Prevention is always better than cure. A good strong healthy plant will better resist an attack by pests or disease. So begin by making sure your soil is in good condition. Sow and plant resistant varieties. Once they are planted out and established keep them happy and they will reward you for your tender loving care.
Avoid drought by mulching, (after a good soaking,) with compost, straw manure etc. It will also help to keep competition from weeds down.
Feed with organic fertiliser: nettle tea, comfrey tea, liquid seaweed etc.
Protect from wind and pest by barriers, bird nets, butterfly netting, fleece and bottles.
Finally give them good companions to grow with. Companion planting helps in many ways: to aid growth to attract beneficial insects for pollination and to attract predators of harmful pests such as aphids. Root extrusions are thought to repel or attract other plants.
In short keep the balance with nature and nature will look after the rest.
Further reading :‑ Organic Gardening ‑ Roy Lacey
Complete Book of Companion Gardening ‑ Bob Flowerdew