CO-Horts Blog

Friday, June 1, 2018

Fireblight running rampant

Posted by Carol O'Meara, Boulder County Extension

Symptoms of Fireblight include browning/blackened leaves
drooping in a classic 'shepherd's crook'

Pulling into my drive, I noticed the Honeycrisp apple tree we love didn’t look quite right.  From a small distance away, wilt was evident around its branches and a sinking feeling settled in my stomach.  Upon closer inspection, my worst fears were confirmed: this tree has fireblight.

To have an apple tree is to risk infection from this deadly disease, one that includes oozing bacteria, curled, brown leaves, inedible fruit, and spreading cankers.  This year, with several hail storms coming just as the tree was in bloom proved fatal.  Temperatures and moisture played a role in the infestation and my tree is entirely engulfed and without hope for recovery.

My tree isn’t alone.  In the past week, samples and emails have been brought to our office by people in similar situations, conversations sound more like support groups, and the disease is everywhere I look.  My mind cues up the dramatic, Hitchcockesque music each time I see another blighted tree.

It’s a banner season for fireblight, a bacterial disease that is especially destructive to apple, pear, quince and crabapple. It attacks in spring, when temperatures reach 65 degrees F and frequent rain occurs.  Bacteria overwintered in cankers on the tree resume activity, multiplying rapidly.  Hail drives the bacteria around and into woody tissues.

Fireblight bacteria moving back into the twigs, blackening the wood
Our wet early summer weather created good conditions for this damaging disease, and masses of bacteria have been forced up through cracks and bark pores to the bark surface, forming a sweet, gummy exudate called bacterial ooze. Insects are pickingd up the bacteria on their bodies and carrying it to opening blossoms where it infects trees. 
Girdling cankers – areas of disease on the wood - eventually develop from branch or blossom infections.  Leaves wilt, darken and curl to form a shepherd’s crook. This gives the tree a fire-scorched appearance, thus the name "fire blight." 

There is no cure for this disease, so prevention is the best solution. Remove and destroy newly infected young twigs as soon as possible, so that your tree doesn’t become the mother ship for disease in the neighborhood.  Do this when no rain is predicted for at least two weeks.  It may be best to leave pruning until winter when the bacteria are not active. In young twigs, make cuts at least 12 inches below the dark, visible edge of infection to avoid slicing into the bacteria. Remove all blighted twigs and cankered branches. Prune larger limbs about 6 to 12 inches below the edge of visible infection.

After each cut, surface sterilize all tools used in pruning. Spray tools with Lysol or dip tools in 70-percent ethyl alcohol, or a 10-percent bleach solution.  Bleach can rust tools, so if you use this to sterilize your pruners, wash them after you’re done and apply a light tool oil to keep them rust-free.

Be on the lookout for apple scab, a fungus that attacks leaves and fruit, which also favors cool, wet weather.  You’ll seeing the rapid spread of this disease across apples and crabapples.  At first, leaves get yellow or dark olive-colored spots, then turn yellow and fall off.  Fruit develops dark, greasy-looking spots that then become sunken. 

The disease overwinters on fallen leaves, so clean the area during fall.  Avoid overhead watering that can splash spores around. 


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