WSJM June 15, 2005 – Orange rust of raspberry

This is Al Gaus the Berrien County Extension Educator for fruit and ornamentals reporting from the MSU Extension office in Berrien County.

Today, I would like to talk about a common disease of certain raspberries that is showing up in the area as orange spots under the leaves.

Orange rust is a fungal pathogen that attacks brambles. The most common occurrences are seen on black raspberries, purple raspberries, and both erect and trailing blackberries. It does not infect any of the red raspberries. Orange rust is a rather unusual fungus in that it is systemic in the plant. It will infect all parts of a plant. Once a plant is infected by orange rust, there is no cure.

In general, leaf symptoms begin in June. This year is no exception. Within the last couple of weeks, there have been several reports of orange rust in our area. The first visible sign of orange rust is that the new canes are weak, and stunted. Several shoots grow from a given shoot giving the plant a bunchy, broom-like appearance. The leaves on infected plants are small, yellowish, and have orangish, rust-colored spots on the underside of the leaf. At first these spots are waxy but as the fungus grows, these spots become powdery. This powdery substance is the spores that spread the fungus to other plants. The spores are shed over a 2- to 3-week period thus causing new infections wherever they land. The spore producing leaves eventually wither and drop off in early summer. This leads one to think the plant will be okay. However, as mentioned before, it is still infected.

Even though there is no curative, the fungicide myclobutanil (sold as Nova) does an excellent job of suppressing orange rust and preventing nearby plants from getting it. Nonchemical approaches should also be utilized. Things such as removing and burning any plants that show symptoms and removing all nearby wild raspberry plants. Your first goal should be protecting your plants with a fungicide to keep from getting rust. Once you have it on a few plants, your new goal should be to keep it from spreading to others.

That is all for today, this has been Al Gaus for Michigan State University Extension in Berrien County.