Rhubarb leaf spots
Leaf spots are commonly found on rhubarb plants. This is more of an aesthetic problem, and will not severely damage the plant. Rhubarb is the main host plant of both Ascochyta rhei and Ramularia rhei, two different pathogenic fungi that cause leaf spots.
Ascochyta leaf spot– Leaf infection with Ascochyta rhei first appears as small, green-yellow, irregular spots that are less than one half inch in diameter on the upper leaf surface. The leaf develops a mosaic appearance as the lesions unite. Later, the spots develop white centers surrounded by reddish margins, bordered by a grey-green zone. Reproductive spores develop deep in the plant tissue and are rarely seen in the spots. In a few days, the infected spots turn brown, die, and fall out, producing a "shot-hole" appearance. Aschochyta does not cause stalk infections.
Ramularia leaf spot– Leaf infection with Ramularia rhei appears first as small red dots that gradually enlarge to form circular lesions a half-inch or more in diameter. Larger spots become white to tan with purplish halos. The larger spots turn tan in color and become sunken lesions in the stalk tissue. Stalk infections occur later, appearing as small spots that elongate as the stalk grows. White fungus develops in the centers of spots on both leaves and stalks, becoming brown as the tissue dies. Stalk infection is a key symptom of ramularia infection.
Both leaf spot pathogens produce spores that spread by wind and splashing water to susceptible plants where they cause new infections 10-14 days later. Spores develop in diseased leaves or stalks that have fallen to the ground and remain in the debris from season to season. Close examination of infected tissue shows small, round, black bodies (sclerotia) embedded in the surface beneath white fungus. The sclerotia germinate when appropriate conditions of temperature, moisture, and light are present. Ascochyta and ramularia fungi spread by infected rootstock.
Both leaf spot pathogens develop characteristic reproductive spores that can be identified using laboratory methods.
Ascochyta and ramularia are controlled with good sanitation practices. To minimize leaf spots, select and plant healthy propagation stock in sunny, well-drained, fertile soil. Maintain good weed control. Remove infected leaves as soon as disease appears and destroy all plant debris following the first frost. Although fungicides are rarely needed, copper compounds are labeled to control both types of leaf spots.
Howard R. J., J. A. Garland, and W. L. Seaman, eds. Diseases and Pests of Vegetable Crops in Canada. The Canadian Phytopathological Society & Entomological Society of Canada, 1994.