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Leaf Spots Follow Cool Wet Spring Weather (05/25/17)

If April showers bring May flowers, what do May showers bring? No – the answer is not wet pilgrims!

Leaf Spots Follow Cool Wet Spring Weather

If April showers bring May flowers, what do May showers bring?  No – the answer is not wet pilgrims!  May showers can bring leaf spotting diseases to trees.  Cool, wet weather encourages fungus diseases to infect and develop in tender, new leaves and needles.  One very common example of this is anthracnose in our deciduous trees.  Each species of deciduous tree that grows here - like oak, ash, elm, basswood, birch, buckeye, maple, and walnut - has its own anthracnose disease.  The parts of the state that have been enjoyed cool, damp weather lately may soon see evidence of these leaf diseases.

Anthracnose leaf infections show up usually as brown spots, blotches, distortions, and dead areas in leaves.  On some trees, especially when infected later in the season, the spots may be smaller and surrounded by either a dark or a yellow halo.  Severe infections will result in leaf drop.  After warmer weather arrives, the leaves will mature and the tree will send out more leaves and shoots.

Anthracnose diseases stress trees, especially when the trees are infected year after year.  Anthracnose can also contribute to declining vigor in trees that are already stressed by other conditions or tree pests.  Although treatment usually is not warranted, trees can be protected by the application of protective fungicides if treated early enough.  By the time you see symptoms, it is too late.

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 Other leaf and needle diseases favored by cool, wet weather include leaf curl of plum, oak, and chokecherry; apple scab, and spruce needlecast.  You can find more information, including control recommendations for these and other common tree diseases in North Dakota in the NDSU Extension publication “Tree Diagnostic Series”.

Lezlee Johnson

North Dakota Forest Service

Forest Health Manager

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