Hybrid astilbe (Astilbe x arendsii) is a perennial that blooms in summer. When in bloom the multibranched flower spikes have vivid color. The leaves are lacey and fernlike and grow in clumps 2 to 4 feet high. Astilbe requires moisture-rich soil and prefers part shade. As with any plant, if not cared for properly, problems can develop.
Powder mildew is a plant fungus which grows on the surface of astilbe leaves. Not only does it look unsightly, but the mildew fungus leaches nutrients away from the plant, all the time sending out spores to spread the disease, according to the Connecticut Agriculture Experiment Station's (CAES) Plant Pest Handbook. Powdery mildew can be controlled with fungicides at the first sign.
Astilbes with leaf spot develop circular dark brown spots with white to gray centers beginning on the lower leaves and moving up the stem. Infected leaves curl up and become dry. The first defense is to keep the plants healthy by fertilizing and watering, CAES advises. It’s best to water in the morning so the leaves have a chance to dry before evening. Affected leaves should be removed as soon as noticed. Fungicides can be applied in the spring when new growth appears.
Fusilarium wilt infects the tissue of astilbe plants. It causes wilt symptoms by restricting water flow. Symptoms include wilt on one side of the plant, leaves that are distorted and yellow, then turn brown, and eventually dry up. These drought-like symptoms occur from the wilt even if the plant is watered properly. Wilt control involves throwing away infected plants and replacing the soil around the plant.
The adults feed on a variety of plants, but the astilbe is a favorite, warns CAES. The beetle is reddish-brown and begins to appear in late in June, peaking in July. Eggs are laid in the soil and the larvae begin to feed on the roots near the surface and work their way down. They winter in the soil and emerge in April. They enter the pupal stage in May and June and after two to four weeks become adults. Pesticides can be used as preventative sprays early in the season.
Black Vine Weevil
The larvae of this insect attacks astilbe plants by eating their roots. They devour the small roots and then move inside to the larger roots, weakening the roots until the plant dies. The adult weevil is black with yellow hairs on the wings. Weevils feed at night and are hard to find because of the shape of the astilbe leaf. Adults and larvae survive the winter, re-appearing from May through July. The first defense again the weevil is treating the soil with beneficial insects such as pathogenic nematodes to control the larvae. If infection is severe, insecticides are the next defense. Spray every three weeks during May, June and July.
According to Cornell University Extension, astilbe require constant moisture to thrive. Water during dry spells. If the plants do not receive enough water, foliage and flowers become crispy and brown and the plant may go dormant until drought conditions are over, or until spring.