Colorado is said to be the worst state in the US for fireblight, and 2018 was considered by many to be one of the worst years in Colorado. Fireblight is a serious disease affecting apples, crabapples, pears, Mountain Ash and hawthorn, and sometimes quince and pyracantha (supposedly up to 73 species of plants).
It is caused by the bacteria Erwinia amylovora, native to North America. The bacteria becomes active in spring, when it oozes through cracks and wounds in infected trees and spreads in the presence of moisture and warmth, through wind and the activity of bees and flies. These vectors carry the bacteria to open blossoms, open wounds, pruning cuts, hail damage and even into the stomata pores in the leaves. The infection may take 3 weeks to appear. The symptoms include blackened new growth, often with hooked ends, brown or black leaves, dried-up immature fruits, reddish-orange bark and orange to black sunken cankers. Sometimes only branch tips die, sometimes whole branchs and occasionally the whole tree. Often the spread is faster in young growth less than 3-5 years old.
Fireblight is a very difficult disease to manage because there is no known cure. However, as bad as it often looks, it can usually be managed and the problem is often made worse by people over-reacting to what they see. In general, I would say, don’t just ignore fireblight, and don’t start whacking away making big cuts. I started working with fireblight 44 years ago, when I managed a small apple orchard in Iowa. It had been planted at the bottom of a hill so all the rain drained to it. I learned that apples with “wet feet” cannot be saved from fireblight and fungal diseases. As an arborist in Boulder I dealt with fireblight for another 30 years. But because there are so many different views on treating fireblight, I interviewed several respected arborists to get a range of opinions.
Steve Pfeifer has worked with fireblight in Boulder for 30 years. He says the main approach should be making proper cuts, cutting back to the branch collar, sometimes called The Shigo Method. He does not believe in disinfecting his tools except between jobs. He does not follow a rule about how far back to cut, but cuts back to wood that is white and not discolored. He believes in injecting trees with phosphorus acid, which is a nutrient and a “plant resistance activator”. He has seen improvement from fireblight with these approaches and has saved trees people thought were doomed. Steve is writing a booklet on fireblight, and though mostly retired, still enjoys working with fruit trees.
Josh Morin, managing partner in Taddiken Tree Service has over 20 years experience as an arborist. His approach is to remove infected tissue in the dormant period when the fireblight is least likely to be spread. If there are only one or two spots, he will remove them in the summer, when it is hot and dry, and will use alcohol or Lysol to disinfect his tool. He says when you prune out infected wood in the dormant season, you don’t have to be aggressive and make cuts that are too big.
Josh says we’re not going to eradicate fireblight here, but we can manage it. Except in extreme cases, it is not necessary to cut an apple tree down—”that’s killing the patient.” Watering to prevent drought stress is important, as is avoiding high nitrogen fertilizers. He likes helping people with their trees in this difficult environment.
Fred Berkelhammer of Berkelhammer Tree Service relies on his experience of 34 years of pruning trees in Boulder County. He says the best time to prune out fireblight is in winter after the sunken cankers have formed, and you can see where to remove the infection. Even our summers often have showers, and making pruning cuts in moist conditions opens entries when the infection can spread. He says he has almost never seen an apple die of fireblight, and that a lot of fireblight is only in the tips. The tree looks bad, like acne, but often the tree can overcome the disease itself. He removes 6″-8″ of infected wood where possible. He never sterilizes his tools in winter and maintains that there is no scientific literature that supports the idea that fireblight is spread on pruning tools.
Fred believes it is important to support trees with proper water by giving a lot of water, seldom. Frequent sprinkling just stressed the trees by driving out the oxygen. He doesn’t spray for fireblight and see the trees in his care looking good.
Chad Street is the owner of Bees and Trees. He prunes trees with fireblight after the leaves fall, sometimes in February after the cold weather. He will remove 6″-12″ of infected wood unless that would leave the tree “just stubs”, because “you’re just not going to get it all.” He disinfects his tools with Lysol, even occasionally in winter. He thinks that phosphorus acid helps with fireblight, but he applies it by spraying on the branches before bud-break. To prevent fireblight, he believes in winter watering, not using too much nitrogen and not waiting too long to do proper maintenance pruning. He prefers a gentle nutrition program like applying compost and corn gluten.
Being a tree lover myself, I really appreciate that these arborists, who spend their lives caring for trees, are not in a hurry to cut down an apple covered in fireblight, but are willing to try care and give them some time to recover. Many of the trees with fireblight in 2018 had shown resistance up to the hail and moist and stressful conditions.
In addition to these good suggestions, I would like to add some from my own experience and research. Fireblight infection is related to stress, and so cultural conditions that reduce stress will support health, like: deep watering when needed, good air circulation by correct siting and periodic thinning of branches, avoiding herbicides, fungicides and pesticides that kill soil life, mulching under trees, avoiding chemical fertilizers, but applying organic fertilizers in September along with minerals from rock dust or seaweed. Proper nutrition in fall can make the tissue less digestible to disease organisms.
Also it is important to choose trees with strong rootstocks. Most dwarfing rootstocks weaken the tree making them more vulnerable to drought, diseases, pests, blow-over and short life. Geneva and M7 are the best semi-dwarf rootstocks; M9 and M26 are the most susceptible. A standard rootstock that can let the tree grow to 20′-25′ is stronger and more resistant to all diseases. They can be pruned to favor horizontal growth in order to keep the height to 15′-20′ or even shorter.
I believe in keeping my pruning cuts as small as possible because small wounds heal faster, and pruning cuts are more entry points for the disease. In really bad years, I will spread a tarp under my very susceptible Jonathan apple, and in addition to pruning cuts, simply snap off little bits of fireblight at the ends of branches with my fingers. Fred Berkelhammer calls this “tip fireblight”, and I have seen that tiny bits of fireblight often don’t spread into the branches. Fireblight in pear is managed in the same way but is more susceptible and the disease cankers may girdle the branches and kill the tree. Diseased prunings can be burned, buried or landfilled.
Research by a team from the US Department of Agriculture discovered that fireblight does Not overwinter in the vascular system. Therefore, when trees are dormant, only blighted shoots and cankers need to be removed. Overpruning leads to overproduction of new shoots that are the most vulnerable to fireblight. Washington State University says fireblight pruning in winter can be made at the next “horticulturally sensible” site below a canker. Therefore cutting back 12″-16″ as is often recommended is not necessary.
This will be the last year antibiotics will be allowed on certified organic orchards. The old lime-sulfur sprays both thin the blossoms and kill fireblight bacteria, but is tricky to use and not sufficient as a solution. There are some newer biological remedies like Blossom Protect, a yeast, that can be helpful. And there is a Bacillus subtilis that is fermented to produce an antibacterial solution. And Potassium phosphite can reduce infection. Some studies offer impressive results like 43% reduction with one product and 70% with two or more products and several applications timed to contact the open blossoms, again and again. Formulas like: lime-sulfur plus fish oil and two applications of Blossom Protect followed by Seranade B. subtilis seem much too difficult and expensive for the average homeowner. However there may be better solutions coming.
It is easy to recommend fireblight resistant varieties, but there is so much disagreement about which ones are resistant that there are no sure bets because so much depends on cultural conditions. Even a highly resistant variety can get fireblight growing in an over-watered lawn. Here is a list of resistant varieties I have collected from several local sources.
Fireblight-Resistant Varieties of Apple
Pears with some resistance
Crabapples with resistance
VERY SUSCEPTIBLE VARIETIES