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Fire blight is a destructive bacterial disease of apple, pear and other related species such as crabapple, hawthorn, quince and mountain ash. It causes severe blighting of blossoms, shoots, limbs and fruit, and can kill young trees. Outbreaks of fire blight occur sporadically in British Columbia pear and apple orchards. Risk of fire blight is strongly related to weather conditions, history of fire blight in the area, and susceptibility of the host tree.
Fire blight symptoms may appear on the blossoms, shoots, branches, trunk and rootstock. Blighted blossoms appear wilted, shriveled and brown or black. Young fruitlets are also very susceptible and appear water soaked and slightly off-colour soon after infection. Fruitlets quickly turn brown to black and eventually shrivel up.
Blighted pear shoots are black in colour, while infected apple shoots are usually a lighter shade of brown. Infected shoots (or "strikes") wilt rapidly, and often form a shepherd's crook at their tips. During warm and humid or rainy weather drops of milky to amber coloured bacterial ooze frequently appear on the blighted shoots and fruit. Blighted leaves may remain attached to the tree throughout the winter. When shoots attached to scaffold limbs or trunks are attacked, the pathogen may spread into the structural wood causing cankers. In susceptible hosts or young trees the disease may travel rapidly down branches causing girdling and death of the branches or sometimes the main trunk.
Cankers appear as slightly darker, water soaked areas in the wood, which may produce amber coloured bacterial ooze that runs down the bark. Reddish brown streaks may be seen in the cambium under the bark of diseased branches. Later in the season the bark often cracks around the margins of the canker.
|Blossom blight on pear. Note blackening of pedicels
|Shoot blight on apple. Note browning of leaves and
shepherd's crook and at end of shoot.
|Droplets of amber coloured bacterial ooze on fire blight
affected pear shoot
|Fire blight-infected apple fruitlet, with bacterial ooze|
|Shoot blight on pear. Note blackened leaves and fruitlets||Fire blight canker on apple|
Fire blight bacteria overwinter primarily in cankers on infected trees. During spring and early summer, cankers that were not removed the previous season may produce bacterial ooze, which may or may not be visible. This ooze, consisting of millions of bacterial cells, is spread to blossoms by insects such as flies, ants, and beetles. If weather conditions are warm and humid, the bacteria are able to multiply rapidly in the blossom nectar. Bacteria can then be spread very efficiently from blossom to blossom by honey bees. This gives fire blight explosive potential if the conditions for blossom infection are met.
Once blossoms are infected, the bacteria can quickly spread into shoots and branches. Blossom infections become visible as "strikes", or dying shoots in anywhere from 1 to 4 weeks, depending on the temperature.
Infected shoots provide additional sources of fire blight bacteria, which can be spread by rain, (especially wind-blown rain), insects, and contaminated pruning tools. Secondary infections may continue to occur throughout the growing season. The worst epidemics always follow blossom infection. However, it is possible for twig or shoot infection to occur in orchards where little or no blossom infection was found. In these situations the disease pressure is usually low and the damage is more easily controlled. Bacteria can enter the host through both wounds and natural openings such as lenticels. Hail storms often result in severe fire blight outbreaks if inoculum is present in an orchard. Wind damaged leaves are also susceptible to infection.
There are several biopesticides registered for suppression of fire blight on apple and pear (see table below). These products are blossom protectants, used during bloom to reduce the incidence of blossom blight. Most of these products consist of live, beneficial bacteria or yeasts which colonize blossoms and help to prevent fire blight bacteria from getting established. These biopesticides should be used as part of an integrated fire blight management program, which includes using a risk assessment model, as well as cultural controls and antibiotic sprays when necessary. For best results, apply during early bloom at the beginning of a warming trend, and follow with Streptomycin or Kasumin 2 - 3 days later if warm temperatures continue to favour blossom infection.
Biological fungicides are often sensitive to fungicides. Check labels for tank mix precautions. Double Nickel can be safely combined with Cueva (copper octanoate) for improved effectiveness.
Fire blight overwinters in old dormant cankers in the trees. Removal of cankers is a critical step in preventing and controlling fire blight. Make cuts 15-30 cm below the canker margin. A separate operation to prune out cankers is recommended. They are most visible on bright, sunny days.
Current season infections should be cut out and burned, with oozing cankers always being removed first. Make cuts well below any visible signs of infection (40 cm) and dip tools in a disinfectant between each cut. Soaking the contaminated blade in either full strength or 1:5 dilution of household bleach, Lysol™ Disinfectant or Pine-Sol™ will destroy the fire blight organism. Commercial disinfectants such as Chemprocide™ are also available; follow label instructions. Scout frequently for new infections, and remove them as quickly as possible. Give priority to young plantings and susceptible varieties.
If disinfecting tools after every cut is not practical due to extensive infection, the “ugly stub” method of pruning may be followed, and has proven effective in other areas. Cuts are made 20-30 cm below visible symptoms in wood that is at least 2 years old using unsteriziled tools. However instead of pruning to a healthy branch union, leave a stub of 10-12 cm in length. Small cankers may form on these stubs, but they can be removed during winter pruning. Marking the stubs at the time of the initial cut with brightly coloured spray paint will make locating the “ugly stubs” much easier for later removal.
If fire blight is a problem, reduce soil moisture as much as feasible and reduce nitrogen fertilizer applications.
Do not run overhead irrigation during bloom when fire blight risk is high.
Do not plant susceptible apple cultivars next to susceptible pear plantings.
Remove alternate fire blight hosts, such as hawthorn, mountain ash and firethorn from the vicinity of the orchard.
Fire blight blossom infection is favoured by warm temperatures along with rain, dew or high humidity. In the fruit growing areas of B.C. prevailing low temperatures during blossom usually limit the development of fire blight. However in recent years hot spells during main bloom have become more common.
Monitor temperatures during the blossom period. If temperatures reach the low to mid 20's C during the blossom period, fire blight risk is high. Several warm days in a row during bloom will greatly increase the risk.
Blossom protectants and early season sprays – If weather conditions favour fire blight, protectant blossom sprays may be necessary to prevent or reduce blossom infection. Products registered for blossom blight control include antibiotics, biofungicides and copper fungicides (see table). The growth regulator Apogee is registered for shoot blight suppression, but must also be applied early in the season. Note - there are no curative treatments for fire blight. The following treatments are preventative and will not eradicate infections after they have occurred.
|Product||Crops||Type of product||Timing||Pre-Harvest Interval||Notes|
|apple, pear||antibiotic||bloom||Apple: 50 days;
Pear: 30 days
|Preventitive. Provides 2-3 days of blossom protection. Maximum 3 applications per season. Do not tank mix with other pesticides. Do not apply later than 14 days after petal fall. Keep product refrigerated.
Resistance Management: streptomycin resistance has been detected in the Okanagan. Alternate with other fire blight products for resistance management. Never spray streptomycin after an outbreak has already produced shoot blight. Resistance is most likely to occur with repeated use and when pathogen populations are high.
|apple, pear||antibiotic||bloom||90 days||Preventitive. Provides 2-3 days of blossom protection. Maximum 4 applications per season. Do not apply after petal fall. Alternate with other fire blight products for resistance management.|
|Blossom Protect (Aureobasidium pullulans)||apple, pear||biological||bloom||n/a||Preventative. Provides 2-3 days of blossom protection. Apply up to 4 times at 10%, 40%, 70% and 90% open blossoms, or up to 5 times when a forecasting model indicates risk of infection. Do not tank mix with other fungicides. Follow label instructions for mixing the 2 components in the product package, which include a citric acid buffer and the active ingredient (a mixture of two strains of living yeast cells)..|
|apple, pear||biological||bloom, post-bloom||0 days||Preventative. Suppression only. Compatible with copper. Can be applied post-bloom for shoot blight management.|
|apple, pear||biological||bloom||0 days||Preventative. Suppression only. Apply at early (1-5%) bloom. Repeat at 4-7 day intervals during high risk periods during bloom.|
|Copper Spray or Copper Oxychloride,
|pear||fungicide & bactericide||bloom, dormant||n/a||May cause russeting. Do not apply to Anjou pears or apples. Russeting may occur on Bartletts if they are wet when sprayed with fixed copper.|
|apple, pear||fungicide & bactericide||bloom, post-bloom, dormant||1 day||Preventative. Less likely to cause russet injury than other coppers. Can be applied post-bloom for shoot blight management. Do not exceed the 1% rate for apple varieties susceptible to russeting. When fruit is present on light coloured varieties consider dropping rate to a 0.8% solution. Use highest rate for dormant application.|
|apple||growth regulator||2.5-7.2 cm growth||45 days||Suppresses fire blight by decreasing host susceptibility. Does not control blossom blight. For best results, apply when shoot growth reaches 2.5 to 7.2 cm. Depending on the cultivar and weather conditions, this may occur during bloom to petal fall. Repeat at 14-21 day intervals, up to a maximum of 4 applications per season. Apogee is a strong growth regulator and will limit shoot growth. Not registered for use on pear.|
Fire blight models - There are several models used to forecast fire blight risk based on daily maximum and minimum temperatures (e.g. Maryblyt, Cougarblight). The Cougarblight model is available on the Washington State University website, or ask your field advisor for a copy. The weather network operated by Growers Supply Co. Ltd. and Field Service has numerous wireless weather stations strategically placed throughout the interior apple growing region. This service provides disease risk information to growers through their packinghouse field service.
Blossom protectants are of limited use once the blossom phase has been completed, and there is little benefit to continued applications after bloom. Streptomycin may be used up to 14 days after petal fall to protect late or rat-tail bloom. Cueva and Double Nickel can be used all season to help manage shoot blight or as rescue sprays immediately following hail damage.
Apogee is very effective for reducing shoot blight. However this product must be applied before symptoms of shoot blight are present. Consider using on apple blocks with a history of fire blight where shoot growth suppression would also be beneficial.
A delayed dormant application of fixed copper (Copper Spray) at the rate of 4 kg/ha or or copper hydroxide (Parasol Fl) at the rate of 4.7 L/ha may be applied in the spring at silvertip to greentip. Bordeaux mixture (copper sulphate + hydrated lime) may also be used. Use a high volume spray to obtain good coverage of all tree surfaces in the block, including resistant varieties. Dormant copper application will reduce the early colonization of tree surfaces by fire blight bacteria. The addition of one percent of 60-70 second emulsifiable (summer) oil to the dormant copper spray has been shown to improve control. Cueva (copper octanoate) can also be used as a dormant spray. Use the highest label rate for dormant application.
Updated November, 2018