Traditional gardening wisdom teaches that certain plants grown together create mutual strength and disease resistance. Companion planting is one way to create a disease-resistant tomato plant.
The National Sustainable Agriculture Information Services provide charts and lists of companion planting. Tomatoes become more disease free and pest resistant when planted near onions, nasturtiums, marigolds, asparagus, carrots, parsley, and cucumber. Basil repels whiteflies. Dill and borage repel hornworms.