Leaves may be dull red or yellow, and stalks eventually collapse. It may be several months after initial infection before aboveground symptoms are apparent. Rogue out affected plants and the soil adjacent to the plants. Fungal Leaf Spot Leaf Spots are caused by Ascochyta rhei and Ramularia rhei , fungi that can over-winter on old rhubarb leaves and stalks. Leaf spots are common on field-grown rhubarb in areas with frequent rains in the early spring when leaves and stalks are actively growing.
Ascochyta infections appear as small, yellow-green infections on the upper leaf surface.
Later, spots merge to give a mosaic appearance. The spots' centers turn white and fall out, giving the leaf a tattered appearence. Ramularia leaf spots begin as small red dots which enlarge to become white to tan circular spots with purple borders. Similar spots commonly occur on the plant's stalks. The white spots are surrounded by a red band bordered by light green.