With frequent rains and cooler conditions common in the spring, it can be difficult to maintain sufficient apple scab fungicide protection for the rapidly expanding foliage under such conditions.

Refer to Table 3–13. Characteristics of apple scab fungicides in the new 2014–2015 Publication 360, Guide to Fruit Production for details on pre- and post-infection activity, retention and redistribution properties of registered scab fungicides.

Here are some important notes to remember for apple scab management in wet conditions:

  • If rain is predicted, apply a fungicide with very good protectant activity, such as Manzate, Dithane, Penncozeb, Polyram, Maestro, Supra Captan, or Folpan just prior to rainfall, particularly to susceptible cultivars and high inoculum blocks. These products will also provide 18-24 hours of post-infection activity, possibly more in lower temperatures.
  • With ½ to 1 inch of rain, effectiveness of protectant fungicides can be drastically reduced and redistribution may be impaired.
  • In prolonged wetting events, it is better to re-apply fungicides during a break in the rain to provide temporary protection, than to not spray at all, even if conditions are still wet. A product with good retention properties, such as Dithane, Manzate and Penncozeb will withstand heavier rains.
  • The length of time required for infection depends upon the temperature during the wetting period. Refer to Table 3–12. Relationship of temperature and moisture of apple scab infection in Publication 360 to determine whether an infection period has occurred. Add together wet periods caused by intermittent rain, unless these periods were separated by more than 10 hours of dry, sunny weather.
  • Apply a fungicide again once conditions are finally dry to replace residues washed off by rain.
  • Fungicides with good or very good post-infection activity, such as Vangard, Scala and Syllit may be used if protectant fungicides have been washed off or for kick-back activity. These products will provide post-infection activity of 48 hours in warm temperatures to 72 hours in cooler temperatures, from the start of the infection period. Do not use these products after tight cluster and be sure to tank-mix with a protectant fungicide for resistance management.
  • Early season, foliage is rapidly growing. Be sure to keep new tissue covered by maintaining tight spray intervals.
  • During wetting events, don't worry about powdery mildew protection. Rain washes away powdery mildew spores. Return to using fungicides with mildew activity once things dry up again.
  • Strong winds will cause spray to drift away from the intended target. If possible, spray at night or early morning or lower spray trajectory into the wind to minimize drift. However, take extra care to ensure good spray coverage to the top of the tree.

For more information on early season fungicide considerations, see the latest Orchard Network Newsletter (Volume 18, Issue 2).