From:
http://cemariposa.ucdavis.edu/newsletterfiles/Backyard_Horticulture470.htm
ROTATING YOUR GARDEN
BACKYARD HORTICULTURE
By Gary W. Hickman, Horticulture Advisor
University of California Cooperative Extension, Mariposa County
Before parts of your summer vegetable garden are replaced with winter plants, make a note of where each crop is now planted. This will help later if you have had trouble with some diseases this summer. By having such a garden map, you will be able to rotate crops next season. A particular vegetable should be grown in the same location only once every two or three years. In fact, other vegetables of the same plant family should not be planted in the same place. For example, if you grow cantaloupe this year, stay away from pumpkin, cucumber, and all the rest of the melons for that spot in the garden next time. Corn is an exception; it is in a plant group basically by itself, so any vegetable can follow it. Peas and beans are in the same family group. Tomatoes are grown in many home gardens in Mariposa County. The tomato plant family also includes pepper, okra, potato, and eggplant. The tuberous, or bulb, plants are all relatives for rotating purposes, including beets, carrots, garlic, and onion. The last major group of plants are the leafy vegetables. This includes cabbage, lettuce, radish, spinach, and turnip.
The reason behind rotation of different groups of plants is that most of the plants in the same family group have many common diseases. Many of these diseases are difficult, or impossible, to control using chemicals. The best solution for control, then, is by crop rotation.
Another good way to plant for a disease-free garden is to use resistant varieties. For our particular area, there are several disease-resistant tomato varieties. They all have VF or VFN in their names to indicate resistance to verticillium, fusarium, and nematodes. For many crops, however, there are yet no specific disease-resistant varieties. For these, simply make sure that the variety used is adapted to grow in our area. Lettuce, which can be planted now in warmer locations, comes in many varieties. Remember not to plant your winter lettuce where any leafy vegetables grew this summer. Another winter garden crop is carrots. Nantes, Imperator, Chantenay, and Amsterdam are all good varieties.
Aside from crop rotation and selecting the best varieties, proper watering is the next most important factor in garden disease control. Deep, infrequent, furrow, or drip watering in the morning will greatly reduce bacterial and fungal problems. This will keep the soil moist down in the root zone without saturating. It will also keep the leaves dry, which results in fewer foliage disease problems.
Sometimes, despite all your best efforts, you can still get diseases in your home garden. In some cases, chemicals will help prevent the spread of the infection. Curing an already infected plant is usually not possible. If this happens, the best control is to remove the diseased plant and destroy it. Then remember to rotate your crops next season.
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Disease Prevention in Home Vegetable Gardens:
From:
http://extension.missouri.edu/explorepdf/agguides/hort/g06202.pdf