Fire blight

We have a huge problem with ornamental pear dieback in the San Francisco region, which has been increasing over the last few years. I'm looking for help/suggestions/links to research.

Callery pears are incredibly beautiful and one of the few trees with spectacular spring flowering as well as brilliant fall color. People have loved them and planted them in excess.

I think there is more going on than just fire-blight, or maybe there is a more aggressive strain. I've tried to get a couple of UC extension scientists to answer this; If it was the wino industry or native oaks, UC would be all over it, but they don't take ornamental pears seriously enough IMO, or maybe just can't get the funding. There are hundreds of thousands of P. calleryana and kawakamii planted throughout the region and some cities are getting devastated with one of their most prevalent street trees and residential favorites, or used to be.

I've been dealing with fireblight since the 70's and it used to be tip dieback and occasionally a little deeper advancement down a branch or two. We could prune it out early and that was it until the next spring. But the dieback in pears is now a major major issue and can be totally devastating and can cause complete mortality in some cases. It continues to advance through summer and fall. I've noticed that the foliage that flags turning color early in September/October, which used to be just that, turning early, is now progressing into fireblight appearing dieback, but without the blackening stems and shepards crook tips that is characteristic of fireblight. Why?
I'm wondering if is there any possibility that something there is not actually Erwinia a., and will get past an OTC treatment.

The best control I've achieved is with experimental repeated, multi solution treatments. Agrifos/pentrabark basal bark, OTC injection, and repeat canopy sprays with Agrifos and Double Nickel biological fungicide. But this intensive work is just not affordable and I won't even ask customers to pay what it would take.
I've been passing up on jobs because I haven't been confident that I could control this issue, especially trees that have already shown signs of infection the previous seasons. I'm putting my faith in early ArborOTC this coming season and hoping that will get me the best suppression possible at somewhat affordable rates.

Any ideas or referrals out there from Treebuzz members?

FYI. This link is for one of the most complete discussion on Fire-blight that I have seen, but of course it's orchard oriented and not considering the ornamental varieties:


Well-Known Member
Without the epinasty (shepherd's crooks) and blackening or charred appearance of the shoots, I'd be slow to diagnose fire blight as the culprit. I haven't seen pear branch canker dieback [Diplodia] in the field, but apparently it is happening in northern CA.

In much of the US, callery pears are treated with derision, particularly by tree care workers. Mostly due to structural problems and having been over-planted.

With your interaction with University extension, what do they say about samples submitted to their diagnostic lab? I don't have experience with northern CA. Ventura County Extension and that part of CA seems to be pretty much on-the-ball and responsive.

Not much help, I'm afraid.
Thanks KT for your response, That is helpful, and may be a possibility. Another associate has suggested possible botryosphaeria canker, but diplodia would make sense.

This spring I'm going to renew this pursuit with UCCE.
KT: That research article link is now getting some attention; I'm digging in to find out what is going on with flowering pears here. But my issue is regional and this information may apply more to your fireblight discussion.
I just receive the following heads up: "...There is another bacterium called Pseudomonas syringae which causes symptoms that resemble so much to symptoms of fire blight - but they are actually not. This other bacterium causes disease Pseudomonas Blight of Trees and Shrubs and it is not as easy to identify and tackle as fire blight. ... It is easy to switch this Pseudomonas blight with fire blight and thus misdiagnose the disease. The prime differences between these two diseases encompass: when they occur, weather conditions that favor them, and how they spread. Here is more information below:"


Well-Known Member
True enough, and P. syringae has a really broad host range. Its probably the most studied phylloplane/facultative bacterial pathogen of plants. I have to admit that my experience with it has been more from the classroom and from text books than from diseases in the field!
Regarding preventive treatment for fire blight: I received this from a PhD researcher with Arborjet. No, I have no connection to AJ except as a customer/applicator, but this is an example of why I am gaining more respect for their product development and customer support. I've had very little success in getting feedback from actual researchers with other product companies in the past, except when Marianne was at Mauget. It's good to hear it from the source, adds some credibility and indicates a taking of responsibility, IMO.

"...I am plant pathologist in Arborjet. There is no doubt that ArborOTC, which I tested for three years at Michigan State University, is very effective for control of fire blight. I tested it under both high and low fire blight infection pressures, on shoots and on flowers and it provided great control of shoot blight severity of 84% which was stable across the whole season after single injection of labeled rate. As Marianne said we have ongoing fields studies which will bring more recommendations in the future. ArborOTC stops progression of fire blight infection in shoots that started and prevents the establishment of new infections. For infection of flowers - it is also very effective, but I would recommend you to inject earlier, i.e. during transition of your CA winter to spring (just before spring starts). This injection, early before spring starts, will secure that more of ArborOTC is accumulated in flowers. Flowers transpire much less than shoots and need more time to accumulate any injected compound to make an effect on disease. Any injection should be started only if you know that tree is not under drought stress. If you do not have enough water in the soil - provide it to the tree. This is very important because it allows your injected product to move faster into the tree crown and accumulate in higher amount to express its effect...
Further, there is a difference between ArborOTC and MycoJect. ArborOTC is a better formulation that moves far much more easily through the tree - this formulation secures its good effect."

BTW, the points he makes about adequate soil moisture is a very important consideration for all systemic chemistry applications: stem injections, basal bark with 'Pentrabark', soil drenches, even foliar applications of systemics and fertilizers. Got to have water!
I'm not going to claim to be an expert on this at all, but I've learned quite a lot recently. I've got a couple of clients with small orchards and I've been fighting fireblight for the last two years on one of them. I've been with my company for 2 seasons and I don't have prior records from this property so I don't know if they've had bad years previously. On my first season, it wasn't horrible, a couple of strikes here and there that I pruned out quickly. You've gotta prune them out because they will ooze bacteria that will spread by insects, rain, people, etc. This past season was worse, but from what I've heard from others I didn't have it incredibly bad. These are production/ornamental at a high end estate and looks are as important as yield. I've got a pretty good fruit tree program set up with about 8-10 treatments per season. It's one of those 'Oh sh*t!' problems because you feel like you're doing good, nice green full foliage one day, come back 3 days later and Oh sh*t! There are brown 6" curled over dead tips every where. The feeling of panic sets in and I wonder how bad this is going to be because you see more each day after you've pruned out what you can see.
Anyhow, my program starts with a hort oil at 2% with copper. Fungicides and insecticides through most of the remaining season. This past season, finished the season with a hort oil and copper treatment.
After the last two seasons, I didn't want another Oh sh*t! so I met with a consulting fruit specialist. He came out today and we discussed pruning, cultural issues and pest management. He suggested that I start with two hort oil and copper treatments. 4% oil on the first and 2% on the second. He also helped me put together different programs for apples, pears, and stone fruit that I'm still working on.
Additionally on fireblight, he suggested to follow a blight forecast model and if rainfall or any moist conditions such as heavy fog or dew is predicted, you've got to spray with streptomycin. The copper kills any bacteria on the trunk and limbs and it and the strep will help to significantly reduce blossom blight. This can be followed up with two applications of Apogee growth regulator as soon as the label allows (I believe it's green-tip). The copper sprays early and strep will help prevent the fireblight infections of the blossoms, while the growth regulator will reduce tip growth at the beginning of the season and somehow helps to prevent the twig blight that comes later and brings the Oh sh*ts!
Again, I'm not claiming to be an expert, I'm just passing on some info as it was presented to me (according to my memory, I don't have my notes). I feel like I know a good deal about trees, but I'm always amazed at how much more I don't know. I love reading KT's posts because it makes me do more research so I can truly understand half of what he's talking about. When I met with this fruit specialist today, I took tons of notes and it was like taking a whole class on fruit trees all in one afternoon. My head is still spinning with all of the new info and ways of looking at fruit trees and trees in general.
Another note in terms of pruning. You're not supposed to prune during the part of the season when you see the strikes, but you've gotta get rid of 'em or it'll spread. At this time I use lysol between cuts. It's a pain, but I don't want to take a chance. I also cut below the infection, but leave a stub if it's on a larger limb just in case I do spread it. Not sure if it makes a diff, but my convoluted theory is that if I accidentally spread it hopefully it won't spread beyond the stub. When I'm pruning in winter, I clean out the stubs and any fireblight cankers that I can find. I've read that you don't have to sterilize in winter since the bacteria are not active, but if pruning root suckers sterilize anyway.
Hope that all helped and doesn't conflict with anyone's non-chemical approach. If anyone's got a better remedy, I'm all ears. Oh yeah, and for ornamental apples or pears I'm pretty sure that you could use copper throughout the infection period if you don't want to use strep since you aren't concerned as much with the fruit. Copper won't damage the fruit, but it will cause it to russet, making it non-salable. A good, old-time chemical.
I also got help with brown rot on peaches and apricot, but that's another thread on another day I guess.


Super Moderator
Staff member
35 - 40 years ago I had about 35 fruit trees; many developed fire blight.
I am now sure that I spread it by pruning, and not properly disinfecting between cuts.
(I also used tetracycline, which you now probably can not easily get without a license.)

My problem now would be that I still don't know how to properly disinfect in a reasonable amount of time.
I have actually looked on-line, but just got discouraged.

Due to mass transfer thru sawdust (between saw teeth), thru sap & resin, & goo, etc. the disinfection agent takes time to contact the bacteria.
Then, there is the time to react - time, temperature, concentrations, etc.
This would not be seconds, or even a few minutes per cut ! ! !

There are quite a few research reports out there; check them out. Good Luck !

Maybe ........... something like rotating many saw blades, soaking in a bucket of bleach, etc ???

Or Perhaps injecting the tree with something ???? .....
Last edited:

Tom Dunlap

Here from the beginning
I was looking at the Vermeer/Sherrill catalog
And saw a bottle of the product with the ISA logo. I zoomed the picture but it pixilated

Seems odd...


Super Moderator
Staff member
See previous posts:

"It seems there are at least 3 major concerns.

1. Mass Transfer (noted above).
The disinfectant needs to penetrate into all of the infected material.
Experiment: Spread a very thin layer of sawdust on a flat surface; wet the top thoroughly with alcohol; wait & then check bottom side !

2. Reaction Time (Chemical / Biological)
These are NOT instantaneous.
Even autoclaves with very high temperatures & pressures, require extensive time at those extreme conditions.

3. Disinfection Material Efficacy
Isopropyl alcohol, bleach, pesticides, etc are not all equally effective.

I think these are some reasons that the disinfectant / preventative material is applied to the tree's cut.

If you just disinfect between trees, or jobs, are we just propagating the disease within the tree, or job ?

I certainly understand this is extremely time consuming.

P.S. I think I killed a lot of apple, peach, plum, pear & apricot trees 35 years ago !
(They were all mine !)


Well-Known Member
Another fire blight question!

A small neighborhood who we have pruned trees for in the past, they have roughly 60 Apple trees, mostly fruit bearing, some ornamental. Due to crazy hail events, fire blight has ravaged through the area in the last month. Many of their trees were hit bad! They were advised by a consulting arb to remove many of the worst stricken in order to sanitize the area in hopes that the blight wont spread. This seems illogical to me, I think that many of the trees on the chopping block have a chance to make a go of it. Does this seem reasonable to y'all? Chop em down? Simply for the sake of discussion, the trees will be chopped no doubt!