Submitted by: Jeanne Himmelein

According to Jan Byrne
MSU Diagnostic Services
Cedar-apple rust is evident

Cedar-apple rust caused by the fungus Gymnosporangium juniperi-virginianae, forms galls on juniper and red cedar. For most of the year, these galls are greenish-brown and relatively inconspicuous. In the spring during warm, rainy weather, like we had this week, orange, gelatinous, tentacle-like projections emerge from these galls. The galls are then readily observed and can be quite disturbing to many people. Diagnostic Services has received several samples and calls this week regarding cedar-apple rust.

This pathogen alternates between two hosts, apple (or crabapple) and juniper or cedar, to complete its lifecycle. Teliospores produced on the tentacles germinate and give rise to another spore type, basidiospores, which are spread by wind to the alternate host. Once spore production is complete, the gelatinous substance will dry and fall from the tree, however the gall will remain for several more years, possibly repeating the cycle in other years.

Galls that produced tentacles this spring are likely the result of infections that took place almost two years ago. Juniper are infected in late summer, the following spring gall formation begins but galls typically don't release spores until the following spring. The symptoms or signs of infection produced on apple are not nearly as dramatic. Both leaves and fruit can be infected, which can result in defoliation and significant crop reduction of susceptible varieties.

Managing this disease depends largely on the situation. In situations where there are individual junipers in the landscape, it may be feasible to prune-out galls. Trees should be checked closely each fall for the presence of newly forming galls. The severity of disease depends on the proximity of the alternate host. In nursery or orchard situations, alternate hosts that are nearby should be removed, if possible. Wild plants should not be overlooked. In the nursery, fungicide control options include applications of triadimefon or thiophante-methyl + mancozeb while the disease is present and ferbam during the growing season.