Fire blight hit Pennsylvania pretty hard during 2014 and 2015 and there are still a lot of reminders of the disease lurking in the orchard: cankers. If these cankers aren't removed, not only will they provide a source of bacteria to cause blossom blight during the coming season, but canker blight is guaranteed to occur, which will ultimately perpetuate the disease further creating even more cankers.
What is a canker?
Some folks may think cankers are a "good thing," assuming a canker is a plant's defense response: this is incorrect. Cankers are localized dead areas of bark and underlying wood on twigs, branches, and trunks. Cankers can be caused by living things (fungi, bacteria) or nonliving things (hail, high or low temperature, injury). For a canker to occur, a wound (entry point) is needed. It is important to remove cankers because they serve as the overwintering source for fungal spores and bacteria that cause diseases during the season, as well as create a nice environment for other fungi you do not want hanging around, such as fungi causing fruit rot.
Spotting cankers in the orchard
Cankered wood has distinct characteristics that can be easily recognized when pruning. Be on the lookout for:
- Localized roughened or cracked bark, especially around wounds, branch stubs, old pruning cuts.
- Bark that is darker than the surrounding bark tissue, which is healthy.
- Roughened/darkened areas appearing "wrinkled" or "sunken."
- Small pimple-like fungal spore forming structures - may be red, dark brown, or black (depending on the fungus).
- Wood-decay fungi, which attack dead wood and often appear as white protrusions growing out of the bark.
Photos: Kari Peter, Gary Moorman
Fire blight disease cycle
Growers need to understand where bacteria overwinter and the canker blight phase of fire blight. Bacteria overwinter in living tissue surrounding cankers formed at the base of spurs or shoots killed the previous season. Cankers also will form where cuts were made to remove infected shoots during the growing season. Bacterial populations are influenced by temperature and can grow in a range of 50°F to 90°F. Bacteria will begin to multiply at canker margins early spring, typically between tight cluster and early pink, and begin to ooze, and the ooze contains trillions of bacteria. This is important because this is when the bacteria are first available for dispersal in the orchard. The oozing bacteria jumpstarts the different phases of fire blight during the season: blossom blight, shoot blight, canker blight, trauma blight, and rootstock blight.
The canker blight phase is often a head scratcher and, consequently, grossly underestimated for its ability to cause damage in the orchard. Canker blight develops due to renewed activity by the bacteria at the margins of overwintering cankers from the previous season and occurs regularly every year where the disease is established. In other words, if cankers are left in your trees, you can count on canker blight.
The first symptom can be detected by cutting into the bark at the canker margin where you will see a narrow zone of water-soaked green or diffuse brown tissue at the margin between the necrotic tissue of the canker and the surrounding healthy bark. The bacteria move systemically (inside of the tree) from the canker into nearby growing, succulent vegetative tissue. Often times, water sprouts close to active canker sites will develop a distinct yellow to orange color and begin to wilt.
Canker blight symptoms are often overlooked because of their similarity to the more familiar shoot tip (shoot blight) infections that occur later. Another distinct feature is canker blight "shoot blight" will appear as if the infection is starting from the base of the shoot, as opposed to the shoot tip, which is characteristic of typical shoot blight.
Canker blight: infection starts at the base of the shoot and leaves. Photo: Kari Peter
In years when blossom infection events do not occur or have been well controlled, active canker sites serve as the primary source of bacteria for a continuing epidemic of secondary shoot blight infections that can lead to major limb, fruit and tree losses. Such sources of bacteria can also be important for new orchards with no history of fire blight when they occur in older, surrounding orchards from which the bacteria can be moved into young orchards by wind, blowing rain and certain insect species.
When cankers are left in older trees, they can look like this--a ~36 year old York tree. Photo: K. Peter
General pruning strategies
The only cure for fire blight cankers is cutting them out of the trees. By removing cankers you will help reduce the incidence of new infections. When pruning this season, it's best to pay extra attention to those orchard blocks with a known history of fire blight. This may require you to visit orchard blocks more than once, especially during different lighting of the day, to be able to spot cankers that could have been missed during the initial round of pruning. If trees were pruned during the season to remove fire blight strikes, you will most likely see a canker at the site where you pruned. Don't forget to remove this canker.
When you see a canker, prune 6 - 12 inches from the canker's visible edge into 2 year old wood or older since older wood is more resistant to the bacteria. This will be easier in larger trees and more challenging in smaller dwarf trees. Since the bacteria are dormant during the winter, disinfecting pruning tools is not necessary. If cankers are established in the trunk of the tree or infected trees are very young (newly planted - 3 years old), it is best to remove the entire tree. Regardless of cultivar or rootstock, these particular circumstances prove to be a losing battle. When it comes to cankered wood, it is best to burn all tissue when possible to ensure destroying any overwintering bacteria.
Prune 6 - 12 inches from the canker's visible edge. Photo: K. Peter
If you know you left some cankers when pruning or pruned trees hard…
Many trees this winter may have to have a lot of branches pruned due to fire blight infection from the last season. When these trees come out of dormancy, they will want to grow. Unfortunately, if there are any cankers remaining in the trees, there will be a very high risk of canker blight/shoot blight due to this new growth: when the tree is pumping nutrients to produce new growth, bacteria will also be in the pipeline to those same growing areas. Not to mention, these leftover cankers will be sources of bacteria to infect younger blocks nearby.
To suppress shoot blight this season, I highly recommend the use of prohexadione-calcium (Apogee or its equivalent). Cankers or no cankers (especially in a mixed-aged orchard), the use of prohexadione-calcium is a reliable fire blight management tool. Since the bacteria are most damaging to succulent growth, prohexadione-calcium will harden off shoots making them less susceptible to shoot blight. It takes 10 - 14 days for the chemical to take effect, so it is recommended to begin applications at king bloom petal fall and continue to spray every two weeks for a total of 3 - 4 applications.
As far as rate per acre to apply, here is a good explanation for an approach thanks to the folks at Michigan State:
Rate per acre is usually calculated on a tree row volume basis and can be adjusted to two-thirds of the label full rate. The two-thirds rate is the starting rate growers should consider if they don't have any experience using Apogee in the past. Past experience on your block will indicate if this rate is too high or too low per acre. This suggested two-thirds rate per acre is a season-long rate per acre.
For example, if your trees are at 75 percent tree row volume, then 24 ounces per acre is the seasonal rate (48 x 0.75 x 2/3). Best results occur when seasonal rate is split into three or four sprays, for example, 8 + 8 + 8 ounces per acre. When fire blight is a severe risk, the first application at king bloom petal fall timing should be increased, perhaps as much as 150 percent of a split rate. For example, increase from 8 ounces per acre to 12 ounces per acre. Subsequent sprays, the second and third sprays, could be reduced, so the seasonal application would be 12 + 6 + 6 = 24 ounces per season.
Cankers are host to undesirable fungi
You may come across some fire blight cankers that appear "orange." This is Nectria twig blight or Nectria canker. Characterized by bright orange colored spores, Nectria fungi only grows on dead wood, such as fire blight cankers, and will not invade healthy tissue. This fungal pathogen is considered a wound invader. If anything, this fungus helps you easily spot fire blight cankers. Nectria is removed when you prune out the fire blight canker.
Nectria (bright orange spores) only grows on dead wood (fire blight cankers) and will not invade healthy tissue. Photo: K. Peter
Cankers are dead wood and removing cankers from trees is an excellent cultural control for managing diseases, especially fire blight. Canker blight will occur this season if you leave cankers in the tree, especially if they are old, larger trees. If leftover cankers are suspected after pruning this winter, applications of prohexadione-calcium (Apogee) early in the season is an additional tool for the fire blight management toolbox to help control vegetative growth and suppress fire blight spread among shoots and within shoots. To ensure no overwintering fire blight bacteria will be hanging around next season, it is recommended to burn all infected tissue.