By Tim Smith, WSU Extension Specialist Emeritus; Tianna DuPont, WSU Extension Specialist
This year ties the record for significant fire blight pressure. Warm temperatures and humidity during bloom last year meant lots of innocula in the area this year. Infection periods during bloom and late bloom this year combined with high levels of innocula resulted in more infections this year. Here are responses to questions growers have been asking this year as they cut fire blight out of the orchard.
I’m seeing shoot blight infections now almost two months after bloom, where are they coming from?
It takes time for Fire Blight pathogen cells to build up in plant tissues to sufficient numbers to infect. Bloom infections may take three to five weeks to become noticeable. By that time pathogen cells have already traveled down through the phloem cells of the plant. When they accumulate in young susceptible tissue (1-2 year old new growth) shoot blight infections occur. This can take a number of weeks. This year cooler temperatures have promoted a longer period of shoot expansion which may be why we keep seeing more new infections.
Most shoot blight infections come from pathogen cells already in the plant tissue from flower infections. However, when young shoot tips are damaged by wind, hail, or insect feeding they are also susceptible to new infections. If an insect or wind blown rain move bacterial cells from droplets of fire blight ooze onto a wound (even one too small to see) it can infect the plant.
How far down from the visibly infected area should I cut when I am removing an infected shoot?
An infected shoot has many millions to billions of pathogen cells where they are most concentrated towards the tip. By cutting this branch we hope to remove many of these cells so that they cannot flow down into the tree where they possibly reach the root or other susceptible tissue. Cut 12 to 18 inches below the noticeably infected area in order to remove the highest concentration of pathogen cells. You will not be able to remove all of them.
Tim Smith, WSU Professor Emeritus also recommends cutting 4-6 inches from the main trunk or leader when you can, a ‘ugly cut.’ Where a stub is left a new shoot will develop and pathogen cells which are left are more likely to infect this new growth but not have time to produce more ineffective ooze rather than form a canker on the trunk which is more difficult to remove. Then it is important that pruners in winter look for these will ugly cuts during winter pruning and remove small cankers on the new shoots that developed before spring and a new infection period.
I’m cutting this year’s infections out, should I remove them from the orchard?
Infected cut branches will continue to harbor the fire blight pathogen as long as the tissue remains alive. Depending on the diameter of the tissue this can be for weeks. While the tissue and thus the pathogen is still alive the cankers can continue to emit ooze full of bacterial cells. When insects are drawn to this ooze they can move it onto susceptible young shoot tissue. Remove infected plant material from the orchard. It is better for workers to bring tissue to the ends of rows where it can be picked up in a wagon rather than bring wagons down the rows where they can rub and damage young tissue.
How long can fire blight live in cut wood?
How long a canker remains active in cut branches depends on the caliper of the wood. Once the branch is completely dead and dry the bacteria can no longer survive. Larger diameter branches will tend to stay ‘alive’ longer. For example, Professor Tim Smith had a cut branch in his office all winter which started to ooze in the spring.
Should I spray my infected orchard in summer to prevent new shoot infections?
Antiobiotics should not be sprayed on non-flowering trees. Oxytet products work by stopping the reproduction of pathogen cells, they don’t kill the cells and so will not be effective sprayed on foliage. Spraying strep type products repeatedly in summer is a recipe for resistance. Soluble coppers such as Previsto and Cueva do kill pathogen cells and can help protect susceptible leaf tissue when there is a lot of innocula in the orchard from a bloom infection or hail is on the horizon. But remember when the tree is growing fast new leaf tissue quickly outgrows a recent spray.
Should I have my crew disinfect the pruning shears as they cut out blight?
Conventional wisdom says yes. However, since much of the wood we are cutting is more than two years old it is less susceptible to infection. The problem comes when disinfecting slows the crew down so much that the work does not get done.
After I cut out infections in summer will I have to prune blight again in the winter.
Yes. Summer pruning reduces the infected material and thus the innocula but will never get it all. Plan to return in the winter for an additional pruning.
Tree Fruit Extension Specialist, WSU Tree Fruit Research and Extension Center
Tree Fruit Extension Specialist Emeritus