Die-off of acres of Indian plums

Asked August 30, 2019, 7:01 PM EDT

This is a question that came up for the Master Gardener Volunteers, but we are at a loss - an absolute dead end.

The client and her neighbor share over two acres on wild (unlandscaped) land. Many of the trees/shrubs in that are are Indian plums. She, and the plums, have been there for 40 years, with no problems.

This year something is causing a massive die-off. She estimates over 90% of Indian plums are affected, but the other trees remain healthy. The leaves turn brown, but stay on the branches. There is no sign of a massive insect infestation. She brought in a sample of a plant that was drying, and while there were a few leaf hoppers, there wasn’t a whole lot of them. There did appear to be black spot on a few of the leaves, but she indicated that most leaves didn’t show black spot symptoms before they turned brown. Suckers continue to come up from the roots of the dead trees.

The references I can find say that, except for lacewings and leaf spot fungus (Cylindrosporium nuttallii) Indian plum is very disease and insect free. The leaves with black spot didn’t look like images of nuttallii leaf spot fungus, but that doesn’t rule it out.

Do you have any idea of what might be going on? I don’t know if a nuttallii infection could cause a die-off over two acres in a single summer. Should I suggest she take an affected leaf to have the fungus identified?

Any help to make us look less clueless would be appreciated.

Clackamas County Oregon

1 Response

I am not familiar with any disease or insect issues that would cause massive die off of Indian plum. The literature does say that it is a relatively short-lived shrub. And that regeneration requires disturbance. So, given that the shrubs are at least 40 years old, it could just be that they are all about the same age (since the last disturbance that regenerated them) and have reached the end of their life span. Also, the heat and drought extremes over the last 5 years could be hastening the decline at the end of the line, as we see with some other species.

If you want to pursue further investigation of possible diseases, harvesting a whole plant, and providing samples of all parts to the OSU Plant Clinic would be an option.