Rhubarb lesions harmless; remove infected stalks
Q: We planted an autumn blaze in our yard about three years ago. This spring, we noticed green and red beads on the leaves. My husband thought he heard that this is a common disease in autumn blaze maples, but he's not sure if we're going to lose the tree. Can you give us some information on this problem?
A: This is an erinium mite infestation. These mites will not harm the tree. The problem will come and go through the years depending on weather conditions and a predatory mite population. There is no practical spray that can be used to control these mites, so just enjoy one of the minor curiosities of nature. The two organisms (maples and mites) evolved together a long time ago.
Q: My rhubarb leaves have small red spots on them. What is causing this and how should I treat it?
A: Rhubarb leaf spots are not unusual this season because of all the cool, rainy weather we've been having. There are two fungi that have been identified as causing the problem, but they are not a cause for alarm. Ascochyta leaf spot infection first appears on the upper leaf surface as small, green-yellow, irregular spots that are less than 1/2 inch in diameter.
The leaf develops a mosaic appearance as the lesions unite. Later, the spots develop white centers surrounded by reddish margins that are bordered by a grey-green zone.
Reproductive spores develop deep in the plant tissue, so they are rarely seen in the spots. In a few days, the infected spots turn brown, die and fall out. This produces a shot-hole appearance. Aschochyta does not cause stalk infections. Ramularia leaf spot appears as small red dots that gradually enlarge to form circular lesions 1/2-inch or more in diameter.
Larger spots become white to tan with purplish halos. The larger spots turn tan and become sunken lesions in the stalk tissue. Stalk infections occur later.
The lesions appear as small spots that elongate as the stalk grows. White fungus develops in the center of the spots on the leaves and stalks. The white turns brown as the tissue dies. In both cases, it is best to remove the infected stalks and leaves.
Usually, fungicides are not needed. However, if the disease has progressed too far and removing the stalks and leaves is not practical, then applying a copper-based fungicide is recommended. It also might be a good idea to divide the plants if they have been in one place for more than five years.
To contact Ron Smith for answers to your questions, write to Ron Smith, NDSU Department of Plant Sciences, Dept. 7670, Box 6050, Fargo, ND 58108-6050 or e-mail firstname.lastname@example.org.