HORTICULTURE
Prune Out Fire Blight in the Winter
SW Michigan District Extension Educator for Horticulture & Marketing
I often wonder where fire blight comes from in the Spring? I have now spent eleven years looking for signs of fire blight in the early spring. I very seldom see anything until after bloom when fire blight explodes in an orchard. Often the fire blight strikes are localized in several areas in an orchard. If I get to the orchard early enough, when the symptoms are just starting, I almost always find shoot blight symptoms on a limb that has an old canker from last year. This old canker is located in the area where the other fire blight symptoms are found. The shoot blight symptoms from old cankers appear before shoot blight from blossom infections. The old canker was the source of the infection. By the time shoot blight symptoms are wide spread it is almost impossible to find the initial source of the infection. Fire blight bacteria do not survive well outside the tree and do not overwinter outside of a host. That means the fireblight must come from trees that are already infected. Next years fire blight will come from trees that had active infections last year. Bacterial ooze from overwintering cankers will be the cause of infections during bloom.
I cannot predict what fire blight will be like next year but I do know that if you make the effort to remove all the fire blight in your orchard when you are pruning this winter that you will reduce the amount of fire blight in your orchard after bloom. I believe that growers need to reduce fire blight when they can. When conditions are right for fire blight the disease runs faster than growers and their workers can work to slow the disease.
Winter is the perfect time to remove fire blight. The disease in inactive and you are less likely to spread the disease with your pruning tools. Even if you do cut into wood that contains bacteria their numbers are small and they are unlikely to survive on the exposed surfaces of tools or pruning cuts so transferring the bacteria by pruning is every unlikely. I suggest that you prune out all the visible symptoms of fire blight. Affected tissues are easy to recognize. They appear black and dry. Often the leaves are still on in the early winter and the leaf stems often remain until spring.
Fire blight forms cankers, sunken areas on the trunk and shoots. These cankers are the main areas where the bacteria survive the winter. These cankers are always associated with shoots that were killed last year. The fire blight quickly kills young tissue and moves down the shoot to larger stems. One year or younger shoots may not cause a canker when they are growing out of much older tissue such as the trunk and old scaffold limbs. As growers move to more dwarfing rootstocks these older tissues are less common in our orchards. I do not worry much about small strikes if there is not a canker at the base of the dead shoot. If the canker ends above the union shoot union, cut the whole shoot off.
All cankers should be cut out. It is commonly accepted that if the canker has a well-defined margin, the tree has walled off the infection and further movement is unlikely. While this is generally true, I have seen enough movement from old cankers that I hesitate to recommend leaving canker if you think they are walled off. It is very hard to tell. The walling off of a canker does tell you how far back to cut. If the canker is well defined you can make the cut close to the edge of the canker. If it looks like the canker has spread past the margin or the canker has a poorly defined margin then you should cut off more. I usually recommend at least 12 inches or to the next branch whichever is more. The fact that the canker margin is not well defined indicates that the canker was growing late in the season. If the edge of the canker is well defined on all sides then growth of the canker stopped in the summer and the tree continued to grow after the canker stopped growing.
Two years ago I visited an orchard in the spring that had widespread fire blight the year before. I was looking for cankers and wanted to document the beginning of the infection cycle by seeing the beginning of oozing at bloom time. The grower had made an effort to remove all the cankers and I found very few cankers in the orchard that he and his pruning crew had missed. None of the cankers I was observing began oozing but a cut branch next to one of the cankers did. This indicated to me that the canker near the cut that had not really been walled off. There was very little fire blight in that orchard in the spring. It would have been easy for the grower to remove the fire blight in the early season because that year there was very little blossom blight and cutting it out early would have bee worthwhile.
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On large limbs with small cankers, I have seen growers clean out cankers with a chain saw by sculpting out the cankers. Another recommendation is burn the cankers with a torch. When using fire it is important to burn the affected tissue. Completely char the edges of the canker.
After you have made every effort to cut out fire blight it is a good idea to walk through the orchard a few more times to look for cankers that were missed the first time. Since you are often looking up and into the sun when pruning it is easy to miss a canker that you would have easily seen at another time of the day. It is a good idea to walk through the orchard in the morning and evening so that the sun is in different positions in the sky and lights the tree from different angles.
In the past I have seen older orchards where the grower walked away from severely infected older trees and the blight killed all the young wood less than 3 or 4 years old. By late July the trees had grown new shoots and you could hardly tell they were infected from the road. Some growers pruned out all the fire blight affected wood that winter and others waited a year and did their pruning after the tree's growth had settled down. Both strategies seem to work well and I think waiting a year was cheaper but you do have a much-increased risk the following spring if you leave it in for a year. This is not a strategy that I would recommend if there were young trees close by. Bees are the major way the bacteria are spread at bloom time and the bees can spread it into adjacent orchards.
Trees that show symptoms of fire blight for several years in row should be removed and burned. This continuous infection is an indication that the fire blight bacteria is systemic in the tree and it will continue express symptoms each year and serve as an infection source inoculating the orchard every year. Continuous pruning of the same infected trees year after year is not sustainable. The trees become unproductive due to the infection and constant pruning, and only serve as a source of inoculum for the rest of the orchard.
I think it is very important to check all your orchards for fire blight strikes. Many of the new varieties are very susceptible to fire blight and if you ignore the problem when it is small and manageable, it will grow into a disaster if conditions are right. Pruning out strikes during years when fireblight is not a major problem allows growers to reduce the small amounts of bacteria present in their orchards in the years that the bloom conditions are right for the spread of the disease. Do not wait until fire blight is your major problem before you begin to control it.
There are antibiotics that most grower use during bloom to reduce the spread of this disease but relying on antibiotic sprays at bloom and ignoring other culture practices to reduce the disease only sets you up for an unpleasant surprise if the antibiotics sprays fail due to timing, conditions or resistance.
