VT Apple IPM: Yup, fire blight risk is on the rise this week

As I mentioned yesterday, fire blight risk is on the rise in all orchards in Vermont.

Too long / didn’t read synopsis: if you have open blooms (and you likely do), check NEWA and be ready with a streptomycin application this week.

With increasing warmth, the population of bacteria that causes fire blight is rapidly multiplying. Let’s look at the conditions required for infection:

1. Open blossoms.

2. Wetting. Everywhere in the state has a chance for some wetting this week, including heavy dew or even a spray application

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.

In the few hours since I first checked the models this morning, they have already increased risk for this week, at least at my South Burlington site. I recommend that all growers with high-risk blocks (young trees of susceptible varieties, sites with a history of FB infection in the past couple of years) apply streptomycin to all blooming blocks (don’t forget those lagging ‘rat tail’ blooms) tomorrow (Monday 5/17) or Tuesday. Remember, a treated blossom is a treated blossom, so if an infection event extends more than 48 hours (you have 24 hours protection before and after the application time), you only need to re-treat if more blooms have opened. For many orchards at full bloom, that may mean that you’re just treating once, but for cooler sites where bloom is just getting going, that may mean to applications.

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.

I do want to mention another material that may provide some other protection. Apogee is a plant growth regulator that is applied around 1-3” shoot growth (i.e., now) that helps to shorten internodes and reduce the need for summer pruning. It also causes a thickening of cell walls that contributes to reduced shoot blight infection. For young, trellised trees where you may not want to stunt vegetative growth, some reduction in shoot blight has been found using ½ rates. The rate calculation for Apogee is fairly complicated and based on relative need for vigor control and tree row volume. Please see the label for more information.

Your best bet is to get a strep treatment on this week if you have any concern about this disease. For a good reader on fire blight, see: https://ag.umass.edu/sites/ag.umass.edu/files/fact-sheets/pdf/a4fruitnotesspring2015fireblight.pdf

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