Due to the cool, wet spring, we have seen a number of diseased rhubarb samples in the Plant Disease Clinic. Most of these samples are infected with Ascochyta leaf spot or anthracnose stalk rot.
The first indications of Ascochyta leaf spot are numerous small yellowish-green areas on the upper surface of the leaves. Within a week of initial symptoms, the leaf tissue turns brown and dies resulting in angular spots. These spots have white centers surrounded by red zone and then a grayish-green zone. Often the dead tissue will drop out giving the leaves a shot-hole appearance which may be confused with insect feeding.
A second disease problem that has been common this year is anthracnose stalk rot. First indications of this disease are wilted leaves and large, water-soaked lesions on the stems. The lesions quickly enlarge and turn black. The stems may have a twisted appearance and the whole stem may collapse.
Both of these diseases can be controlled with good sanitation practices. Remove and dispose of infected tissue during the summer and after the first frost. In the case of Ascochyta leaf spot, stems with infected leaves may still be harvested and should be taken first whenever possible. Since both diseases overwinter in infected plant tissue, good sanitation practices should control most of the disease problems.
In addition to sanitation, the plants should be fertilized heavily as soon as growth appears the next spring. Another mild application should be made as soon as stalk harvest is completed. These two diseases are more prevalent in plants that are stressed and any measures to improve growing conditions should decrease injury caused by disease.
This article originally appeared in the June 30, 1995 issue, p. 100.