Bacteria and viruses are a part of everyday life ‐ for plants as well as humans. Some may have positive effects but many are harmful. While insects and many fungi are visible to the naked eye, you can’t actually see bacteria and viruses. Learning to recognise and treat vegetables infected with these types of diseases can be tricky.
For an introduction to plant diseases in vegetables, and information on fungal diseases, see “Fungal Diseases in Vegetables”.
Bacterial Diseases In Vegetables
Bacteria are single‐celled organisms, many of which secrete a sticky or slimy substance. Bacterial diseases are not as common in vegetables as fungal diseases or viruses. Bacterial diseases, in general, only start if the vegetable has a wound or cut, from insect damage, for example.
Don’t forget there are many “good” bacteria. Just as the bacteria used to make cheese and yoghurt don’t harm people (and the acidophilus bacteria in yoghurt is thought to be beneficial), the nitrogen‐fixing bacteria on the roots of legumes are a helpful and necessary part of the ecosystem. On the other hand, some bacteria cause diseases in vegetables, such as bacterial wilt and scab.
Bacterial wilt, which will cause vegetables to suddenly wilt and die, is carried by insects such as cucumber beetles. If your plant is drooping and you suspect it has bacterial wilt, cut the stem open. If you see threads of slime, this is most likely the culprit. To prevent, control beetles early in the season.
Scab affects potatoes, beetroot, radishes, and other root crops. Telltale signs are rough, brown, corky spots on vegetable roots or tubers. While the disease affects the appearance of the produce, it’s perfectly fine to eat the vegetables. To prevent scab, keep foliage as dry as possible, and don’t overcrowd plants, providing good air circulation. Highly alkaline, sandy soil promotes scab. Don’t lime soil before planting potatoes to prevent scab.
Viral Diseases In Vegetables
Viruses are microscopic amounts of genetic material (RNA or DNA) encased in a protein shell. To reproduce, they infect other living things. Plants, animals, people ‐ even fungi and bacteria ‐ can get viruses. The virus attaches to the host and reproduce inside the plant’s cells. One plant can host millions of virus particles.
Viruses are probably the hardest types to diagnose of garden diseases. Viruses may affect brassicas, lettuce, potatoes, and tomatoes, among other vegetables. Plants affected by viral diseases may exhibit a splotchy, mottled, light green and yellow “mosaic” pattern. Leaves may be crinkled or curled.
Viral diseases in vegetables include bean mosaic, cauliflower mosaic, and cucumber mosaic. Note that the viruses are named after the plant that they were first found on: just because the virus is called cucumber mosaic doesn’t mean it just affects cucumbers. Viruses are hard to get rid of, and there are no cures for plant viruses. Viruses can be spread by “vectors” ‐ infected insects that carry the disease, including aphids, thrips, whiteflies, and leafhoppers. They can also be spread by infected plants or contaminated hands or garden tools. Prevention is the best solution. Treat aphid and other insect infestations promptly. While a few “healthy” aphids feeding on your garden plants won’t cause much in the way of garden diseases, if they carry a mosaic virus, the virus could kill the plants.
If you smoke cigarettes or use other tobacco products, keep them away from the garden and wash your hands before gardening. Cigars, cigarettes, and pipe tobacco can be infected with the tobacco mosaic virus and can communicate the vegetable disease to plants in the nightshade family, including tomatoes and peppers. Even handling tobacco infected with a mosaic virus can easily spread it to garden plants.
Cucumber Mosaic can affect not only cucumbers, but tomatoes, peppers, squash, spinach, celery, and beetroot. Plant mosaic‐resistant varieties. Control aphids, which carry the virus. Promptly remove diseased plant matter.
General Advice on Fighting Garden and Vegetable Diseases
- Fungal garden diseases are spread by water. Don’t water the garden late in the day ‐ foliage should be dry by nightfall. Don’t work in the garden when it’s wet. Avoid overcrowding plants and promote good air flow.
- Practice good garden hygiene. Wash soil off tools after use. Thoroughly clean and disinfect tools and plant pots at the end of the season. Wash your hands after handling diseased plant matter to avoid infecting healthy plants with vegetable diseases.
- Manage insect infestations. See “Insects” and “Garden Pests: Safe and Effective Solutions” for more information.
- Use crop rotation. See “Vegetable Crop Rotation” for tips.
- Destroy diseased plant material. Don’t put them in the compost. (Compost with an internal temperature of at least 72°C (160°F) will kill most pathogenic fungi, bacteria, and nematodes, but why risk it? Don’t bury diseased plant matter in the yard ‐ diseases may overwinter. Wrap it securely and put it in the trash.
- Look for seed and plant varieties that are disease resistant. The letters after the name can tell you they are resistant to verticillium wilt (V), fusarium wilt (F), nematodes (N), tobacco mosaic virus (T) and alternaria (A). The tomato “Roma VF”, for example, is resistant to vericillium wilt and fusarium wilt.
- Use organic biofungicides and biopesticides (to fight insects that spread bacteria and viruses), which are available from garden centres and catalogue companies.
Knowledge of common garden diseases can help you treat an infection before it gets out of hand. Prompt action may keep a vegetable disease in check, and help you save your crop.