Pocket BotanistCheck these first
- Look for white, dusty patches on the leaves.
- Check if the plants are crowded too closely together.
- Feel the soil to see if it is staying too wet.
Likely causes
Powdery mildew
Most likelyHow to tell: White, dusty spots appear on the leaves and stems, eventually spreading to cover the whole plant. It looks like someone dusted the fennel with flour.
What to do: Remove the most heavily infected leaves to stop the spread. Spray the remaining plant with a mixture of one tablespoon of baking soda in a gallon of water.
Poor air circulation
PossibleHow to tell: The plants are crowded together, preventing the leaves from drying out after rain or watering. This creates the perfect environment for fungus.
What to do: Thin out your fennel plants to ensure they have at least 12 inches of space between them. Prune away any foliage that is touching the ground.
When to worry
It is serious if the spots cover most of the foliage, as this prevents photosynthesis. Minor spots can be managed with pruning.
Stopping it coming back
Water at the base of the plant rather than overhead to keep the leaves dry. Ensure your garden has good airflow by spacing plants properly.
Not sure which one it is? Photograph your florence fennel.
This page lists the usual causes. Pocket Botanist looks at your actual leaf and tells you the likely cause, how serious it is, and what to try first. Identification and a health check are free.
Questions people ask
- Is powdery mildew harmful to eat?
- It is not toxic, but it ruins the flavor and texture of the fennel. It is best to discard heavily infected parts.
- Does this mean my soil is bad?
- Not necessarily. Powdery mildew is usually a result of humidity and lack of airflow rather than soil quality.
More problems with Florence fennel
Back to the Florence fennel guide
This article was generated with AI from the plant’s care profile and is general information, not horticultural advice. Read about how we build these guides.