Borer may be source of plum problems

We have a purple plum tree that has given us very tasty plums for a number of years. This year, however, its branches are burdened down with hundreds of plums that won’t ripen. The plums are various shades of purple, but not the deep purple of sweet plums. Some are green near the stem, and purple farther down the fruit. We’ve given the tree plenty of water. What’s going on with our tree?

— John

It concerns me a bit how you’re describing your plum tree. A tree that sets TONS of fruit that is undersized and won’t ripen often indicates a tree that’s under a significant amount of stress. There can be a whole bunch of possible reasons for this (watering, soils, physical injury, etc.) but far and away the most common one is a little devil insect called the peach tree borer.

Peach tree borer is the most common serious insect pest in fruit trees like yours. In spite of the name, this insect is a problem on members of the genus Prunus that set a “stone type” fruit. This would include peaches, plums, cherries, apricots and almonds. It can also affect their ornamental relatives such as cistena plum, pink flowering almond, purple leaf plums, European bird cherries, Schubert chokecherry, flowering cherries, double flowering plum, Nanking cherries and more.

This sneaky little guy tunnels around under the bark like other shade tree borers, but it does it low down on the tree near ground level where many people don’t notice the problem until serious damage or even death of the tree occurs. As with most all borers, treatment involves wetting the bark of the tree where this borer is active. However, it’s also very important to carefully time the applications to achieve good control.

The way to treat the problem is to drench around the tree using a watering can or a bucket of permethrin. On edible fruit trees like yours (even though it’s sold as an ornamental, you’re eating the delicious fruit off of your purple leaf plum), you want to use the 2.5 percent concentrate of permethrin according to label directions. There’s a 38 percent concentrate available that we commonly use for borer control, but it’s not labeled for use on edibles. You can only use it on ornamental plants with the problem.

We recommend that you apply the drench around your trees approximately June 1, June 20, July 10, Aug. 1 and again on Aug. 20. Wet the bottom 6 to 8 inches of the trunk and soak the ground immediately around the trunk. It really doesn’t take all that much of the insecticide solution to treat each tree. If you have any susceptible ornamentals you want to treat with the 38 percent permethrin, you only have to apply it three times. I’d figure on doing it about June 1, July 1 and Aug. 1.


I wanted to know if Honey Crisp apple trees grow this area, or is it too warm. I saw they need 800 chilling hours. What do you think? Also can they pollinate with an older established tree?

— Bev


Honeycrisp apples will grow in this area. In fact, they’re grown commercially in western Colorado. One thing about this variety is that they ripen best in cooler areas. I don’t know how many are being grown down here in the Grand Valley. I do know that they’re grown at higher elevations around Cedaredge and the North Fork Valley. This doesn’t mean they won’t grow down here, it’s just that their flavor is developed best when it’s cooler.

Dennis Hill is the nursery manager at Bookcliff Gardens, Send questions to Bookcliff Gardens, 755 26 Road, Grand Junction 81506; or email .(JavaScript must be enabled to view this email address).


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