CO-Horts Blog

Thursday, July 24, 2014

A Darn Good Year for Fire Blight

Posted by: Alison O'Connor, Larimer County Extension

It seems that the summer of 2014 has been the "best year ever" for fire blight. I've never seen it so much and for awhile, our Extension Office phones were ringing off the hook about people seeing brown branches and crooked twigs from this bacterial disease. Fortunately CSU has a very good Fact Sheet on the subject.

If you haven't seen fire blight, it's characterized by the tip of the branch curling over, resembling a Shepard's crook:
The Shepard's crook of fire blight
If you look further down the branch, most of the leaves will be brown and you'll see a distinct darkening along the stem where the canker has infected tissues:
The bacteria turns the branch/stem a dark brown/black color
So what is fire blight? Well, as I mentioned, it's a bacterial infection caused by Erwinia amylovora that only occurs on plants in the Roseaceae family (apples, crabapples, mountain-ash, hawthorn, etc.). Remember, the Roseaceae family is one of the largest families in our landscapes, so it can affect a number of plants.

Infection can occur in a number of ways...through cracks in the bark or pores in leaves, on insect bodies, by splashing of the spores via rainfall or irrigation and through other natural openings. There seems to be a correlation between fire blight and hail-damaged trees from the previous summer. The bacteria kills cambial tissue (where the xylem and phloem are located) and continues to move down the branch.
Everything brown on this tree is fire blight
It's important to note that the tree will try to ward off the spread of the bacteria. Some species and cultivars are better at doing this than others. While a tree may be labeled as "resistant" to fire blight, it can still get the bacterial infection, but it's better at "sealing" off tissues to stop infection. The bacteria survives winter inside infected tissues and tends to ramp up during warm, wet springs. There is generally bacterial ooze associated with this disease.

So what can you do? Well, the best control is to plant resistant varieties. But that's easier said than done. You can prune out infected tissues during the growing season, but it's crucial that you sanitize your pruning tools between each and every cut. That means you have to spray them with Lysol or Listerine or dip them in a 10% bleach solution. Does this sound like a pain to you? It is. I've done it.

Or you can live with the ugliness of the tree this summer and prune out infected areas during the dormant season. By waiting, you don't have to sanitize. But you have to wait.

Regardless of when you prune, the current recommendation is to cut 8-12" below the edge of visible infection. This is a lot of extra branch you're taking off. And if the infection has moved into the main trunk of the tree, it's best to make a final pruning cut at the base.

For a personal anecdote, I was just at the Gardens on Spring Creek in Fort Collins. One of their apples was absolutely annihilated with fire blight. The director and the city forester were discussing the tree and they came to the conclusion to remove the tree. After making all the pruning cuts to remove the infected branches, the tree would have looked unsightly and wouldn't resemble the proper shape.

There are some pesticides that can be used, both as a dormant control and during flowering. The University of Minnesota Fact Sheet has information on the various types of pesticides that can be used to prevent new infections.

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