Apple Alert: Rain Rain Go Away……

— Written By

…..the apple growers need to spray.

Image of flooded fieldSince last Tuesday (May 15) some orchards in Henderson County, NC have received over 7 inches of rain. Great news if you’re a pathogen, not so much if you’re an apple grower. Many of us were taught that captan needs to be reapplied when greater than 2 inches of rain has occurred. A 2008 study by Xu et al., has found that any rainfall greater than 1 mm (0.04 inches) can reduce captan residues by an average of 50%!

General Guidelines for Apple Disease Control During Rainy Periods

  • Locally systemic/single-site fungicides (e.g.Flint, Inspire Super, Merivon, Aprovia): IF these products are able to completely dry and be absorbed by plant tissue prior to a rain event, then their residual activity will likely be longer than protectant fungicides
  • Spraying captan, mancozeb, and ziram during a light rain is better than not spraying at all. In some cases, the rain can even help in redistributing the fungicide.
  • You’ll likely have to apply fungicides more than normal during this period. Assume that trees are no longer protected after 2 inches of rain.

In regards to fungal diseases of apple, sustained management of Glomerella leaf spot/bitter rot and apple scab during this period is critical. As I mentioned in last week’s 2nd Cover weekly update primary scab lesions have been observed on developing leaves and cluster/spur leaves in our research orchard. I have also been receiving some reports of scab in commercial orchards throughout Western NC. Early scab lesions can be easy to miss. Even just a few primary lesions lead to an epidemic under the right conditions (such as moderate temperatures and 7 inches of rain…….). Resistance to older S.I. (FRAC 3) fungicides in populations of the apple scab pathogen, Venturia inaequalis, is fairly prevalent throughout the eastern US. Considering the potential for high scab pressure in orchards where primary infections occurred, consider an application of Aprovia (Group 7) + mancozeb or captan or Inspire Super (FRAC 3 + 9) + mancozeb or captan as soon as possible. Both Aprovia and Inspire Super will provide at least 24 hours of kick-back activity.

With all of this scab talk, make sure you do not forget about Glomerella leaf spot and bitter rot. On Glomerella and scab susceptible cultivars, I suggest choosing Aprovia over Inspire Super and applying it in tank mixture with a half rate of captan, mancozeb or both, and also include 4 pts ProPhyt for enhanced disease control. Keeping residues on the trees is important, even if it means getting on the sprayer in light rain.

Written By

Photo of Sara Villani, N.C. Cooperative ExtensionDr. Sara VillaniExtension Specialist (Apple and Ornamental Plant Pathology) (828) 687-0570 (Office) sara_villani@ncsu.eduEntomology and Plant Pathology - NC State University
Updated on Jun 13, 2018
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