Vegetable gardens provide a tempting target for disease-causing agents including viruses, bacteria, fungi, phytoplasmas (single-celled organisms similar to bacteria but lacking a rigid cell wall) and nematodes. Vegetable plants afflicted with disease may die, have stunted growth or lack productivity. Plant diseases specific to susceptible plant families may be transmitted via insect or animal hosts, be dormant in the soil or carried by wind or water.
Environmental stressors such as high humidity, extreme temperatures and nutrient deficiencies make plants more vulnerable to disease.
Fungal agents, the most common cause of vegetable plant diseases, include powdery mildew, damping-off, anthracnose, late blight and black spot. Although sometimes plant specific, most fungi can infect many different plants. Fungi produce spores that travel via wind and water to spread their attacks.
Viral infections, challenging to diagnose and to treat, cause a mottled appearance on plant leaves and include common problems like the mosaic viruses (lettuce mosaic, cucumber mosaic, and tobacco mosaic). Mosaic viruses, named for the plant type on which they were first discovered, can infect other plants in your garden. Tomato spotted wilt virus attacks many vegetable garden plants, not just tomatoes.
Bacterial diseases affecting garden vegetables include leaf spot, black rot, cankers, common blight, scab, bacterial wilt and bacterial speck.
Phytoplasmas, a relatively new category of agents causing plant disease, causes yellows disease on plants. Today plant pathologists diagnose damage from phytoplasmas using advanced instrumentation.
Nematodes, microscopic round worms, live in the soil. Beneficial nematodes aid in decomposing organic material. However, harmful nematodes attack plant cells and extract content, leaving the plant with yellowing leaves, stunted growth, wilting and reduced productivity.
Organic gardeners treat fungal infections in vegetable gardens using sulfur. Chemical treatments for fungal infections include fungicides such as captan, mancozeb and maneb. Alternately, remove damaged leaves and plants and burn or dispose of plant materials. Virus infections on vegetable plants do not respond to treatment. The plants should be removed and destroyed. Bacterial infections can be treated with sprays containing copper compounds.
Keeping down insect populations minimizes disease spread. Phytoplasmas infection requires eliminating insect vectors and removing diseased plants. No nematicides are available to the home vegetable gardener.
Cornell University Vegetable MD provides an excellent resource for identifying any disease causing problems in your vegetable garden. This website (see Resources) offers fact sheets for the most commonly grown home vegetables, whichinclude a description and pictures as well as a diagnostic disease key for tomatoes and cucurbits.
Many plant diseases respond favorably to weather conditions related to geography. High humidity in southern gardens favors several fungi. Relatively mild winters allow many disease-carrying pests and soil diseases to over-winter in the soil. Winds can increase the spread of airborne fungi, and different geographic regions have unique insects that vector plant diseases.
The most successful strategy for combating plant diseases requires prevention. Buy disease-resistant varieties for cultivation. Provide plants with sufficient space to allow air to flow around the plant. Use drip irrigation to keep water drops off of plant leaves.
Rotate crops each year so that disease susceptible plants are not grown in the same soil for two or three years. Clean and disinfect your gardening tools. If removing diseased plants, do not compost the plant materials--burn or place in landfills.