Quince rust is caused by the fungus Gymnosporangium claviceps. The quince rust fungus requires two hosts, both a juniper or cedar and a pomaceous host, to complete its disease cycle. Pomaceous hosts include hawthorn, serviceberry, flowering quince, mountain ash, cotoneaster, pear, and susceptible apple varieties.
On pomaceous hosts symptoms are most common on fruits, but also occur on twigs, buds, and leaves. Infected fruit are often misshapen and have sunken lesions near the blossom end. The fungus causes swellings or distortions on twigs and petioles.
The quince rust fungus infects leaves, twigs, branches, and trunks of junipers or cedars. It produces elongate, swollen, rough cankers on twigs and branches. The fungus generally does little damage to cedar or junipers.
To control quince rust, prune any visible cankers on junipers or gall-like areas on pomaceous hosts. Protective fungicide sprays may be applied in the spring. Chlorothalonil is effective against rust. For the homeowner, this would most likely be available as an Ortho product labeled as Daconil 2787 or "Ortho Multipurpose Fungicide." Read the label to determine if the host in question is listed on the label. Also read the label for timing and rates and safety precautions.
This article originally appeared in the May 5, 1993 issue, p. 59.