Garden vegetables belong to families of related plants.

These families tend to share pests and diseases. Rotating families can keep you garden healthy.

Pests and disease spores overwinter in the soil and on crops residues. You can prevent many problems by hot-composting crops residues rather than leaving them in place. Moving each crop family from year to year helps them get a good start without last year's pests waiting for the new seedlings.
Here is a gardener's list of families:

Parsley Family (Umbellifers):

carrots, dill, fennel, parsley, parsnips, cilantro, caraway, celery, cumin

Aster Family (Composites):

lettuce, chickory, sunflowers, artichokes, cardoon, endive

Cabbage family (Brassicas):
broccoli, cabbage, cauliflower, mustard, Asian greens, Brussels sprouts, Turnips, radishes, cress.

Tomato family (nightshades):
tomatoes, potatoes, peppers, eggplants, tobacco, petunias

Squash family (cucurbits):  
winter and summer squash, cucumbers, melons, watermelon

Bean family (Legumes):  

peas, beans, cowpeas, vetch, favas, soybeans, astragalus, sweetpeas

Spinach family (goosefoot or chenopods): 

spinach, chard, beet, quinoa, lambsquarters, magentaspreen, epazote

Hibiscus family (mallows): 

Okra, hollyhocks, marsh mallow, hibiscus, jamaica, Egyptian spinach, jute

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