Fire Blight- a Nasty Disease

We are seeing some unfortunate incidents of fire blight in our area, very much connected to the extended rainy spring we experienced. Fire Blight (Erwinia amylovora) is a devastating, yet common bacterial disease affecting an estimated 75 species of plants in the Rosaceae family. Most susceptible are apple, crabapple, pear and ornamental pear trees, however other host plants include cotoneaster, hawthorn, flowering quince, raspberry, blackberry, mountain ash and spiraea.

The fire blight bacterium over winters in cankers on infected host plants. In spring, when temperatures get above 60 degrees and conditions are wet and humid, the cankers begin to ooze, attracting insects to it. This is the primary way the disease is spread- ooze to insect to blossoms. Other major means of infections are open wounds, perhaps caused by storm damage, rain splash and pruning equipment.

Symptoms of fire blight are wilting shoots, cankers on branches and blackened leaves, giving the trees a scorched appearance! Young twigs and branches die from the terminal end and may bend at the tip to look like a shepherds hook, while the dead leaves remain on the branches; both ugly, tell-tale signs. As the bacteria spreads, cankers appear as sunken or cracked area on branches; and the cycle is ready to repeat.

Very unfortunately, there is no cure for this disease, so prevention and early management are key. Known defenses are

  • To plant disease resistant varieties
  • Fertilize only if warranted via soil testing; excess nitrogen encourages lush new growth which is more susceptible to the disease
  • Monitor often in spring and again in mid-winter. Early detection is critical for management of disease.

If infection does occur, prune infected twigs and branches. Recommended time to prune out fire blight infections is mid- winter when bacteria is in dormant state. However, remove rapidly advancing infections on young, very susceptible cultivars of pear, Asian pear and some apple varieties as soon as possible in the spring when weather is dry.

When pruning during growing season, disinfect pruning tools after each cut with rubbing alcohol or a 10% bleach solution (one part household bleach to nine parts water, equivalent to 1.5 cups bleach to 1 gallon of water) to avoid spreading the disease. All cuts should be at least 8-12 inches below the site of infection.

If a fire blight canker appears on a major limb and is less than 50 percent around the branch, you can try to stop the further spread of disease by scraping off the infected area of the inner and outer bark, clear down to the wood. Treat this area with a 70 percent alcohol solution.

Take a deep breathe, and then go ahead and remove trees that have cankers on the trunk, infection is covering more than one-third of the tree, or has been systematic for several years in a row. These things indicate the fire blight bacteria is systemic in the tree and will only continue to serve as an infection source to nearby host plants. Burn or bury infected plant material.

Obviously, this is a sad subject and a huge loss to homeowners, especially as this disease has such poor prognosis. Copper compounds and antibiotics are available for commercial growers, but cost, complicated spray cycles, labor extensive applications and limited effectiveness make these options unavailable and impractical for the homeowner.

If you suspect your tree has fire blight or have more questions about this disease, call the Master Gardeners at your local Extension office or send a sample to the U of I Plant Clinic. You can also learn more about disease resistant varieties, or other plant questions you may have.