Fire blight risk for Thursday and Friday of this week

By Terence Bradshaw
May 16, 2017

Okay, we’ve been hinting around about it for weeks, and the risk for fire blight will be upon us later this week. Warm weather today, and hot weather Wednesday and Thursday will allow for substantial increase in populations of E. amylovera, the bacteria that causes fire blight. Once that population is sufficiently high (characterized as the EIP or epiphytic infection potential in NEWA, 100 is considered high enough to cause infection), you need three factors for infection to occur: open wounds (i.e. blossoms or fresh pruning cuts); mean temperature above 60°F; and wetting, even a spray or dew event can be enough to move bacteria into susceptible openings.

This risk is pretty much widespread across the state. The thing to remember is that protective measures, i.e. application of streptomycin in most orchards, or caustic materials/biological controls in organic orchards, must be applied to open wounds or blossoms within 24 hours before or after infection. So if we assume that an infection event occurs in a rain event Thursday afternoon, you’ll want strep or another material (really, if your orchard isn’t certified organic, strep is the only material to consider) on by mid-day Friday. It will be hot Thursday, so blossoms will be opening all day (or petals falling on cultivars that have finished bloom already) and potential for phytoxicity will be greater. The later you can go, the better to make sure you cover any blossoms that open, but don’t delay and miss it. It’s also going to be on the windy side, so there’s that.

Harbour is the strep material available to most growers. I would recommend applying at 1-2 pounds per acre based on tree canopy volume, if in doubt, err on the lower end. I also recommend including a wetting agent like Regulaid or LI-700. If possible, apply on its own, without other fungicides or insecticides (you are in bloom, so no insecticides anyway). You will get leaf yellowing from this application, expect it and the tree will soon grow out of it. One application Wednesday PM-Friday AM should cover you. Once any particular blossom is treated, it’s protected. Temperatures are expected to drop after Friday, which lowers risk, but if high risk continues through the weekend and you keep having blossoms open, then a second application Saturday-Monday (I have no good idea of what the weather will be five+ days from now) may be warranted.

Trees that are at full petal fall are not susceptible, but straggling late blooms can be infection sites. Ideally, the whole orchard would be treated. If you need to prioritize, go first for cultivars that had fire blight last year or highly susceptible cultivars (Gala, Ginger Gold, Honeycrisp, Cortland, Paulared, etc), and of course blocks that are in bloom. Application well after Thursday’s infection event, on Saturday or later, will not give protection against it. Also, despite the wording on the Harbour label, continued treatment every 10-14 days after bloom is not recommended, and doing so is a) expensive, b) a waste, and, most importantly, c) the best way to develop antibiotic resistance in E. amylovera populations.

For organic growers, Actinovate, BlightBan A506, Bloomtine Biological, Blossom Protect, Double Nickel, Regalia, and Serenade are labeled biological controls. I can’t vouch for any of their effectiveness, and I know that some growers apply copper or lime sulfur (warning- both are very phytotoxic) during bloom to manage the disease but again, no promises are offered from me there. If you can, I would suggest Serenade or one of the other biological materials if you have it on-hand and to watch carefully for symptoms which will require cutting out.

I’ll be following up on this as things develop. The key here is to treat for this week’s infection, then relax.


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