With all of the recent rains and humidity, expect lots of fungal diseases this summer.
In general, healthy plants resist fungus diseases all by themselves. Please remember that most fungicides that are sold are rather dangerous for human health.
For most fungal diseases like powdery mildew on cucumbers, pumpkins, zucchinis etc. use Eco fungicide. It is more or less baking powder – i.e. very safe. This spray is also registered for rust diseases too.
For fungal root rot use Yates Anti-rot. This is more or less food acid that is in lots of foods that we eat and it is very safe indeed.
Be careful with other fungicides. Those two are by far the safest of all in my opinion.