Fire Blight is a very common problem along the Front Range of Colorado and many people wonder how to treat for it, or if it’s even possible. Treatment is often possible when detected early. First let’s cover some of the basics:
Causes of Fire Blight
Fire Blight is caused by a native bacteria Erwinia amylovora that overwinters in plant cankers and is spread by air, insects, rain, pruning equipment and animals. The usual mode of entry into a plant is through the flowers; the bacteria can build to great numbers on flower blossoms and subsequently enter the plant. From there it travels down the xylem and infects the twig. It can continue to travel down a branch and to the trunk where it forms cankers and builds up enough cells on the surface of the bark to be visible with the naked eye.
Fire Blight can also enter a plant through any kind of wound on the leaves or bark. This could be caused by pruning, animal feeding (voles and squirrels), insect feeding (such as caterpillars and borers, probably not aphids and mites), and hail. In Northern Colorado, hail is a big problem for susceptible plants.
Many plants in the Rose family are susceptible. In particular: Pear, apple, crabapple, mountain ash (but not other types of ash), serviceberry, hawthorn, pyracantha, quince, plum and cherry are all susceptible to one degree or another. Some cultivars are resistant, some seem to get it no matter what. Plants not in the Rose family will not and can not get Fire Blight, thankfully.
The best way to manage Fire Blight is to avoid planting susceptible trees
First signs and symptoms appear as in the large image above. Shoots will start to die back and the leaves will look scorched as if by fire. The leaves will curl up and get crispy brown, and stay on the branch for many months. The smaller branches tend to discolor bronze/ orange/ black and take on a wrinkled appearance (next image). Bark may also form papery bronze flakes. To me this change in appearance helps me identify Fire Blight from freeze die-back, where branches will not take on the bronze/ orange or wrinkled characteristic.
Once fully established, cankers (sunken discolored areas) will form in stems and branches where the bacteria multiply prolifically. Any apparently healthy tissue beyond (outward) the canker will eventually be affected. The entire area in this image is a canker.
I do not believe that Fire Blight cankers can be cured. Therefore, all infected areas should be pruned out thoroughly and carefully in late summer, fall or winter. This may mean the entire tree, in which case it’s pretty hopeless, although trees can sometimes live several years or more with extensive Fire Blight infection. There are lots of good resources online talking about how to prune out FB so I won’t get into it here, but the takeaway is it needs to be pruned out aggressively or it will only get worse.
Treatment of Fire Blight
Treatment should really only be viewed as a preventative measure. If your tree has FB, it should be pruned out completely and then we can look at doing a treatment. It doesn’t really matter which is done first, but pruning has to be done in addition to a treatment. Basically, there are two options: A spray of the flowers to help prevent new infection through blossoms, and a systemic trunk injection of antibiotics.
Fire Blight spray: Can only be done in the spring while the tree is flowering. Pretty useless any other time of year except maybe immediately after a hail event. Copper sulfate, Bordeaux solution, horticultural oil, potassium bicarbonate and Stroptomycin are some of the more common sprays used and also probably the least effective. I prefer Oxy-tetracycline or Streptomycin if an antibiotic is desired, or the biological Bacillus subtillis if an Organic treatment is wanted. Both are quite effective at reducing the local Fire Blight population and therefore helping prevent new infection.
Fire Blight trunk injection: This treatment can be done in mid to late spring and should last a couple months or more. It can protect the plant from new infection not just through flower blossoms but also the other modes of entry such as hail and animal damage because it is systemic and reaches all parts of the tree. I’ve found it to be generally effective and worth the expense. The only problem is it takes time to do the treatment and so the cost per tree is higher than the spray. For one or two trees it’s easily doable but much more than that and it can be prohibitively expensive. One small-medium tree is about $130 depending on the size, as of 2018.
This picture shows the trunk-injected treatment being done on an ornamental pear.