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I have fruit trees that I spray with Imidan and Captan. I try to do this during dry spells, but often the weather will change or my only opportunity to spray is right before rain is expected.

Will these chemicals last through a soft rain, hard rain or no rain?

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For the Imidan, I've pulled up the label. Reading carefully, there is no mention of rain for it's use on fruit trees. For deciduous trees it mentions not to spray if the leaves won't be able to dry before the rain falls. While that does not appear to be required for your fruit tree application, I'd go ahead and make sure they have enough time to dry anyways as you don't want your chemical to wash off in the rain and be wasted or worse run off into your ground water.

I've also pulled up several fact sheets for Captan, and while I can't judge it's effectiveness, it is a water soluble powder which means it will likely be of reduced effectiveness if it rains immediately after application because it will be washed off the leaves before it can be fully absorbed.

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  • Thanks for the input. I usually get about 24 hours or more until it rains, so hopefully that will be long enough. The trees could use the rain, so hopefully they drink up the spray quickly.
    – Airn5475
    Jun 25 '13 at 16:43
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Captan isn't going to hang around anyway, rain or shine:

What happens to captan outdoors?

The half-life of captan in soil ranges from less than 1 to 10 days (1, 9). See Half-life box.

Captan’s half-life on plants ranges from 3 to 13 days (1).

The half-life of captan is less than one day in water (1).

Captan is stationary to slightly mobile in various soils (1).

http://npic.orst.edu/factsheets/captangen.pdf

For Imidan:

This pesticide is toxic to fish and wildlife. For terrestrial uses, do not apply directly to water, to areas where surface water is present or to intertidal areas below the mean high water mark. Do not contaminate water by cleaning of equipment or disposal of wastes. Drift and runoff from treated areas may be hazardous to aquatic organisms in adjacent aquatic sites. This product is toxic to bees exposed to direct treatment. Do not apply this product while bees are actively visiting the treatment area.

http://www.ces.ncsu.edu/fletcher/programs/xmas/pesticides/labels/Imidan-msds.pdf

Seems to me that water does not reduce its effectiveness, though it may not stay where you put it.

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