Ideal Time to Spray

Fungicides need to stick to the surface of branches and leaves, either to have time to be absorbed or to create a protective coating, so the ideal time to spray trees is when they are dry. Immediately after a rain, fungicide sprays will not be as effective because rainwater is still dripping off the tree and running down the trunk, washing away some of the chemical. The tree does not need to be completely dry, though. As long as there is approximately three or four hours for the spray to dry before the next rain, fungicide can be applied and still be effective.

The Type of Fungicide Matters

Whether or not it is necessary to spray shortly after a rain depends on what type of fungicide was previously used and how long ago it was used. Systemic fungicides are absorbed into the tree, so once they are properly dried, rain will only have a marginal effect and a normal spray schedule can be kept. Protectant fungicides coat the surface of a plant. If they are washed off, they are no longer effective. However, the amount of rain received is relevant to how soon the protectant should be used again.

Protectant Fungicides

Protectant fungicides do not easily wash off once they are dry. Several rules of thumb have been developed to help determine how soon protectant fungicides should be sprayed after rain. If you have received less than 1 inch of rain since the last spray, the remaining residue is still effective. Between 1 and 2 inches of rain reduces the effectiveness by half, so read the product label and cut in half the number of days until the next scheduled application. If you get more than 2 inches of rain, you should respray as soon as possible.

Curative Fungicides

Some fungicides, such as dodine, also have curative properties. Not only do they perform their primary protective function, but act against fungus already in fruit trees if applied within a short window after infection. Dodine is able to act against infections less than 36 hours old. Myclobutanil has curative activity for 96 hours after infection. If a curative fungicide is labeled for use against the common fungus infections in your area, it allows you to delay spraying until the fruit tree has properly dried after rain and still have effective control.