When is best time to spray fungicide and how many treatments per year are recommended
Livingston County Michigan
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This publication outlines the management of needlecast:
The fungicide must be applied to the new growth when the buds are elongating, or when that new green growth occurs. The fungicide prevents the new growth from being infected. Two - three applications annually are usually required. One can limit the spread of needlecast by ensuring air flow through the branches. If the tree has branches that lay on the ground, you might want to consider pruning those off so that air can flow under the tree, preventing the moisture from furthering infection. Doesn't have to be much, even a foot or two of space without branches can help (and allow you to continue to retain privacy).
I hope this helps. Please feel free to reach out if you have any questions.