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Ash rust

Cynthia Ash

wrinkled, brown spotted ash leaves on cardboard

Photo: Chad Behrendt

orange spots on leaves

Photo: Chad Behrendt

Ash rust: Orange-colored lesions on leaves.

Ash rust is distributed throughout most of eastern North America and has been seen sporadically in Minnesota. Ash rust can infect several species of ash including green, white, and black. The disease, caused by the fungus Puccinia sparganioides, spends part of its life cycle on ash and the rest on cordgrass in marshes.

In the spring during warm wet weather, spores are released from infected grasses and windblown to nearby ash trees. Infection results in conspicuous swellings on leaves, twigs and petioles. Infected leaves may be distorted, necrotic (brown or dead), and/or wilted with heavily infected leaves dropping from the tree. Swellings on the infected petioles and upper leaf surfaces of ash trees develop yellow to orange spots in late June. These spots contain masses of orange powdery spores which are windblown from ash trees to cord grasses. The fungus then infects the grasses to complete its life cycle and overwinter.

Usually, ash trees are not seriously damaged by this disease. Therefore no preventive fungicides are recommended. However, successive infections can weaken plants making them susceptible to stressful environmental factors. Cultural practices such as watering during dry periods, mulching, and fertilizing may increase vigor of stressed trees. Pruning infections will remove spores that can reinfect cordgrass. This may lessen the number of infections the following year. Removal of all nearby cordgrass will disrupt the life cycle of the fungus, thereby preventing new infections. However, removal of grasses is not always feasible.

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