Written by Tianna DuPont, WSU Extenson. May 2018.
Unfortunately for the third year in a row we have had multiple severe fire blight infection periods during bloom. Warm conditions followed by a rainfall event during bloom are prime conditions for fire blight infections. As folks are cutting strikes out of their orchards there have been multiple questions and so here is a review on cutting blight.
- Cut AT LEAST 12-18” below the noticeably infected area (summer cutting).
- On susceptible varieties or young/vigorous trees remove whole limbs or whole trees.
- On young trees with many strikes remove the whole tree.
- Cut EARLY to increase the likelihood you will save the tree and not spread the disease.
- Leave a stub ‘ugly cut.’
- In orchards with so many strikes that most of the tree would need to be removed, severe pruning can stimulate new growth that can become infected. In these older blocks consider waiting until fall/winter to cut or walking away (removing block).
- Cutting quickly is more important than sanitizing tools.
- Actigard is a plant stimulator. Do NOT spray the cut. Spray a concentrate on 1.5 feet or more of limb/trunk below the cut if used.
There are two reasons to cut out fire blight strikes: 1) to remove the innocula (bacterial ooze which can stimulate secondary infections; 2) try to save the tree from systemic infections which can travel through the tree killing limbs and even the rootstock in susceptible trees (young, vigorous, or susceptible varieties, ie Concord pear).
Remove the innocula. Infected clusters and shoots can ooze bacteria. When this ooze is moved around the orchard it can cause additional infections.
Remove bacterial cells to try to stop systemic infections in the tree: new strikes, rootstock blight.
An infected shoot has many millions to billions of pathogen cells where they are most concentrated towards the tip. By cutting this branch we hope to remove many of these cells so that they cannot flow down into the tree where they possibly reach the root or other susceptible tissue. Cut AT LEAST 12 to 18 inches below the noticeably infected area in order to remove the highest concentration of pathogen cells. You will not be able to remove all of them. It is a numbers game. Hopefully you remove enough to keep the infection at bay.
Growers often debate how far to cut. Cut hard, AT LEAST 12-18” below the noticeably infected area. Usually remove the entire limb back to a 4-6” stub from the trunk. Fire blight travels more quickly in one and two year old wood. Cutting back to ‘old wood’ (3+ years) makes it more likely that a large enough number of bacterial cells are cut. How far you cut depends on the age, vigor and variety of the tree. Young vigorous trees are more susceptible and need to be cut harder and sometimes removed. Extremely susceptible varieties/rootstocks also need to be cut harder or removed. For example, Bosc/Concord/Bartlett all have to be cut back much further. An analogy used is cancer. It is important to cut out as much of the infected cells as you can to decrease the likelihood of reinfection and the disease going systemic. Unfortunately, research does not have all the facts for cutting blight. Below is an example of what one grower tells his crew.
Rules for cutting mature trees:
- Cut at least 12-18” below the canker. Usually remove the entire limb back to a 4-6” stub from the trunk.
- If the rebound shoot from a cut gets blight, cut tree to one nurse limb.
- If a rebound shoot gets blight, rip out the tree (The rootstock is likely infected).
Rules for cutting young trees:
- Cut hard, at least 12-18” below the canker. That usually means a foot down the central leader from the branch that was infected.
- Three strikes and you are out. Young trees (1-4) years with three strikes or more should likely be removed.
- If a rebound shoot gets blight, rip out the tree (rootstock is likely infected).
Leave a Stub
Do not make the cut all the way back to another healthy limb or spur. Instead, leave a 4-inch-long (“ugly”) stub. Mark stubs to make them easy to locate and (spray paint works well) and remove any new cankers that develop during dormant pruning. Conventional pruning all the way to leader/limb leaves cankers regrowth in the tree. With a stub, new cankers will form in the stub, which can be cut off the next winter.
Cutting out strikes as soon as feasible can help reduce diseased trees. For example, a study in Canada (Tousaint et al, 2008) showed that cutting upon disease detection was significantly better than starting late (in August) or waiting till winter. See Figure 3. In another study in Washington cutting as soon as wilting was noticeable in Bartletts reduced the amount of material cut six times compared to waiting 2 weeks (Covey and Fischer 1992).
When not to cut.
In orchards with so many strikes that most of the tree would need to be removed, severe pruning can stimulate new growth that can become infected (Shtienberg et al. 2003). In these older blocks consider waiting until fall/winter to cut or walking away (removing block).
Dr. Ken Johnson, Oregon State University, has found that painting a concentrated solution of Actigard Acibenzolar-S-methyl (ASM, Actigard 50 WG) on trees after cutting out infection reduced the severity of re-occurring fire blight cankers. Actigard is a product that triggers the plant’s defense system. Its mode of action is to mimic the plant hormone, salicylic acid, which is responsible for priming the plant’s defense system. With Actigard applications, both the proportion of trees in which fire blight came back and the rate of canker expansion was reduced (Johnson and Temple 2016, 2017) (Figure 4-5).
During the summer, cut out blight when you see it. Removing a strike can greatly reduce further damage on the tree, especially if you catch the strike early. Apply concentrated Actigard (generally with a small sprayer) with an up and down motion to a ½ meter length of the central leader or major scaffold near where the blight was (Figure 6). The material must be applied to a large enough surface area to have sufficient active ingredient move into the tree where it can stimulate the plant ‘immune system.” Use the labeled rate of 1 oz/ 1 quart
of water mixed with silicone based penetrant (1%). One quart will treat approximately 50 cuts. IMPORTANT: This product does not work to kill the bacteria directly. Applying directly to the cuts made will NOT be effective. Applying to tools to sanitize is NOT effective and an illegal use.
- Apply concentrated Actigard on 1.5 foot of central leader/main scaffold (BELOW) the cut.
- Edits Paint trunks of replacement trees (avoid buds and roots).
- When pulling a tree paint the two trees adjacent to the one pulled.
- In 1st or 2nd leaf trees pull and replace – don’t prune.
- 3rd-10th leaf prune + ASM paint.
Should I spray my infected orchard in summer to prevent new shoot infections?
Coppers such as Previsto and Cueva kill pathogen cells and can help protect susceptible leaf tissue when there is a lot of innocula in the orchard from a bloom infection or hail that is on the horizon. But remember when the tree is growing fast new leaf tissue quickly outgrows a recent spray. These products will not ‘cure’ fire blight which is already inside the tree.
Should I sanitize my tools?
Though careful sanitation is mentioned almost everywhere in the written literature, currently researchers believe that sanitation makes little difference when cutting blight. The blight bacteria stream down the tree way ahead of the visible symptoms. When you cut the current strikes, the bacteria are still inside the bark below that cut, but in numbers too low to cause symptoms. However, as bacteria move through the tree they can concentrate in susceptible tissue such as young wood at shoot tips or susceptible rootstock and cause new infections. For example, Tousaint et al, 2008 (Figure 3) saw no benefit to sanitizing tools while cutting blight compared to not sanitizing.
Covey, R. P., and W. R. Fischer. 1992. TIMELY CUTTING OF FIRE BLIGHT INFECTIONS REDUCES LOSSES. Edited by T. Deckers. Vol. 273, Fifth International Workshop on Fireblight. Wageningen: Int Soc Horticultural Science.
Kleinhempel, H., and M. Nachtigall. 1987. “Disinfection of Pruning Shears for the Prevention of Fire Blight Transmission.” Acta Horticulturae no. 217:211.
Lecomte, P. 1992. RISK OF FIRE BLIGHT INFECTION ASSOCIATED WITH PRUNING OF PEAR TREES. Edited by T. Deckers. Vol. 273, Fifth International Workshop on Fireblight. Wageningen: Int Soc Horticultural Science.
Steiner, P. W., and P. Suleman. 1993. “A physiological model for fire blight symptom development in vegetative apple tissues infected by Erwinia Amylovora.” Acta Horticulturae no. 338:217.
Toussaint, V., and V. Philion. 2008. “Natural epidemic of fire blight in a newly planted orchard and effect of pruning on disease development.” In Proceedings of the Eleventh International Workshop on Fire Blight, edited by K. B. Johnson and V. O. Stockwell, 313-+. Leuven 1: Int Soc Horticultural Science.
Shtienberg, D., M. Zilberstaine, D. Oppenheim, S. Levi, H. Shwartz, and G. Kritzman. 2003. “New considerations for pruning in management of fire blight in pears.” Plant Disease 87 (9):1083-1088. doi: 10.1094/pdis.2003.87.9.1083.
WSU Extension Specialist
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