Most garden diseases are caused by fungi -- microscopic relatives of the common garden mushroom. Mature fungi release millions of spores that are then carried on the wind or otherwise get transferred to our plants. And if the conditions suit them, the spores germinate and penetrate leaf tissue, creating spots, lesions, or other symptoms. Most fungi need moisture to germinate, so damp leaves are an open invitation. Some diseases are soil-borne and are transported to foliage when raindrops splash soil onto plants. Preventing infection is the best way to control fungal diseases.
1. Choose resistant varieties. Plant breeders have created varieties with built-in resistance to certain diseases. For example, look for tomato varieties with V, F, or T after the names; these indicate resistance to verticillium, fusarium, and tobacco mosaic diseases.
2. Space plants properly. Adequate spacing allows good air circulation, which helps keep foliage dry.
3. Water the soil, not the foliage. Drip irrigation and soaker hoses are better than overhead sprinklers.
4. Apply mulch. Soil-borne diseases can be thwarted with a layer of mulch, which prevents soil from splashing onto leaves.
5. Rotate crops. Plants in the same family are often attacked by the same diseases, so rotate annual crops. For example, avoid planting solanaceous plants, including tomato, pepper, potato, and eggplant, in the same garden bed year after year. Rather, rotate with plants in a different family, such as cucurbits (squash, cucumber, pumpkin).
6. Remove and destroy diseased plant tissue. If only a few leaves are affected, prune them off and discard (don't compost; composting may not kill the disease organims). Sterilize pruning shears with rubbing alcohol between cuts.
7. Don't overfertilize. Plants pumped full of fast-acting nutrients (especially nitrogen) grow quickly, but this succulent growth is very attractive to disease organisms. Aim for slow, steady growth by providing slow-release nutrients.
Fungi aren't the only disease-causing microorganisms. Some diseases, such as fire blight, are caused by bacteria. Others, such as tobacco mosaic, are caused by viruses. Bacterial and viral diseases are difficult, if not impossible, to control in the home garden. Sometimes environmental conditions, such as a late frost, cause symptoms similar to disease infection so rule these out first.
Consider the plant, the time of year, and the degree of damage before resorting to pesticide sprays. A small amount of damage on tomato foliage at the end of the season may be tolerable, while the same damage in early summer may warrant control.
Diseases can spread quickly if not contained, so observe plants frequently. Note that most plant diseases are host-specific; that is, they attack similar plants or plants in the same family. For example, the powdery mildew species that attacks tall phlox is different than the one that attacks cucumbers.