Hello Bob,

Thank you for using the Ask an Expert service.

What do you mean when you say you noticed fungus on the leaves? Did you notice whitish or pinkish masses on the underside of the leaves? Different colored fungal-looking growth? Or did you just notice spots on the leaves?

Since you are concerned enough about your tree's health to do multiple fungicide applications, I would recommend mailing in a sample in to the MSU Plant and Pest Diagnostic Services in East Lansing, MI. Please review the instructions on the website about submitting plant samples so that you send in an appropriate specimen. Basically, send a few branches with leaves attached, not just the leaves themselves, and try to include leaves in various stages of decline, and not completely dead ones. It is $20 per case, plus postage, for routine plant analysis.

Confirming the issue(s) that your tree has would help with appropriate timing of any pesticide applications, or let you know whether pesticide applications are even worth the effort. Timing is very important for pesticide application. You have been using Ortho Garden Disease Control fungicide which has the active ingredient chlorothalonil. This active ingredient is a protectant against fungal infection. If you get good spray coverage on the leaves (good coverage is very important), chlorothalonil provides a protective barrier over the surface of the leaves that prevents the fungal spores that land there from germinating and thus causing disease and infection. It doesn’t kill the fungi that have already infected the leaves. So, if you spray after you already notice symptoms this will not help with the infection that is already in the leaves. It might be difficult to get full coverage of the entire tree and both sides of all the leaves. Chlorothalonil is not systemic, and thus does not travel through the plant to parts that you have not sprayed.

Always read and follow all label directions when using pesticides, such as fungicides, and make sure the plant that you are trying to protect and the pest (ex. disease) you are trying to protect against are BOTH on the label. This is another reason to verify what the disease or other problem is.

I’m not sure if the holes in the leaves are “shotholes” from a fungal OR bacterial disease, or from insects. Could be more than one thing. Bacterial leaf spot is a possibility, in which case fungicides would not be helpful.

Some resources:

Bacterial Spot of Stone Fruits from Ohio State University Extension

IPM Series: Ornamental Fruit Trees from University of Maryland Extension. Plum is in the Prunus genus.

Cherry leaf spot from Michigan State University (MSU) Integrated Pest Management

Please let me know if you have further questions.

Regards,

Irene

Cherry Shot Hole on Flowering Cherries from the University of Maryland Extension

You can also always consider contacting an International Society of Arboriculture (ISA)-certified arborist, especially if you have other tree questions too.

https://www.treesaregood.org/findanarborist