Bacterial leaf spot is a disease affecting several plants of interest to gardeners:
- Leafy plants in the cabbage family.
The disease is caused by several species of Xanthomonas bacteria. Not all types of Xanthomonas can affect all plants.
This bacterial disease is common east of the Rocky Mountains, as it thrives on warm, wet weather. The bacteria spend the winter in old leaves and cankers on twigs. Bacterial leaf spot can be introduced into a previously healthy garden through diseased seeds or seedlings.
Overuse of high-nitrogen fertilizers, whether chemical or organic, tend to increase the plant’s susceptibility to bacterial leaf spot. Plants with wounded tissue are more severely affected than healthy plants.
- Small, angular pale green to yellow spots on leaves, particularly near tip.
- Dry, flaky substance on leaves.
- “Shotgun” holes on leaves that widen over time.
- Yellowing of old leaves.
- Premature leaf drop.
- Sunken dark spots or cracks on fruit.
- Fruit drop.
Bacterial leaf spot is a serious problem in vegetables, as it can ruin the produce. There is no organic cure for this disease. Bactericides can be used, but are generally not effective. Removal and destruction of infected plants is the best solution.
Fortunately, bacterial leaf spot is much less serious in fruit trees. Bactericides can be used to prevent the spread of the disease, but not to treat it. Allowing the disease to run its course is suggested, provided that steps are taken to improve orchard health and sanitation (see below).
Diseased ornamental plants should be destroyed promptly.
Choosing disease-resistant plant varieties from reliable sources is the first step to preventing bacterial leaf spot. Rotate garden crops, and space plants far enough apart for good airflow.
Because high-nitrogen fertilizer is a major contributor to the development of bacterial leaf spot, keep the nitrogen levels low. If you are concerned that your plants may need the extra nitrogen, test the soil nutrient levels before fertilizing.
Other measures for preventing bacterial leaf spot fall into the category of maintaining garden sanitation:
- Support plants to keep their fruits and leaves off of the ground.
- Do not work with plants when wet.
- Do not use overhead sprinklers in the garden.
- Keep the weeds in check.
- Remove plant debris in the fall.