What causes bacterial disease fire blight?
Thanks to our unusually wet, cool spring in 2016, I’ve noticed lots of bacterial and fungal issues in local landscapes and gardens. And with unusual weather comes unusual, or at least less common, questions about plants with unusual symptoms.
Cool, wet conditions are perfect for the growth of many fungi and bacteria that we don’t see most years, so we forget about them. But the spores are there, just waiting for conditions to be right, then they burst onto the scene, to take advantage of their uncommon good fortune.
Oak leaf blister was rampant on our live oaks again this year, and black spot on our roses was also particularly bad.
And in Adkins, some of George Byrnes’ cotoneasters have browning branches, while others look fine. This damage looks like fire blight, a bacterial disease common to certain plant families, and devastating to some fruit trees, such as pear and apple.
George has had these plants about 10 years and pruned them this spring. Well, George, the pruning likely did not have anything to do with your fire blight problem. This bacterium normally develops in cankers in the woody tissue, then moves to infect tender new growth as it’s developing and more vulnerable to invasion.
When it rains in spring, spores get bounced from stems and branches onto flowers and new growth, infecting the tissue and starting the process over again. Fire blight is cyclical, with alternate plant hosts, so it may not even be a problem most years.
In your situation, I’d recommend pruning out the affected areas, making sure to clean up any leaf or branch litter around the plant. Late winter next year, check the bark for any oozing cankers, and prune out those areas, making sure to toss all of that infected plant material in the garbage.
With regular checking, you should be able to get the issue under control and make it manageable, without having to do anything more than cultural maintenance.
Fire blight is also common on spiraea, which we’ve seen in other gardens. After years of extended drought and heat, we forget that diseases were ever even an issue in our gardens, and when they return, they catch us by surprise and we wonder what we did wrong, or how we could’ve prevented the problem in the first place.
But take heart, in most cases, there isn’t much we could’ve done, outside of planting less susceptible species. So if the problems persist, consider removing these plants and replacing them with species that are not susceptible to fire blight.