You only have to use preventive fungicide when the conditions of wet humid weather prevail. On dry days it is less likely to be a problem.
Cultural things you can do
Make sure plants are pruned so they allow air to flow between them and that they are spaced properly
Get resistant plants. Some plants are very prone to problems but some cultivars have more built in disease resistance
like glossy leaves on roses.
Good sanitation, drip irrigation, and mulching help prevent fungal spores from splashing up from the ground onto the plants.
Trellis plants up and remove lower leaves. It improves air ciculation and gets the leaves off the ground. I prefer to space plants farther apart and use cages for plants. It is less work than a line or pole trellis and gives the vines more space to spread out.
Most plants with naturally glossy leaves have fewer issues with fungus.
Plants with large hairy or matte leaves or bumpy leaves will have more problems because they are designed to hold and trap water on their leaves. Tomatoes, eggplant, peppers, cucurbits, fuschia, basil have leaves which collect water and have fungal issues when the weather is humid and the leaves don't dry well. Sulfur is a good general purpose preventive fungicide but sulfur and oil are difficult to use in the heat of summer, you need to be careful to apply it in the cool part of the day and you don't want to use it on a very hot day or it may burn the plants. Whatever you use, it will have to be repeated after rain for the duration of the humid weather. Some plants are sensitive so always test it on a leaf first to make sure it won't burn the plant.
Once fungal infections take hold it is often futile to try to cure the plant. Remove or treat as soon as possible to prevent the spores from spreading.
If powdery mildew is the most common fungus you are dealing with sulfur is fine to use as a preventive
Milk and baking soda could work too. They both change the environment on the leaf by changing pH. If you use a milk with some fat in it, it will also help repel water off the leaves. Baking soda helps the leaves dry faster.
Natural fungicides cinnamon, peroxide can be used as a preventive on some plants.
If the problem is with downy mildew. Downy mildew likes cooler wet weather. Copper sulfate is usually a good choice for prevention. Also removing leaves as soon as they show signs of becoming infected.
Downside with sulfur
Do not use it within two weeks before or after an oil
Should not be used if the day temperature is likely to exceed 85 degrees
Needs to be reapplied after rain since it gets washed off.
Copper is toxic and can build up.
Sometimes it is best to have a couple of fungicides available.
Captan is good for coating seeds that are prone to dampening off.
Dithane is a broad spectrum fungicide with low toxicity and has been around for many years. It does not get washed off easily if it rains.
Chlorothalanil (daconil) Sometimes fungicides work better in combination and some fungi are more susceptible to different fungicides and may be resistant to others.
Some problems like black spot on roses or rust in ornamentals are best treated with systemic fungicides.