This is a big week ahead. Many orchards are at tight cluster bud stage, and with the warm and eventually hot weather coming up, bud stages are going to move fast. Anything you want to do prebloom should be done this week. That means foliar nutrient– zinc, boron, and nitrogen should be applied to improve bud and blossom viability. Oil may be applied for mite management, too, up to 1% up until tight cluster-early pink. But I would not mix oil with nutrients nor captan/sulfur. Nutrients are more important now.
With bloom and hot weather approaching, the biggest disease consideration – no scab is expected for the week with the dry weather, but apply a fungicide before expected wetting periods- is fire blight. This has the potential to be a big infection period.
Let’s look at the conditions required for infection:
1. Open blossoms.
2. Wetting. This doesn’t have to be a rain, a heavy dew or spray application could be enough to cause infection.
3. Heat during and after the wetting event. Later this week ‘they’ are predicting temperatures in the 80s with chance of thundershowers.
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. NEWA models suggest that EIP sufficient for infection could be reaches statewide by Thursday.
Follow the NEWA model daily to best assess fire blight risk for your 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) be ready to apply streptomycin to all blooming blocks.. 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 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 be ready 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
This is the week to get your codling moth traps up. Traps should be hung at pink and checked daily until first catch is seen. That capture date will be the biofix you use when calculating degree days for subsequent management actions.
Finally, if you have irrigation, this is the week to make sure it’s running.
Stay cool.
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