Agapanthus, known as Lily of the Nile, is a herald of summer. Blue, funnel-shaped flowers cluster atop the plant's long stalks, which spring out of strappy green leaves. This perennial is a favorite with hummingbirds, and it looks great in containers or mass plantings. Generally, agapanthus is a low-maintenance plant with few problems, though it is susceptible to a few diseases.
Boytrytis (Botrytis cinerea) is a fungal disease that appears during warm and damp weather. It appears as gray or brown lesions on the plant and may attack the flowers, preventing them from opening. Other symptoms include red stripes on the leaves and stems. It is not fatal to the plant, and there is no cure. Botrytis-resistant varieties of agapanthus are available.
Powdery mildew is a fungal growth that appears as a dusty gray or white circles on plant surfaces. The coating expands and grows together, producing a mat of mildew. Spray sulfur fungicide or neem oil on your plant before the infection is too widespread. Collect and destroy all infected leaves or branches, and make sure your plant is receiving enough air circulation to reduce humidity.
Tomato Spotted Wilt Virus
The tomato spotted wilt virus occasionally will infect aganpanthus. Symptoms include irregular blotches, concentric ring and line patterns, and streaks on the leaves. The virus will cause the agapanthus to become severely stunted and fail to flower. Young plants may turn brown and collapse. The virus is transmitted by thrips, tiny insects with fringed wings. Spray plants with insecticidal soap, and control weeds near your plants.
The fungus Macrophoma agapanthus may attack agapanthus's foliage, causing the leaves to die back. Symptoms include dead-tipped, lighter green olive leaves in the middle of the plant. Get rid of the fungus with a fungicide containing mancozeb or captab.