The surface of the leaves may develop numerous yellow spots (Fig. 1). However the most obvious symptoms are the orangebrown pustules on the undersides of the leaves (Fig. 2) which are characteristic signs of a rust infection. These pustules may also form on the upper side of the leaves, on stems, and on green flower parts. Hollyhock rust tends to become more severe as the summer progresses, killing most of the leaves on infected plants by early fall.
Large numbers of tiny rustcolored teliospores develop in the rust pustules. These spores are carried by splashing rain and air currents to nearby healthy plant parts and cause new infections. The fungus overwinters in infected plant debris. In the spring new infectious spores are formed on infected plant debris that cause infection on the newly emerging leaves.
For maximum protection begin fungicide treatments in early spring when the first leaves are expanding. Homeowners in New York State may apply fungicides containing chlorothalonil, sulfur, neem oil, or myclobutanil, if needed, according to label directions. Be certain any formulation(s) of pesticide(s) you purchase are registered for the intended use. Sulfur may damage leaves if air temperature exceeds 30°C (85°F) within 24 hours of spray application. When plants are dry, pick off and destroy any leaves or other plant parts as soon as signs of rust infection are noticed during the growing season.
Created, KLS, 05/02; Updated, SLJ, 3/09
This publication contains pesticide recommendations. Changes in pesticide regulations occur constantly, some materials mentioned may no longer be available, and some uses may no longer be legal. All pesticides distributed, sold, and/or applied in New York State must be registered with the New York State Department of Environmental Conservation (DEC). Questions concerning the legality and/or registration status for pesticide use in New York State should be directed to the appropriate Cornell Cooperative Extension Specialist or your regional DEC office. READ THE LABEL BEFORE APPLYING ANY PESTICIDE. __________________________________________________________________________________
The Plant Disease Diagnostic Clinic at Cornell University is located at 334 Plant Science Building, Ithaca, NY, 14853. Phone: 607-255-7850, Fax: 607-255-4471, Email: firstname.lastname@example.org or email@example.com