Leaf spots and rust or mildew discoloration are fungus diseases. They favor moist overnight conditions on the leaves, which can be caused by irrigation in late afternoon or early evening.
Snails and slugs are best controlled by baits. Metaldehyde baits are very effective. However, slug baits containing iron phosphate are safer to use around children and pets.
Two of my favorite organic pesticides that are very safe to use are neem oil and spinosad. Neem oil controls many common sucking insects and also controls most leaf diseases such as mildew, rust and black spot. Spinosad controls many chewing insects, including apple worms and cabbage worms that feed on all cabbage-family vegetables including broccoli and cauliflower. Repeat applications at biweekly intervals are necessary in both cases. Spinosad is effective on lace bugs and also has systemic properties so it can reach inside plant tissue and kill insects like borers.
Both products are also toxic to bees and should not be applied when they are pollinating flowers. There is little bee activity in the late afternoon and early evening.