Holly bush problems
I have four holly bushes that were healthy for ~14 of the 19 years on my lot. But now 2 of the 4 have an issue that I've not been able to remedy. Initially I thought it was black spot disease and was applying ortho garden fungicide. But recently I'm told it was mites and have been trying Bio-advanced mite and disease spray. This initially seemed to work but now new growth has the same symptoms. I also use Schultz plant food every other week or so but can't stem the issue. Your thoughts please
Oakland County Michigan
American holly and some other trees are intolerant of flooded or wet saturated soils. The dieback may be due to wet soil, especially if your affected shrubs are situated in a low spot or in clay soil. -https://www.canr.msu.edu/news/trees_and_flooding_faqs
Other problems that cause dieback are cankers on branches., or winter kill. The spots on leaves are probably due to our wet spring and could be alternaria or one of a few other diseases. A look at leaves under a microscope can confirm which one, or an arborist may recognize it.
I wouldn’t feed the shrubs every other week. Pushing too much lush growth can make the shrub very attractive to insects, and in fall any tender growth will likely dieback over winter. A soil test is the best way to go, following the test’s recommendation for fertilizer, and applying it in spring as new growth starts. A reference for you-
There are a couple ways to get a diagnosis. You can send your pictures along with clippings from the shrubs showing both diseased and healthy leaves, and clippings of the branches that have died back, to the MSU plant diagnostic service. Their website has the sample submission form, the fee schedule, and instructions on how to submit samples to the lab. You may call them during business hours if you have any questions on how to submit samples. https://pestid.msu.edu
Or you could hire a certified arborist to come on site, go over the shrubs and give you a complete diagnosis. He/she can check for canker, verticillium wilt, root rot, insect pests and determine the exact cause of the leaf spots; and give you a plan of action. Find certified arborists here by zip code- www.treesaregood.org
I hope this is helpful and good luck with your shrubs. Thank you for using our service.