Fire blight concerns 5/15/2018

By Terence Bradshaw

I’ve had a number of people ask my thoughts on fire blight infection potential with today’s rains. I’ve looked at a couple of models, and generally, they hover around the point of calling or not calling an infection. Let’s look at the conditions required for infection:

1. Bloom. If you’re inland or upland and don’t have any open blossoms, then stop reading now. You don’t have any blossom blight risk during this wetting event.

2. Wetting. I think everywhere in the state will receive at least some wetting today that could contribute to infection.

3. Heat during and after the wetting event. This is marginal, but if there’s a high enough bacterial load, then infection could occur.

4. Build-up of sufficient population of the pathogen to trigger infection. This is known as the Epiphytic Infection Potential (EIP) and requires a) an overwintering or introduced pathogen source and b) heat prior to the infection that allows for that bacteria to multiply. This is the part that has been generally just below threshold.

Remember that models are only as good as the information that goes into them. Both the eastern (Maryblyt) and western (Cougarblyt, used by NEWA) models allow for some adjustment based on the level of fire blight that has been in your neighborhood this year and the past two. For orchards with no fire blight in the area last year, EIP does not reach infective threshold in Vermont orchards. But, if you had fire blight in your orchard recently, and have susceptible cultivars (Paulared, Gala, Macoun, Cortland), then a streptomycin spray may be warranted. Remember that strep works 24 hours in either direction, before or after an infection event, and it must contact an open blossom to protect it. There’s potential for another infection event on Thursday that looks to also be marginal, but carry slightly than today’s.

For organic growers, streptomycin is no longer allowed by NOP standards. Some materials that may be effective include lime sulfur, which burns flower tissues so will only help a blossom that is already pollinated; low-rate copper materials like Cueva and Badge, which may russet fruit; and biologicals like Double Nickel or Serenade. None of those are as effective as streptomycin but each may be better than not treating at all in an infection situation.

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