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Fungal Issues in Trees

University of Minnesota Extension, Stearns County News
June 28, 2017        
Source:  Beth Berlin, Extension Educator-Horticulture
University of Minnesota Extension
Stearns, Benton, & Morrison Counties


Fungal Issues in Trees
By Beth Berlin, University of Minnesota Extension

ST. CLOUD, Minn. (06/28/17) — This late spring we had the right conditions for fungal issues to impact many of our trees including maple, oak, and apple trees.  When cool wet conditions occur at the optimal time, fungal spores thrive and multiply on newly emerging leaves.  Symptoms don’t often become noticeable till weeks if not months later. Keep in mind fungal spores are typically tree species specific; therefore those impacted maples are not the same impacting your apple trees. This year there seems to be an influx of different species trees that are now showing symptoms due to many different fungal spores thriving.

Maple trees can suffer from a variety of fungal spore diseases including anthracnose, more specifically tar spot. Tar spot is a fungal disease that is well named because it looks as if someone took a paint brush dipped in tar or black paint and splashed it onto the leaves leaving black blotches.  In other cases general anthracnose, which is the term used for closely related fungi species, can cause spotting or even distortion of the leaves.

Oak trees also seemed to be hit this year with anthracnose; again not the same fungal species that impact the maples, but yet similar.  Oak anthracnose tends to leave leaves distorted or curled down and inward with some discoloration or blackening near the margin.  Other issues observed this year are the oak leaf blister.

Finally apple trees, seem to be showing signs of apple scab. Even resistant varieties such as the Honeycrisp ™ appear to be showing some signs. Apple scab is a fungal disease that impacts all Malus species including apples and crabapples. Symptoms include spotting on the leaves and even fruit. In more severe cases the leaves may then turn yellow and prematurely drop from the tree. 

At this point there is nothing that can be done with any of the tree species fungal diseases. Simply rake up and dispose of any leaves that fall whether that is prematurely this summer but definitely take the time to do a thorough clean-up in the fall.  This will help reduce the number of spores in the area and hopefully help prevent reoccurrence again next year.  Spring weather, specifically cool wet springs, is the primary cause that determines if the fungal spores thrive or not.  In general the tree will survive anthracnose or other fungal issues, however several consecutive years of heavy symptoms can weaken the tree. Continue to keep the tree healthy by watering in drought periods, preventing compaction in root area, and properly timed pruning. Treatment of a fungicide on oaks and maples is rarely necessary unless the tree has experienced complete defoliation. Seek assistance from a certified arborist as timing and application is difficult.  Apple trees should also be properly pruned in late winter to early spring to help provide ample air movement, drying out fungal spores before they have the chance to multiply.  If you’ve experienced several years of apple scab issues, treatment is possible, but timing is critical. Visit and search for “apple scab” to find more information.

PHOTO: Anthracnose on oak tree. Photo credit: Michelle Grabowski, University of Minnesota Extension


Beth Berlin
Extension Educator, Horticulture
(320) 255-6169
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