powdery mildew which easily develop there and are especially difficult to reach with large leaves like pumpkin.
  2. Start applications prior to or when symptoms first develop, and are at a low severity. Fungicides have little to no curative activity, and curative use can promote development of fungicide resistance.
  3. Select fungicides that are labeled for the targeted disease and demonstrated to be effective. Obtain current information on efficacy and resistance. Most selective (single site mode of action) fungicides are at risk for resistance development. Consequently, a product may no longer be as effective as it once was.
  4. Check calculations to ensure the intended rate will be applied.
  5. Include an adjuvant when indicated on a label. There are many factors to consider when choosing an adjuvant, including whether the fungicide must:
    • adhere to the leaf (contact products like copper)
    • penetrate the leaf (systemic and translaminar products)
    • volatilize to spread to other areas (especially the underside of leaves)
  6. Check the spray pH. Use buffers if it is not between 6 and 7. Most pesticides are formulated to work best at a pH near 7. Many formulations include buffers.

More information/prepared by:

Margaret Tuttle McGrath
Associate Professor
Long Island Horticultural Research and Extension Center (LIHREC)
Plant Pathology and Plant-Microbe Biology Section
School of Integrative Plant Science
College of Agriculture and Life Sciences
Cornell University
mtm3@cornell.edu

© 2020 Cornell University. This web resource is designed to enhance access to Cornell's vegetable production resources. This site is a project of the Cornell Vegetable Program Work Team (PWT). Visit the About section for more information on the team. Comments or questions? Email Craig Cramer, Communication Specialist, School of Integrative Plant Science. Some of the informational links provided are not maintained by, nor are the responsibility of, Cornell Cooperative Extension and Cornell University. Mention of commercial products and trade names is for educational and informational purposes only. Manufacturers' instructions change. Read the manufacturers' instructions on the pesticide label carefully before use. Inclusion of information is not intended as an endorsement by Cornell Cooperative Extension or Cornell University, nor is discrimination of excluded information implied. Cornell University is an equal opportunity, affirmative action educator and employer. If you have a disability and are having trouble accessing information on this website or need materials in an alternate format, contact web-accessibility@cornell.edu for assistance.
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