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UW Extension Diagnostics Team Seeing Spots

September 17, 2014
Diseased leaves
UW plant pathologists say leaf-spotting disease organisms can affect all manner of plants.

A growing number of tree and ornamental samples submitted to the University of Wyoming Extension diagnostic team have leaf-spot issues.

A generic term used to describe a number of leaf-spotting disease organisms that can affect all manner of plants, most leaf-spot diseases develop as small, scattered circular-to-oval dead areas in leaves under proper conditions (usually damp weather), says William Stump, extension plant pathologist.

Spots can enlarge and grow together to form large angular to irregular dead areas, and can range in color from tan, brown and yellow to gray and black, and with or without margins.

Stump says late-season leaf spots are typically more unsightly than harmful, while those beginning early in the season can severely weaken trees or shrubs, especially if they occur in two or more successive years.

Stump offers these suggestions to minimize leaf-spot disease:

-- Remove infected leaves and dead twigs before winter sets in.

-- Avoid wetting foliage while watering.

-- Maintain good plant health and avoid over-fertilization.

-- Use fungicides only in cases of severe disease causing defoliation for several consecutive years.

Fungi overwintering in fallen leaves, buds, fruits and twigs can spread leaf-spotting disease. Some fungi have specific hosts or may attack several species and, under proper conditions -- usually extended periods of cool, wet weather --numerous spores are produced that infect leaves, Stump says.

These conditions typically are found in spring or fall for Wyoming’s intermountain regions.

For more information, contact Stump at (307) 766-2062 or email

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