To grow disease free vegetables the best practice is not relying in a single method to control disease in your vegetable garden.
Keeping your vegetables disease free is an ongoing effort. But that chore is made easier when you employ a broad-based effort that doesn't rely solely on one method.
Begin with good soil preparation and proper seed selection. If you transplant, picking healthy plants will keep disease from being introduced and spread. Remove any diseased plant before it can infect others nearby.
Water at appropriate times, usually early in the day. That gives the leaves a chance to dry before nighttime temperatures set in. Watering practices can influence the spread of disease in other ways. Water splashed off one plant onto another exposes the ones nearby to any disease the first carries. It's similar to being near a person with a cold who sneezes into the air. Even rainfall will produce the same effect to some degree. Space your plants out.
Viruses are spread by other forms of contact, too. Insects may carry them from one to the next. So, keeping the insect population under control provides benefits beyond preventing them from eating your plants. But animals and gardeners can spread them, as well. Tobacco mosaic virus may be spread from the gardener's glove and on the legs of rabbits, for example.
Keeping the garden well weeded will lower the prospect of disease. Keeping the area attractive turns out to have health benefits, as well. Many organisms thrive on weeds, then move onto your vegetables. They can be carried by wind, insect, water movement and other means.
Being able to recognize the signs of various diseases, many of which are distinctive to a particular vegetable, can help control them.
Lettuce mold appears as a wet rot at the base when the outer edges touch the ground. The Sclerotinia mold is white and Botrytis mold is gray. Removal of the mold by removing the infected sections, or an entire plant, can help keep the problem down.
Spinach mosaic virus is another common problem with these leafy vegetables. Leaves become mottled, and later may turn yellow. The plant acquires a stunted, wilted appearance. Growing resistant varieties can help keep the problem from occurring in the first place.
Asparagus may suffer from wilt or rot caused by the Fusarium. The spears look spindly and shoots may become yellow. The roots may rot and become discolored. Thinning the crop to eliminated infected plants is desired. Rust is another common asparagus problem, caused by the Puccinia fungus. It appears as red spots on the spears or shoots. The fungus may even survive the winter. Avoid excess watering to keep it at bay.
Leaf spots, blight and other conditions affect tomatoes. They'll usually appear by mid-August, especially if the summer has been cool. Some soil fungi affect tomatoes in particular. Nearby walnut trees can produce a toxin that is harmful, where the roots carry it into the soil. Look out for dark concentric rings on leaves. Keeping the leaves from being wet at night will help reduce these problems.
Recognizing the signs and treating them early will optimize your chances of a healthy crop.