Homegrown Column July 11, 2009

I have a peach tree that is getting rust spots on the leaves. They then turn yellow and fall off. What’s up with that? 
— Tom

Sounds a bit to me like you have a fungal disease called coryneum. I’m not absolutely sure without seeing some leaves, so if you get a chance, bring some by the store and I’ll verify it for you.

Coryneum is a leaf spot-type disease that we see occasionally in members of the genus prunus that set a stone-type fruit. I see it in peaches, apricots and plums most often, but it can also be a problem in cherries as well as all their ornamental relatives.

The disease starts with small reddish spots on the foliage that grow and become purple with a white or light gray center. The center eventually falls out leaving a small hole in the leaf. A common name for this disease is “shot hole disease” because the leaves look like they’ve been hit with bird shot.

Moving from the leaves, the disease also can infect small twigs on the plant. It can cause canker formation and even some minor die-back.

The disease can also affect fruit. In fact, this is probably the most destructive aspect of the disease from a commercial point of view.

The same purplish-red spots appear on the fruit. These eventually turn brown and can lead to the skin of the fruit cracking and the accumulation of gummy sap there.

This disease requires warmer temperatures and high humidity to develop, which is why we don’t see it all that often around here.

You usually see coryneum development where the leaves of the tree are wet by irrigation sprinklers. The water doesn’t always have to get directly on the leaves, even the spray mist from sprinklers can raise the humidity enough for it to get going.

The best long-term solution to this problem is almost always managing water around the plant.

If sprinklers are wetting the leaves, try to adjust them so the leaves stay dry.

If that’s not possible, only run the sprinklers between midnight and sunrise or between 10:00 a.m. and 2:00 p.m. We can almost always take care of the problem this way.

There are some spray options that can help as well. Fungicides containing chlorothalonil or copper can be applied in the spring when the tree is dropping its immature fruits. I like using chlorothalonil because the copper spray sometimes can spot or burn the foliage. In the fall, after the leaves drop, you should rake up all of the fallen leaves, dispose of them, and spray all the branches and the ground beneath the tree with a copper fungicide spray.

If you develop any small lesions on the twigs, they should be cut out before spraying in the fall.

I’ll emphasize again that the key to controlling this problem is keeping the foliage dry. Our naturally dry climate pretty much precludes diseases such as coryneum from becoming problems unless we’re artificially changing things.

I have wireworms infesting my strawberries. What can I use to get rid of them and still enjoy the berries?
— Allen

Wireworms are an extremely difficult insect to control. In fact, there are places that no longer commercially grow crops such as potatoes because of wireworm damage.

The old standbys in the insecticide world that were used for wireworm control (such as chlordane) were banned from the market because of environmental problems and have not been available for years.

Looking through some of the research, the one insecticide that offers some hope for wireworm control is the synthetic pyrethroid bifenthrin.

Unfortunately, I can’t find a product for homeowners that lists wireworm on the label. You might check with the Colorado State Extension office out at the Mesa County Fairgrounds to see if they have a solution I’m not aware of.

Wish I could help more than this.

Dennis Hill is the nursery manager at Bookcliff Gardens, online at http://www.bookcliffgardens.com. Send questions to Bookcliff Gardens, 755 26 Road, Grand Junction 81506; or e-mail .(JavaScript must be enabled to view this email address).


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