Rick: Your question(s) and answer(s) depend on the tree and the disease. In general, trees get certain aerial diseases during times of wet weather and at a given growing stage of the tree. For example, apple scab, which was a problem this last year, was amplified by a very wet spring. Diseases of cherries and many ornamentals operate the same way. Most are treated just before bud break and may be treated once leaves are present if the weather suggests it and/r you had a bad infestation the previous spring/summer. 

If you are dealing with root diseases, the timing might be quite different. 

Here is the easier-to-read label for  your product: https://www3.epa.gov/pesticides/chem_search/ppls/<personal data hidden>1-20170719.pdf 

The next step is to go to the PNW Plant Disease Handbook and read about diseases that you might or might not have on your specific tree species: https://pnwhandbooks.org/plantdisease 

That should give  you decent information to decide on the spray protocols that would be most effective for the trees you want to protect. 

I hope this helps. Chip Bubl
OSU Extension Agent/Columbia County/ retired.