I posted a while back on an Ernst Bosch Apple tree that came down with fireblight. I wanted to share with the group what had happened, although I must say that after reading the latest Illinois Fruits and Vegetable Crop news, I am grateful not to have to deal with the fireblight outbreak that the Midwest is experiencing this year.
We are not in a fireblight prone area due to our cool, dry conditions, but this year, fireblight has broken out in a number of local orchards thanks to a warm and humid, rainy and drizzly week in May. So this has been a learning experience for me. I finally figured out how to deal with the fireblight infection, here is what I did: Since only one of my 200 trees (of which 80 are apple trees on MM111) came down with fireblight, my focus was to eliminate any further disease vectors, so removal of the tree is the only solution. However, I am a scientist by training, and my curiosity got me to play with the tree for a bit to try to understand what is going on. Here are some of the observations: Step one was to cut, but to my dismay, the infection returned, even after several cuts. Disinfecting the shears seemed irrelevant. The morning dew collecting on the cuts would often turn orange, and the drops would carry the infection to lower limbs. Step two was to try to figure out if the re-infection vector was external, or the bacteria was already systemic in the cambium. So for the next sets of cuts, my approach was to coat the cuts with a oily antibiotic paste with a bactrin base (bactrin is a wide spectrum antibiotic). The coating would prevent re-infection from external sources, but not if the bacteria is systemic in the cambium. Sure enough, another set of cuts lead to more infection, the ooze coming from under the paste. It only took 24 hours for the wood to turn orange under the paste, proof that the infection came from inside the tree, not from external sources or from the pruning sheers. Step 3 was to prune back radically, past the node of the infected branch. Upon close inspection, I discovered that the cambium had a darker shade, Cutting into the darker shaded areas revealed healthy wood, but the cut turned orange within 24 hours and would start to ooze. However, cutting several inches below any slightly darkened areas did not cause the wood to turn orange, and no re-infection took place. Step 4 was to douse the tree with antibiotic to make sure there is not any residual fireblight bacteria on the tree. I used a combination of bactrin and retromycin sulfate to treat the tree and surrounding mulch. At this point, it's safe to say the fireblight active infection is gone, although I believe the bacteria is still in the tree, but dormant for now even though there are no visible cankers. Since the tree is so young, I plan on removing and replacing it after the harvest this October. But before I do, I also plan on feeding the tree some heavy nitrogen to get it to grow and see if the new growth will show signs of re-infection. My conclusion for controlling fireblight is the following: the bacteria travels much, much further than 6-12 inches, and one must cut past any infected ambium: Chances are trees could carry fireblight even without showing any active signs of infection. 1) Always coat the cuts with an antibiotic oil-based paste to prevent re-infection from outside vectors. 2) The wood must be placed into a plastic bag immediately and not allowed to touch anything else. So it's a good idea to cover any oozing branch into a plastic bag and break off the branch, then do the pruning so no ooze droplet hits any other part of the tree. 3) Always disinfect the sheers in case the previous cut was into still infected wood. Preferred disinfection is a field heat treatement disinfector, which should be used anyway to prevent any sort of disease spread. You wouldn't want the doctor not to disinfect his surgical instruments before treating you, a plant is no different. 4) Always observe for orange discoloration under the paste for the next 24 to 72 hours. If the orange discoloration returns, the cut was made into more infected tissue, and you have to cut further. 5) After any infected tissue is removed and discarded, the tree needs to be treated externally with an antibiotic to remove any residual bacteria. I cannot vouch that the above would work outside of Santa Cruz county; when I see the pictures of fireblight in the midwest, it makes me wonder how one would even deal with fireblight once it broke out, the only reasonable approach seems radical removal, and preventative spraying in the Spring.
