Tianna DuPont, Aina Baro, Washington State University; Ken Johnson, Oregon State University; Kerik Cox, Cornell University; Kari Peter, Penn State University. May 1, 2023.
It is important to quickly prune out fire blight infected materials soon after an infection occurs to reduce the bacterial ooze which insects and wind can spread through the orchard causing new infections, and to reduce the spread of the pathogen through the tree which can kill the tree. This article summarizes information from a new study.
We conducted ten experiments looking at the success of fire blight removal strategies in WA, OR, PA and NY. Experiments had different scions, rootstocks, vigor and training systems. We compared six therapeutic fire blight removal practices. Each treatment was replicated six to fifteen times.
Fire blight removal treatments:
- Best management practice (BMP): Cut 12 to 18 inches from edge of symptoms into two-year-old wood with sanitized shears
- Aggressive: Cut 2.5 feet from edge of infection with sanitized loppers
- BMP No-sanitize: BMP without sanitizing shears
- Long stub: Leave a 5-inch stub from structural wood, sanitize loppers
- Short stub, flush cut: Cut back to structural wood, sanitize shears
- Breaking: Break at the joint between one- and two-year-old wood by hand
- No-treatment control: No removal of the infections
Which fire blight removal treatments save trees?
Timely summer cutting of fire blight infections can significantly reduce the number of trees that die from fire blight. In five of our experiments, trees died due to fire blight infections. In these same five experiments, all of the pruning strategies reduced the number of trees that died. For example, in all four New York experiments, 100% of the trees receiving no pruning treatment died while zero trees receiving a removal treatment died.
Which fire blight removal strategies reduce new symptoms after pruning?
When pruning blight, we are trying to cut far enough below noticeable symptoms so that the bacterial cells which remain in the plant are not sufficient to cause cankers to re-occur at the cutting point. We also want to prevent the bacteria moving systemically through the infected trees in the vascular tissue and intercellular spaces from causing new symptoms in young tender shoot tips.
The standard practice BMP where we removed fire blight 12 to 18 inches from the edge of blight symptoms into two-year-old wood with sanitized loppers generally reduced new symptoms from canker re-formation or the systemic movement of fire blight bacteria through the tree. BMP reduced new symptoms in seven of nine experiments with significant differences in five of nine experiments. The exception were some experiments where no new symptoms occurred or where trees were very young and vigorous. For example, in the Washington 2020 trial the ‘Cripps Pink’ trees were fourteen-years-old and low vigor, and no new symptoms were recorded. Similarly, in the Pennsylvania 2020 trial the spring was very cool followed by hot temperatures and terminal bud set. In contrast, in the New York 2021 trial, trees were young (three-year-old), vigorous ‘NY 2’ (marketed as Ruby Frost) ’ and none of the treatments significantly reduced the number of new symptoms and cankers that re-formed after initial pruning.
Can aggressive cutting reduce new symptoms after pruning?
We hypothesized that aggressive removal, cutting 2.5 ft below blight symptoms, would remove more bacterial cells in symptomless wood and consequently, reduce the number of new symptoms caused by systemic movement of bacteria to new young susceptible shoots. Generally, the Aggressive treatment did not improve on BMP, where fire blight was removed 12 to 18 inches below noticeable blight symptoms with sanitized loppers. The exception was in a Washington trial in 2019 with ‘Yarlington Mill’ grafts. Even with aggressive removal in vigorous trees multiple passes can be necessary to remove symptoms. For example, in the New York 2019 MAIA1 trial (Evercrisp) trial with vigorous four-year-old trees both BMP and aggressive treatments reduced new blight symptoms threefold when compared to the no-treatment control but still resulted in four to five cankers per tree that reformed after initial pruning.
Can a stub cut keep fire blight cankers from reaching structural wood?
When cankers caused by fire blight infections reach central leaders and main structural branches, growers face the decision to either prune out the canker, removing large parts of the tree resulting in lost productive capacity for several years, or to leave cankers which become the source of new fire blight infections the following spring (Fig 3). Some recommendations suggest an ‘ugly stub cut’ where growers make cuts leaving a 5-inch stub (Fig 4). While small cankers will form on many of these cuts, these cankers can be removed during winter pruning (Suleman and Steiner 1994, Steiner 2000). In two of five experiments where Long Stub and Short Stub treatments were compared, a Long Stub reduced the number of cankers on structural wood with significant reductions in the 2020 WA trial on ‘Cripps Pink’. In trials where leaving a Long Stub was not helpful no cankers formed on structural wood due to low susceptibility (e.g. old Red Delicious interstems) or terminal bud set right after infections occurred (2020 PA).
Is breaking by hand as effective as removal with loppers?
In some orchards managers employ the technique of breaking of current season growth rather than cutting to remove fire blight infected wood. This practice is designed to be quick and avoid the use of loppers. With this strategy, managers break at the joint between one- and two-year-old wood by hand (Fig. 5).
Breaking off diseased branches by hand provided a rapid removal method, but it can result in a greater number of cankers in the orchard at the end of the season with more cankers on structural wood. In the experiment ‘Washington 2021 Cripps Pink’ where four-year-old trees were trained to the wire, treatment Breaking resulted in significantly more re-formed cankers and new symptoms than other cutting treatments, and was similar to the no-treatment control. In three of ten experiments Breaking resulted in significantly more canker tissue left in the tree at the end of the season compared to BMP. A larger number of remaining cankers provide a greater source of inoculum in the following year.
Is it necessary to sanitize loppers when removing fire blight?
Sanitizing pruning shears has been long considered important to prevent dissemination of fire blight infections (Van Der Zwet and Keil 1979). However, in multiple studies sanitizing shears made no difference in preventing new or re-formed canker development as long as cuts were made at the recommended distance below canker margins (Toussaint and Philion 2008; Travis and Kleiner 1997). This is likely because fire blight bacteria can quickly migrate up to several meters beyond the visible symptoms (Suleman 1992), and therefore, it makes little difference if a few bacteria are left on the cutting blade.
In our study, skipping sanitization when making the BMP cuts back 12 to 18 inches beyond visible symptoms did not have a significantly greater number of re-formed symptoms after initial cutting than BMP with sanitation in nine of nine experiments. The quantity of canker left in the tree at the end of the season was also not different between BMP and BMP NO-sanitize. In one of seven experiments BMP No-sanitize had a greater (but not significantly different) number of trees develop rootstock blight.
Sanitizing pruning loppers may reduce transfer of bacteria if managers cut through active ooze. However, in situations where there are many infections throughout the block, the speed at which crews can remove blight is critical to reduce the number of trees that die due to systemic infections. Therefore, the risk of not sanitizing loppers may be less than the risk of going slower. For example, Toussaint and Philion (2008) found that pruning quickly as soon as disease was detected greatly reduced fire blight damage compared to pruning symptoms out starting in August. In Bartlett pear, Covey and Fischer (1990) found that a two-week delay in removal increased the amount of plant material that had to be removed by six-fold.
First to third leaf trees
In very young trees, fire blight infections move quickly through the tree. When infected, removal and destruction of the whole tree is recommended. After removal, a concentrated application of Actigard (acibenzolar-S-methyl) to adjoining trees can increase the plant’s defense system and reduce the likelihood that adjoining trees are also infected.
Applying a concentrated solution of Actigard as part of pruning therapies can reduce the severity of re-occurring fire blight cankers. Oregon State University studies over five years found that applying Actigard when removing fire blight infection can reduce both the proportion of trees in which fire blight re-occurred and the rate of canker expansion (Johnson and Temple 2016, Johnson and Temple 2017). Apply concentrated Actigard with an up and down motion to a 2-foot section of the central leader or major scaffold near where the fire blight infection was removed. Use the labeled rate of 1 oz. per 1 quart of water with 1 percent silicone-based penetrant.
- Timely removal of fire blight cankers can reduce rootstock blight and tree death.
- Pruning 12 to 18 inches below the visibly diseased (cankered) tissue into two-year-old wood generally reduces new symptoms and canker reformation caused by systemic movement of fire blight bacteria through the plant.
- Aggressive removal 2.5 feet below cankers was generally not better than removal at 12 to 18 inches.
- Breaking can leave many active infections in the orchard.
- Leaving a stub can reduce canker re-formation on structural wood.
Washington State University
Associate Professor, Extension Specialist
This work was supported by the Washington Tree Fruit Research Commission grant no CP-19-102A and the United Stated States Department of Agriculture Specialty Crop Research Initiative grant number 2020-51181-32158.
DuPont, S. T., Munir, M., Cox, K., Johnson, K., Peter, K., Baro, A. 2023. Evaluation of Pruning Therapies in Apple Trees with Fire Blight. Journal of Plant Pathology Accepted in press JPPY-D-23-00119.
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