Like most crops, fennel is susceptible to certain diseases, which are in turn promoted by pest damage. Here are some of the diseases you should look out for:
Symptoms: Small flecks on leaves which develop a halo and expand into tannish brown spots. These lesions will cause leaves to wither, curl, and die.
Leaf blight can be introduced through infected seeds and spread by wind or splashing water. This disease usually occurs on younger foliage first.
How to Handle Leaf Blight
Plant disease-free seed and rotate crops regularly. Infected plants should be removed and thrown away instead of composting. You may also need to apply a fungicide spray.
Symptoms: Yellow spots on the upper surface of leaves, white fluffy growth on the underside of leaves, and lesions that become darker as they mature.
Downy mildew affects young, tender leaves. The emergence and spread of this disease is prompted by prolonged leaf wetness.
How to Handle Downy Mildew
Plant disease-free seed, do not overcrowd, and avoid planting fennel near or rotating with other umbelliferous specimens such as dill, parsley, carrots, celery, and parsnips.
Symptoms: Powdery growth on leaves, flowers, and stalks, and leaves becoming yellowish in color. Severe fungal infections can cause fennel flowers to become distorted.
Powdery mildew can spread long distances in the air and the emergence of this disease can be encouraged by high humidity and moderate temperatures (60 to 80 degrees F). This fungal infection is typically most severe in shaded areas.
How to Handle Powdery Mildew
Plant mildew resistant varieties of fennel and avoid excess fertilization. Destroy infected plants (do not compost) and treat the area with a fungicide.
How do you prevent diseases on your fennel plants? Please share your techniques with us in the comments.