Can you get hold of some books by DG Hessayan ( SP ?) he writes the " The expert vegetable gardener , The expert fruit tree gardener , The expert flower gardener etc etc.
In his books he shows some useful disease symptoms on stems, leaves produce & roots
Another place is the Royal Horticultural Society website . Put " RHS" into your search engine , they have a fantastic range of colour pictures & loads of detailed crop info once you locate the plant you require .