Hardy and reliable, most problems with geraniums can be avoided or minimized with good gardening and maintenance habits. To ensure that your plants stay healthy and robust, plant them in a sunny to partly shady location, provide adequate space for air circulation, and give them extra water during dry spells. However, even well-tended geraniums occasionally encounter problems with insects and disease.
Bacterial Leaf Blight
Bacterial leaf blight is a very common disease caused by bacteria that thrive during periods of warm, humid weather. Leaf blight appears as dry brown spots and lesions on infected foliage. Some infected leaves fall off right away; others dry up and may hang from the stem for several weeks. Leaf blight then attacks the remainder of the plant, causing stems to shrivel and become black and rotten. Remove and destroy any infected plants. Leaf blight can overwinter in plant debris and soil. Wear gloves when handling infected foliage. Wash hands well after working with infected plants. Disinfect tools.
Whiteflies are tiny flies that resemble white moths. The adults and larvae live on the underside of the geranium leaves, sucking out the plant sap. Leaves on infested plants turn yellow or become mottled as the entire plant weakens. Spray leaves with insecticidal soap every three days for two weeks. Soak the foliage well and be sure that the undersides of the leaves are covered. Insecticidal soap must come in contact with the insects to be effective.
Two-spotted Spider Mites
Two-spotted spider mites are destructive pests that suck sap from the host's leaves, and weaken the geranium plant. Infected foliage turns yellow or brown with unsightly webbing. To control, spray plants with water three times a day for several days, and make sure to reach the undersides of the leaves. For severe infestations, spray geraniums with insecticidal soap twice a day, every two days. Spider mites thrive in hot, dry weather, causing most of their damage in mid- to late summer.
These small, soft-bodied insects feed on plant sap, reducing the plant's vigor. Aphids can be green, pink, black, gray, or white. Infected leaves, stems and buds appear misshapen. Severe infestations result in copious leaf drop. As they suck, aphids secrete a sugary sap onto the foliage which encourages the development of a black, sooty mold. Wash the pests off the geraniums with a strong spray of water from a hose. Repeat as needed. Spray with insecticidal soap for serious infestations. Aphids can also infest weeds growing around your geraniums, so keep the area well-groomed.
Budworms are small caterpillars which tunnel into unopened buds and eat them from the inside out. Small holes appear in the infected buds. When the flowers open, they are full of holes. Remove and destroy any buds that have holes or brown specks on them. Pick off and destroy any caterpillars. For heavy infestations, spray with Bt (Bacillus thuringiensis). This beneficial bacteria is harmful only to caterpillar pests, who die after ingesting treated buds and foliage.