Deb Babcock: Leaf fungus on aspens, cottonwoods

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Deb Babcock

Deb Babcock's gardening column appears Thursdays in Steamboat Today.

Find more gardening columns here.

— The cool, wet spring we experienced this year has been tough on humans who wish to enjoy outdoor activities but wonderful in most every way for plants.

Every way, that is, except for the growth of fungus on aspen and cottonwood trees.

The fungus often appears in dense groupings of trees where moist conditions allow the fungus to grow and spread among the trees. Fungus is carried by wind and spends winters in the infected leaves left at the base of the trees.

If the fungus occurs early in the growing season, the tree may be able to grow new leaves and store enough food to make it through a Steamboat winter. However, if it loses its leaves in midsummer or late in the season, the tree could be seriously damaged and possibly not survive.

Marssonina leaf spot appears on cottonwood and aspen tree leaves as dark brown spots that may fuse a whole branch of leaves together into a large, black, dead patch. The fungus is carried by wind and affects emerging leaves.

Septoria leaf spot can cause premature leaf loss on infected trees. Circular spots that are tan in color, with black outlines and sometimes, black dots in the center, show up on infected leaves.

Ink spot, or ciborinia, is another disease where circular patterns of spots are seen on infected leaves, turning them brown by midsummer. Oval, black masses of fungus appear and then disappear by late summer, leaving behind a hole in the leaf.

The fungus venturia causes leaves and shoots of aspen and cottonwoods to blacken, dry and become distorted. The new shoots often curl into a shape resembling a shepherd’s crook, causing distorted growth.

Leaf rust is caused by the fungus melampsora and is recognized by orange pimples on the underside of the leaves. This disease requires a nearby evergreen where the spores are produced and then blown by the wind onto aspen and cottonwood trees.

Aspen cytospora canker is another fungus that affects stressed or weakened trees. This canker can cause the tree to die. If the orange, oozing wound is fresh, cut out the diseased bark to reach, live healthy tissue. If it’s an older canker, just remove loose bark pieces and a callus will form across the dead area. Do not apply any type of wound dressings.

To manage fungus diseases on your aspens and cottonwoods, keep the base area around your trees free of infected leaf litter, twigs and branches. And if stems are infected or cankers have occurred, prune them to reduce new infections. When watering, adjust the spray so the leaves don’t stay wet. Early morning is best so that leaves dry out before evening.

Space your aspens and cottonwoods so that air circulates freely and humidity is kept down.

If you apply fungicide early enough, you might be able to save uninfected leaves and stems, although it won’t help those leaves already infected. Spraying should be done two or three times during the growing season, at two-week intervals.

Deb Babcock is a master gardener through the Routt County Cooperative Extension Office. Call 970-879-0825 with questions.

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