We have received a number of hawthorn and crabapple samples with quince rust in the Plant Disease Clinic. Quince rust is common on these hosts and often results in severe deformation of plant tissue. This rust is often seen with its cousin--another Gymnosporangium rust called cedar-apple rust.
Quince rust is caused by the fungus Gymnosporangium clavipes. The quince rust fungus requires two hosts, both a juniper and pomaceous host (such as apple, crabapple, or hawthorn), to complete its disease cycle. The disease is most destructive on the pomaceous hosts however. Quince rust stunts and kills fruit and causes swelling, distortion and death of twigs and petioles. It can sometimes cause cankers on small branches resulting in twig dieback. On the fruits, the fungus breaks out in little cup-like structures called cluster-cups, from which quantities of bright orange spores are produced. These spores are spread to the alternate hosts, the cedars and junipers. The fungus is perennial on the cedars but annual on the pomaceous hosts.
To control quince rust, prune out the cankered areas on the pomaceous hosts and any gall-like areas on the junipers. 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. Chlorothalonil is effective against this rust. For the homeowner, this would most likely be available as and 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, application rates and safety precautions.
This article originally appeared in the August 11, 1995 issue, p. 119.