Asked April 05, 2020, 12:33 PM EDT
Ingham County Michigan
Hello, This question has come up before and I'm attaching a previous answer. Thanks for the question, and good luck with your tree.
"It is difficult to tell from the picture. Here are some possible causes:
Lichens- these are usually of various colors- white-ish to green-ish, tan or brown. They are harmless and nothing to worry about. This is the most likely cause of your patches. You can search the internet for color pictures of lichen on trees and compare with what you are seeing.
Birds (sapsuckers or woodpeckers) punch holes in the trunks of various trees, and the milky exudate you are seeing is simply the sap and usually causes no harm. The tree will heal naturally.
A rot fungi or canker fungi (there are many)- these usually start near a wound, crack or pruned area. From your picture, this does not appear to be the case. I am giving you general descriptions of these two diseases so you can look closely at your tree and compare.
Young cankers appear as round flattened, bark covered areas on main trunk or larger branch. Branch stub or other wound is often visible at center of the canker. As cankers age, the flattened surface turns black and bark begins to fall off revealing decaying wood in a target shape pattern below.
Rot fungi- Groups or rows of small (<2 inches wide) semi-circle self fungi along killed branches or on the main trunk. Schizophyllum shelf fungi are white and appear fuzzy on top. Cerrena shelf fungi are white to greenish grey and have concentric rings on the surface.
If your tree is one that has high value for you, you will want to get a definite diagnosis. Here are a couple options to do that: To consult a certified arborist for an on site diagnosis, see www.treesaregood.com and search under their ‘Find a Tree Care Service’ tab at the top of the page. Call 2-3 in your area for pricing. Be sure to ask for a certified arborist, not just a tree trimmer.
Scrape the area lightly and see what is under the white patch. Take more pictures of the areas affected. Try to get a sample of the white patch and take it to your MSU Extension office. If that is not practical, you can send pictures and the sample to MSU Diagnostics Lab in East Lansing. For instructions on how to do that, and their fees, see www.pestid.msu.edu
I hope this gives you some help with your tree. Remember, if the tree has lichen growing on it, there is no treatment or other action required. Thank you for using our service.