A hawthorn sample showing quince rust was recently submitted to the Plant Disease Clinic . The quince rust fungus, Gymnosporangium clavipes, may cause rough, swollen areas on twigs and branches. These canker-like swellings are especially noticeable after the leaves have fallen.
Quince rust infects a number of genera in the pomaceous group, including hawthorn, crabapple, apple, pear, serviceberry, and mountain ash. In addition to causing distortion and death of twigs and petioles, the fungus may stunt and kill fruit. Cup-like structures that produce orange rust spores form on the fruit. Leaves seldom show symptoms.
The quince rust fungus requires two hosts, both a juniper or cedar host and pomaceous host, to complete its disease cycle. The fungus infects leaves, twigs, branches, and trunks on cedar or juniper and causes rough swellings. In the spring, reddish-orange jelly-like masses of spores ooze from these cankered areas and are blown to the pomaceous host.
To control quince rust, prune out the cankered areas on the pomaceous hosts and any gall-like areas on the junipers if practical. Avoid planting pomaceous trees in the vicinity of cedar hosts. Protective fungicide sprays may be applied to the pomaceous hosts in the spring when the orange masses appear on the junipers. Read the label to determine if the host in question is listed on the label. Also read the label for timing, application rates and safety precautions.
This article originally appeared in the December 11, 1998 issue, p. 131.
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