Honestly IME the single best thing you can do is get your soil in superb shape -- fertile, loose, water-retentive, well-drained, weed-free.
The better your plants are growing, the less they are apt to 'catch' diseases and pests, and the better they can keep producing despite some level of damage.
Beyond that, organic pest/disease-control strategies are generally specific to each particular kind of pest or disease, there is not really a one-size-fits-all magic cure.
Good luck, have fun,
Pat