Feeling the Burn of Fire Blight

In the past week, fire blight symptoms have begun to appear in apple orchards throughout Ontario and even in Quebec. Fire blight is caused by Erwinia amylovora, a pathogenic bacteria that infects the blooms, fruit, leaves, shoots, limbs, trunks and rootstocks of apple and pear trees.

So why are we starting to see these symptoms in orchards now? Fire blight often enters the tree through blossoms during bloom. Although many growers applied streptomycin sprays at bloom based on predictive models (Cougar Blight, and MaryBlyt), the extended bloom and secondary bloom that we saw in some regions may have left blossoms vulnerable to attack from bacteria. The symptoms of these early season infections may be appearing now in apple orchards. Fire blight also enters trees through wounds caused by hail, insects, summer pruning, deer feeding, and even high winds. So the infections that are appearing in some areas may be a result of post bloom infections. It is also important to remember that trees (particularly young trees) can be infected the previous year and may not exhibit symptoms of the disease until the subsequent year (personal communication. Dr. Antonet Svircev, AAFC). Once a tree is infected with fire blight, the bacteria move quickly through the current seasons growth into older growth. The bacteria that causes fire blight multiplies in the hot humid conditions we have been experiencing over the past few weeks, rapidly increasing the development of the disease. Young trees (bearing and non-bearing) can be killed by this disease, and older mature trees may survive but show significant damage.

So now that fire blight is present in orchards what options do growers have for managing this serious disease? To prune or not to prune—that is the question all growers are asking. Unfortunately there is no easy answer to this question, since the severity of the infection and the weather conditions suitable for infection both have an impact on the decision.

Pruning the trees at this time of year can have several effects:

• Reduce the innoculum, and prevent the spread of disease
• Increase shoot growth, making the tree even more susceptible to fire blight
• Increase labour costs.

So how do you know whether to prune or not to prune? When dealing with older (>10 year old) vigourous trees that have many fire blight strikes, avoid pruning out shoot and blossom strikes early in the growing season. Extensive early pruning stimulates trees to produce succulent shoots, making the tree more susceptible to infections. When dealing with high vigour trees that have only a few fire blight infections, pruning out the fire blight may help to provide complete control of this disease. In slow growing, low vigour trees, the bacteria generally doesn’t move into major limbs or trunks during the growing season, so lesions can be pruned out when the tree is dormant. This may help save time and money. . In comparison, young vigourous trees are very susceptible to fire blight and the bacteria may move rapidly into the trunk and down to the rootstock, killing the tree. As a result, pruning is recommended on young trees.

In orchards where fire blight is not a major concern this year, growers may still consider pruning fire blight strikes. Pruning out fire blight cankers, under dry conditions and low disease pressure may reduce the amount of bacteria that will be present in the orchard in future years.

If you are considering pruning out infections in a young orchard, you should do so as soon as they appear to prevent the spread of the disease to adjacent trees. Depending on the variety, it may or may not be practical to prune out infections in mature trees. Keep in mind that once the terminal buds have set on apple trees, fire blight stops spreading within infected trees and between trees. However, leaving the disease in the orchard may serve as a source of inoculum, and cause increased infections should a hail storm, severe storm, or high wind damage the trees and provide an entrance wound for the bacteria to infect.

If you decide to prune out fire blight strikes, pruning should not be initiated until terminal bud set has occurred (early to mid August) and remember to make cuts into at least 2-year-old wood where the bacteria is less likely to multiply. Pruning cuts should be about 30 cm (12 inches or more) below the visible symptoms. Always prune during dry weather, since it reduces the potential for spreading the disease. The efficacy of sterilizing pruning shears to prevent the spread of fire blight is debatable, and sterilization may not always be practical. Regardless, pruning tools should be sterilized in a solution of 1 part bleach to 5 parts water, this solution will be effective in about 1 minute of wetting. It should be noted that there is no benefit by spraying cut surfaces on the tree with chlorine products, since the chlorine binds with the wood and is inactivated immediately. See info sheet from the Washington State University on Cutting Fire Blight from Infected Apple and Pear.

In sections where trees are severely affected, it may be more cost-effective to simply remove the whole tree than to prune it. Pruning clippings should be tossed into the row middles and allowed to dry before mowing them down. Avoid hand thinning, bud pinching and other activities until after terminal set, since these activities may help to spread the disease. Monitoring and managing insects (aphids, leafhoppers) that may spread the bacteria or cause wounds for the pathogen to enter tissue is important fire blight management. Orchards with high levels of fire blight this year will have a high inoculum next year, and as a result growers will need to use predictive models (Cougar Blight, and MaryBlyt) to determine if and when applications of streptomycin at bloom are necessary.

Consult the OMAFRA factsheet Fire Blight of Apple and Pear in Ontario, Order No. 02-011 for more information on fire blight.

Related Link

• Info Sheet on Cutting Fire Blight from Apple & Pear, Washington State University
• Fire Blight of Apple and Pear in Ontario, Order No. 02,011