Fennel Rust

Symptoms by part

Causes

• High humidity levels around the foliage.
• Poor air circulation between closely planted fennel stalks.
• Extended periods of leaf wetness from overhead watering or rain.
• Temperatures that remain mild and damp for several days.

Could be confused with

• Aphid infestation · Aphids are visible, moving insects that cluster on stems, whereas rust is a stationary, powdery fungal growth.
• Nutrient deficiency · Deficiencies usually cause uniform yellowing or discoloration across the leaf, rather than the raised, dusty pustules characteristic of rust.

Treatment

1. Prune away and destroy any leaves showing visible orange pustules to reduce the number of spores.
2. Increase spacing between plants to improve airflow and allow foliage to dry out faster after rain.
3. Apply potassium bicarbonate to the affected foliage to alter the pH and inhibit fungal growth, follow the label.
4. Apply sulfur to the leaves during the early stages of infection to suppress further spore development, follow the label.

Without chemicals

• Remove and dispose of infected plant debris in the trash rather than the compost pile.
• Water only at the base of the plant to keep the leaves dry.
• Use a rain cover or cloche during wet spells to protect the foliage from excess moisture.

Chemical options

• Potassium bicarbonate, follow the label.
• Sulfur, follow the label.

Preventing it

• Choose planting sites with full sun and excellent drainage.
• Space plants at least 12 to 18 inches apart to ensure good air movement.
• Rotate crops so that fennel is not planted in the same spot for 3 years.
• Clean garden tools thoroughly after working with infected plants to avoid spreading spores.

When to act

Take action as soon as you spot the first orange pustules on the leaves. If the infection covers more than half of the plant, it is best to remove and destroy the entire plant to protect nearby crops.

Questions people ask

Can I eat fennel that has rust on it?

It is best to avoid eating leaves that show signs of rust, as the fungal spores can be irritating and the plant quality is compromised.

Will the rust go away on its own?

Fennel rust rarely disappears without intervention and will likely worsen if the weather remains humid or wet.

Plants it affects

• Florence Fennel