I always start off most talks about bloom time and spring diseases with the concept of leaf wetness. Although it may seem over-simplified, without leaf wetness (and high humidity), foliar fungi are not able to infect the plant.
Periods of leaf wetness are needed for several functions of fungal dispersion and growth. Windy, rainy weather blows spores from infected tissues to new, healthy tissues. Leaf wetness or high humidity provides favorable conditions for spore germination. Once inside the tissues, the pathogen may grow and reproduce faster in wet and mild weather.
These aspects are why bloom and spring time diseases are of concern when it rains. It is not because rain brings on the disease, but rather that rain favors the disease by providing environmental conditions that favor the fungi. In the summer time, the opposite occurs. The environmental conditions favor plant growth and discourages the growth of many fungi, thus reducing the risk of some diseases.
With four days of intermittent rains with warmish temperatures (~60F), fungal infection of unprotected tissues can be high. This is why fungicides are applied, and should be applied prior to the rain event. Fungicides serve best as a plant protectants – meaning that they must be on the surface of the plant prior to spore germination. As the fungal germ tube grows along the surface of the leaf, it comes in contact with the fungicide, which will kill it.
What if a fungicide spray was missed? Certain chemistries (FRAC 3, 11) provide a few days of “reach back” activity (~2-3 days). This means that they are able to stop the infection of the fungus even though it may have already invaded the plant. These fungicides have the ability to move into the upper laminar layers of the leaf, and kill the fungus as it begins the initial infection. With that said, these fungicides are much more effective if applied prior to spore deposition and initiation of infection.
Most fungicide sprays, if having ample time to dry, provide protection for 10-14 days depending upon the amount and number of rain events. Even in perfect weather, the products will begin to photo-degrade and will lose their effectiveness after two weeks. If rainy conditions return after that period, another spray will be needed to prevent infection of the present fungal pathogens.