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?What is Fire blightFire blight is a destructive bacterial disease of apple and pear, recognized by a severe blighting of blossoms, shoots, limbs and fruit. It also affects several ornamentals in the same family such as hawthorn, mountain ash, cotoneaster, quince and firethorn. Outbreaks of fire blight occur periodically in British Columbia pear orchards. Fire blight has also been an increasing problem in high density apple plantings of newer, more susceptible varieties.
|Shoot blight on young Fuji apple. Note shepherd's crook and browning of leaves at end of shoot.|
?What is Fire blight
|Shoot blight on young Fuji apple. Note shepherd's crook and browning of leaves at end of shoot.|
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.
|Shoot blight on pear.|
Note shepherd's crook and blackening of leaves (Photo courtesy of P. Sholberg, Agriculture & Agri-Food Canada, Summerland Research Station)
|Fire blight-infected apple fruitlet, with bacterial ooze|
|Fire blight infection of inoculated pear fruit. Note bacterial ooze and water-soaked, brown lesions.|
Shoot blight caused by reactivating cankers in the spring is known as canker blight. Bacteria that overwintered in holdover cankers begin to multiply and invade nearby shoots or water sprouts. Invaded shoots wilt and die, and may be mistaken for early shoot blight. Reactivating cankers may also expand to girdle limbs.
Fire blight may also spread into the root area, leading to tree death. Rootstock blight may not exhibit typical fire blight symptoms. It is sometimes mistaken for crown rot.
Apple nursery stock with fire blight infection in the rootstock was observed in 1994 in several Okanagan orchards. Infected trees weakened and died beginning 1 to 3 months after planting. Oozing often occurred near the base of the tree. Cutting into the crown area below the bud union, or in some cases the roots, revealed dead, brown tissue. In some cases, leaves appeared yellow or reddish in colour early in the season -- a general symptom of root problems.
|Fire blight rootstock blight (Gala on Ottawa 3). Note brown discolouration under bark and droplets of amber-coloured bacterial ooze on rootstock.|
Fire blight blossom infection is favoured by moderately high temperatures and rainy or humid weather. Temperatures of 18oC and above favour rapid infection whenever moisture or dew is present. Temperatures below 15.5oC retard blight development. In the Okanagan, prevailing cool temperatures during the main blossom period are normally not favourable for infection. Infection is more likely to take place during secondary bloom when temperatures are higher.
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.
|Blossom blight on pear. Note blackening of pedicels (flower stems).|
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.
One or more separate operations to prune out cankers are recommended. Since cankers may be hard to locate, it is always best to go over the orchard several times. Cankers are most visible on bright, sunny days. Make cuts 15-30 cm below the canker margins. It is not considered necessary to sterilize pruning tools during the dormant season. Do, however, disinfect your tools if spring pruning is extended into late spring when temperatures have warmed up and/or the budburst stage has arrived. These conditions may also reactivate the infectious bacteria in cankers. In fact, an additional inspection for cankers around the budburst stage may reveal cankers that were missed earlier.
Scout for new fire blight strikes every 3 or 4 days. Frequent scouting will aid removal of new infections before they have a chance to invade the structural wood.
Where infections occur on shoots attached to scaffold limbs or the trunk, it is not always possible to cut back 30 cm without sacrificing the limb or even the tree. An option on large trees is to scrape out discoloured inner bark using a hatchet or knife, down to clean wood, and disinfect the cut surface.
Cankers often form at the sites of pruning wounds, where blight was cut out during the summer. Such cankers may be hard to detect. To overcome this problem, it is sometimes recommended that a short (10 cm) stub be left beyond the next healthy spur or branch union when pruning out strikes. Remove the stub later, during dormant pruning. Marking the stubs with bright paint will make them more visible.
|Fire blight canker on apple branch|
During severe epidemics, give priority to young trees and high density plantings. Concentrate on salvaging as much of the tree structure and bearing surface as possible. Excessive pruning during the summer will encourage a late flush of growth, which will be susceptible to continued infections.
Summer pruning (other than removal of strikes) should be avoided during a serious outbreak, due to the danger of spreading the disease. If there is any fire blight in the area, disinfect your tools while summer pruning. Avoid pruning during wet weather or when storms are expected within the next 24 hours.
Root suckers and rootstock sprouts may put the entire tree at risk if they become infected. Common dwarfing rootstocks such as M9 and M26 are highly susceptible to blight. Do not cut rootsuckers or rootstock sprouts during a blight outbreak, because the wounds may become infected. They may be safely removed during the dormant season.
Don't over fertilize the trees. Excess nitrogen causes vigorous shoot growth, which is more susceptible. Nutrient application should be balanced, preferably based on soil and leaf analysis. Application of fertilizer should also be timed to avoid a late flush of growth, because late season infections are more likely to produce cankers that allow the bacteria to overwinter.
Do not run overhead sprinklers while blossom is present on the tree and weather is favorable for fire blight infection. Overhead irrigation may increase fire blight by splashing bacteria around from tree to tree, and by increasing moisture and humidity levels in the canopy. Cutting back on irrigation may also help to slow down over-vigorous trees.
Control insects with sucking mouthparts such as aphids, leafhoppers and pear psylla. These insects can spread fire blight.
For comments on spray timing, refer to sections on blossom protection and forecasting fire blight.
Also see the Tree Fruit Production Guide for application rates.
Streptomycin 17WP is registered for control of fire blight on both apples and pears. It is effective for blossom blight control during warm (over 18oC) temperatures. Streptomycin provides better control than copper, and does not cause fruit russet. It only provides 2 to 3 days of protection because it breaks down quickly when exposed to sunlight.
Streptomycin is most effective when applied as a dilute spray to open blossoms. Better absorption is obtained during slow drying conditions. For best results, do not tank mix streptomycin with other pesticides. Streptomycin has a 50 day pre-harvest interval on apple, and 30 days on pear.
Streptomycin should always be stored in a refrigerator, or effectiveness will decline rapidly. Refrigerated product will keep well for two years, with a gradual decline in effectiveness from two to five years.
Fire blight resistance to streptomycin is known to occur in the Okanagan Valley of British Columbia, and throughout most of the Pacific Northwest of the United States. To help preserve any remaining effectiveness, never use streptomycin after the blossom stage, and do not use more than 3 times per season. Alternate with Serenade, Bloomtime or Blightban 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.
Copper oxychloride 50% WP (Clean Crop Copper Spray, Guardsman copper oxychloride) is registered for blossom blight control on pears, crabapple and other susceptible ornamental trees. It is not registered for blossom protection on apples because it will russet the fruit, and also should not be used on Anjou pears for the same reason. Copper oxychloride should not be used on wet foliage. Bartletts sprayed with copper during wet weather have suffered russeting.
Clean Crop Copper Spray is also registered for dormant (silvertip) or post harvest application to apple. Dormant copper applications have been shown to reduce or delay the production of inoculum in overwintering cankers. The addition of a light (summer) emulsifiable spray oil to the dormant copper spray (1 L per 100 L solution) has been shown to increase the effectiveness of the treatment.
Bordeaux mixture can also be used for fire blight. It is registered for application to the blossom stage of apple and pear, but should be used with caution due to the potential for russeting. Follow the same precautions as listed for fixed copper sprays. Bordeaux mixture must be prepared on the farm by mixing copper sulfate, hydrated lime and water. The addition of lime to the copper sulfate helps to reduce but may not entirely eliminate the phytotoxicity (burning action) that would occur if copper sulfate was used alone.
To prepare Bordeaux mixture, dissolve 1 kg of tri-basic copper sulfate (Clean Crop Copper 53W), or 2 kg of 25.2% copper sulphate in 1000 L water in the spray tank. Turn on the mechanical agitator. Premix 6 kg of hydrated spray lime in a pail in enough water to make a slurry. Pour the lime slurry through a 0.3 mm screen into the spray tank. Allow 15 minutes of mixing before starting to spray, and keep agitator running while spraying. Bordeaux mixture is not compatible with other pesticides. Apply as a dilute spray only, using 3000 L/ha. Do not use as a concentrate. Do not use Streptomycin after using bordeaux mixture, as the high pH of the Bordeaux will break down the streptomycin.
Note: not all brands of copper sulfate are registered as pesticides. Check the label -- it should have instructions on mixing and application for fire blight control.
Caution: Bordeaux mixture is corrosive. Sprayers should be washed thoroughly after use. A protective coat of oil will make clean-up easier by preventing Bordeaux from sticking to paint.
Apogee (prohexadione ca) is a plant growth regulator that is registered for suppression of the shoot blight phase of fire blight on apple. Apogee does not control the blossom blight phase of fire blight, and it is not a bactericide. Apogee will suppress the shoot-blight phase of fire blight when applied 7 to 10 days prior to the onset of conditions favourable for shoot-blight development. For best results, apply when shoot growth reaches 2.5 to 7.2 cm according to label directions. Depending on the cultivar and weather conditions, this may occur during bloom to petal fall. Make subsequent applications at 14-21 day intervals, up to a maximum of 4 applications per season. Apogee is a strong growth regulator and will limit shoot extension. When using the high rate for fire blight suppression, shoot extension may be reduced more than is desired in non-vigorous plantings. Apogee is not registered for use on pear.
Serenade Max, Bloomtime Biological FD and BlightBan C9-1 are three new biopesticides registered for suppression of fire blight on apple and pear. These products are blossom protectants, used during bloom to reduce the incidence of blossom blight. The products consist of beneficial bacteria (Bacillus subtilis in Serenade, and Pantoea agglomerans in Bloomtime and Blightban) which colonize blossoms and help to prevent fire blight bacteria from getting established. For best results, apply during bloom at the beginning of a warming trend, and follow with a streptomycin spray 2 or 3 days later if warm temperatures continue to favour blossom infection. These biopesticides should be used as part of an integrated fire blight suppression program, which includes using a risk assessment model, as well as cultural controls and streptomycin when necessary. Do not use copper just prior to or during bloom, as it is toxic to the biopesticides and can affect their performance. Bloomtime and Blightban should be kept frozen prior to use.
Serenade Max: Begin applications at early (1-5%) bloom if weather conditions are warm enough to be favourable for fire blight. Repeat at 4-7 day intervals during high risk periods during bloom. Best used in rotation with streptomycin for improved control and resistance management. Also registered for suppression of powdery mildew on apple.
Bloomtime Biological FD Biopesticide: Apply at 15-20% bloom, followed by a second application at full bloom to petal fall. Ensure thorough coverage of the blooms. Do not apply after fruit set. Do not exceed two applications per season.
BlightBan C9-1: Apply at 15-20% bloom followed by a second application at first petal fall to full bloom, and a third application at "rattail" bloom for pear, or post petal fall for apples. Ensure thorough coverage. Use the higher rate under high disease pressure. Maximum of three applications per crop season.
Blossom infection may be reduced by spraying with Serenade, BlightBan, Bloomtime, streptomycin or fixed copper. Repeated sprays at 72 hour intervals may be necessary if weather conditions remain warm and moist while blossoms remain on the tree. Do not apply copper to Anjou pears or apples after green tip, due to the danger of fruit russeting. Russeting may also occur on Bartletts if they are wet when sprayed with fixed copper. Copper is not compatible with BlightBan or Bloomtime.
If secondary blossoms could be removed, the risk of fire blight infection would be greatly reduced. It may be feasible to pinch off late bloom on small trees.
|Pear orchard with extensive fire blight infection|
The Cougarblight model, developed by Tim Smith at Washington State University, is very easy to use. It is necessary to note the daily maximum and minimum temperatures while open blossom is present, and determine whether blossoms have been wetted by rain, irrigation or heavy dew. Daily degree-hour estimates are read off a chart based on maximum and minimum temperatures, and added up over a 4-day period. Once a certain threshold is reached, there is risk of blossom infection.
Cougarblight is available on the WSU website at http://www.ncw.wsu.edu/treefruit/fireblight/mdl98c.htm. The model is also available as an Excel spreadsheet. Contact your field advisor for a copy or for more information on forecasting fire blight.
Factors affecting susceptibility:
Table 1. Relative Fire Blight Resistance of Apple Varieties and Rootstocks*
|Most Resistant||Moderately Resistant||Susceptible|
Table 2. Relative Fire Blight Resistance of Pear Varieties and Rootstocks*
|Most Resistant||Moderately Resistant||Susceptible|
|Rootstock||Old Home (OH)|
Old Home x Farmingdale
(except OHF 51)
You can minimize the chances of introducing fire blight into your orchard by selecting nursery stock from a reliable source. Ask your supplier if they have had fire blight problems in the nursery. The varieties and rootstocks you grow will also influence the potential for fire blight.
delayed dormant copper spray
monitor temperature during blossom
spray Serenade, Bloomtime or BlightBan biopesticides to reduce blossom infection if blight-favourable conditions are forecast
spray streptomycin during blossom at 3 day intervals or following biopesticide application only if fire blight risk is high and weather is favourable for blight
scout for and remove strikes at regular intervals
Post bloomscout for and remove strikes at regular intervals
disinfect tools between cuts
control aphids, leafhoppers and pear psylla
Post harvesapply copper sprays if fire blight is active
continue to remove new infections
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