How to Identify and Manage Fire Blight

This disease isn’t diagnosed by identifying the organism, as with an insect, but by examining the damage. Many factors can cause a brown leaf or a dead shoot on an apple tree. My crab apple loses seemingly half of its young shoots each year to breakage from wind, fat birds thinking the branch is big enough, or even chipmunks scurrying up to check this year’s crab apple crop. But those are easy enough to identify once you follow the branch back to the break. 

To scout for fire blight in susceptible trees and shrubs, start by walking around and noting any young growth that appears to be struggling. Since this bacteria needs an entry point to the plant, it’s common for one branch to look ill while all its neighbors look green and lush. That one stem with browning and curling leaves is your clue.

When looking at new growth, browned leaves, and the telltale J-hook or shepherd’s crook shape of the younger, softer portion of the stem are very indicative of fire blight. Cankers on the trunk, especially on younger bark which would not usually have developed ridges yet, are a sign of a more established (and likely fatal) fire blight infection. Dead leaves and fruit typically are withered and browned but remain on the branches and look scorched or rusty brown in color.

If you grew bacteria on a petri dish in high school, you know they multiply rapidly in the right conditions. Droplets of bacterial ooze can start to appear in as little as a few days, furthering the spread. Fortunately, as the new flush of growth in spring and early summer transitions from soft and green to mature and more woody, those tissues become much less susceptible to fire blight infection. It’s one of the reasons to be careful with fertilizer, which we’ll discuss later.

Here are a few more identifying characteristics:

Look at the blossoms

For many of our fruiting trees and ornamentals, the blossom is the entry point, and where you may see the first signs. 

• Infected blossoms appear water-soaked, then wilt and brown. The bacteria move down the flower cluster and into the stem or spur, which will turn brown or black.
• Cool climates can reduce the problem in many plants, because flowering happens before the warm weather needed by the bacteria to grow and spread. But, even in Vermont, it still occurs regularly.
• The browned flowers will remain attached instead of being discarded as the fruit grows.

Checking shoots and stems

Infected young shoots are the easiest characteristic of fire blight to spot. A wilted blossom could be many things, but the “J” hook wilted young shoot tip sticks out like a sore thumb. (Do sore thumbs stick out? If I’ve already banged my thumb on something, I’m usually careful to tuck it away to avoid smacking it again.)

Fire blight bacteria can enter young, new growth through a wound small enough that we don’t notice it, and move quickly into the mature portion of the twig. As the disease progresses, the new shoot will wilt, the leaves will brown and shrivel, and the shoot will bend in a fishhook shape. You may see droplets of white bacterial ooze, although I’ve not noticed that as much. 

Another telltale sign is that the wilted leaves will stay attached to the twig throughout the summer. It makes it easier to find as you walk around, but of course, we’d like to find it earlier rather than later.

Are there cankers on the bark?

Look for spots on the stem, branch, or trunk where the bark is puckered and cracked. They’re easy to see on smooth-stemmed larger branches, but can also appear on those smaller than a pencil, making them difficult to spot. Look for rough, broken bark, as well as staining in dark purple, brown, or black. The bark may also peel, and the sapwood underneath can become stained a reddish-brown.