Plants can suffer from diseases just like people. Identifying and treating vegetable disease can be one of the most challenging aspects of vegetable gardening. With a little knowledge however, you can learn to be your own “plant doctor”, preventing and treating many common vegetable diseases.
This article focuses on fungal diseases. For information on bacterial and viral infections, disease identification, and a summary of tips to help fight all types of garden vegetable diseases, see “Bacterial and Viral Diseases in Vegetables”.
What are Plant Diseases?
Most diseases that attack vegetable plants are fungal, bacterial, or viral. Plant diseases inhibit normal growth and interfere with the natural functions that plants need to perform in order to live and thrive. Diseases caused by fungi or bacteria can inhibit growth by using up the plant’s store of food. Some bacteria can cause blocked pathways, limiting the plant’s uptake of water and nutrients. Viruses may stunt plant growth or cause deformed foliage.
Fungal diseases are the most common type of disease in vegetables. Fungi don’t contain chlorophyll and can’t produce their own food using photosynthesis, so they get their nutrients from other living things. Fungi are made up narrow web‐like strands called hyphae, which release enzymes and absorb food. Most fungi feed on dead and decaying matter, and are beneficial to gardeners as they aid in the decomposition of matter, returning minerals and nutrients to the soil. Some fungal infections are actually a good thing ‐ mycorrhizal fungi infect the roots of many plants, forming a symbiotic relationship with the plant that actually helps the plant absorb nutrients ‐ especially phosphorus ‐ and withstand drought.
Some fungi are destructive in the garden, however. Common fungal diseases in plants include blights, botrytis, clubroot, damping off, leaf and pod spots, mildews, and wilts.
Blights thrive in warm, wet weather and may affect vegetable crops such as potatoes and tomatoes. Potato blight was the cause of the Irish Potato Famine in the 1840s. Blight starts with dark spots which progress to wilting, yellowing leaves, and rotting fruit. If blight occurs, destroy diseased foliage and dispose of any infected potatoes, as the disease can overwinter on diseased tubers. Buy certified seed potatoes, which are free of disease. Practice crop rotation with vegetables.
Botrytis (grey mould) is a common vegetable disease that affects plants such as lettuces and tomatoes. Botrytis likes cold, damp conditions. Brown‐spotted or blotchy leaves are an early sign, followed by a fluffy grey mould that causes the plant tissue underneath to die. As with other fungal diseases, prevent botrytis by keeping plant foliage dry, clearing the vegetable garden of debris at the end of the gardening season, and destroying affected plant matter.
Clubroot is a fungal disease that attacks brassicas such as broccoli, cauliflower, and cabbage. Once clubroot is in the soil it can remain there for up to 20 years. Prevention is the best way to deal with clubroot. Grow your own plants from seed, or be sure to buy seedlings from a reputable grower. Clubroot can only survive in acid soil, so if the soil is already infected, raise the pH above 7.2 to eradicate the disease. See “Soil Amendments” for suggestions.
Damping off is a fungal disease that can affect any type of vegetable plant, and is caused by several different types of soil‐ or water‐borne fungi. Affected seedlings collapse and die, or they may not emerge at all. Infected containers, overcrowding, and poor ventilation are frequent causes of damping off. Clean and sterilise pots and flats, don’t sow seeds too thickly, and don’t over water.
Leaf and pod spots (anthracnose) can occur on beans, as well as vegetables like brassicas, cucumbers, peppers, and tomatoes, creating sunken brownish spots. Remove and dispose of infected plants. Buy disease‐resistant seed varieties.
Downy mildew results in yellowing leaves with a greyish or white mould on their underside. Older leaves have tiny black fungi spots on their leaves, and slowly die in the autumn. Wet foliage can cause fungal diseases such as downy mildew. Don’t water with a sprinkler ‐ use drip irrigation or container watering. Susceptible plants include lettuce, onions, peas, and spinach. Powdery mildew occurs in hot, dry conditions when plants are overcrowded. Use sufficient water to prevent powdery mildew and destroy affected leaves to keep it from spreading.
Fusarium wilt and verticillium wilt are fungal diseases that cause leaves to turn yellow and look wilted. The yellow leaves turn brown and the disease usually starts from the bottom of the plant and works its way up, eventually killing the plant. Fusarium wilt is the most common form of the fungal disease, and is more prevalent in warm climates. Verticillium wilt is more common in cooler regions. Both fusarium wilt and verticillium wilt can be spread by cucumber beetles and squash vine borers. Crop rotation may have little effect on fusarium and verticillium fungi, which can survive in the soil for several years without a host plant. Grow disease‐resistant varieties and destroy infected plant material, disposing of it properly.
See “Bacterial and Viral Diseases in Vegetables” to learn more about identifying and treating common plant diseases.