I have had the good fortune not to have major problems with pests and diseases in my allotment plots.   I hope to keep it that way

• by crop rotation, by using compost - without potato or brassica waste - and horse manure.
• by "good husbandry" - digging in winter, forking in spring, and hoeing in spring/summer to expose pests and bugs to the birds!
• by choosing resistant varieties, where necessary/available.
• by growing several varieties, two or three of each vegetable; I'll use the better variety again and again, and replace the less successful the next season.
• by using appropriate barriers - eg mesh tunnels, and collars from pop bottles.
• by encouraging the "good bugs" - plant nasturtiums, calendula officinalis, and French marigolds (tagetes) - and they go in salads.
• and as a VERY last resort, by spraying brassicas with derris to control caterpillars, and potatoes with bordeaux mixture if blight is a real danger.

I haven't needed to learn very much yet, but my first port of call would be the HDRA.