Disease Update: Fire Blight and Scab Risk Continues This Week

Bloom is occurring in Pennsylvania apple growing regions—fire blight conditions continue, available mature apple scab spores are very high, and an apple scab infection event is predicted for May 10, 2018.
Disease Update: Fire Blight and Scab Risk Continues This Week - News


Bloom is still raging in the orchard. Stay vigilant for disease conditions! Photo: K. Peter, Penn State

Keep trees protected this week!

Reports indicate that a large part of Pennsylvania is experiencing some stage of apple bloom and petal fall right now. Conditions remain favorable for fire blight and apple scab infection periods this week. Here are your nuggets of wisdom for the second week of May when dealing with these two challenging diseases:

Fire blight: Continue to remain vigilant

Our bloom experience is on track to mirror what was observed in 2014 and 2015: Bloom occurred during the first week of May and was followed by persistent warm weather, with the occasional rain event. Both of those years were also very bad fire blight years, too.

Consequently, 2018 is not a year to take chances.

In the last week, we had very warm days for the pollinators to potentially move bacteria around, as well as any bacteria landing on the stigma of the flowers to replicate to very high numbers. Also, the entire region has experienced some kind of rain over the weekend. This means, if trees were not protected with streptomycin, that rain acted as a vehicle to wash the bacteria into the nectaries.

This week, a lot of bloom is still occurring, and ideal fire blight weather continues to persist. The goal now is to continue to keep trees protected, which includes management through petal fall and onward until the shoots reach terminal bud set. Please do not to wait for symptoms to show up to start managing the disease. That’s like trying to stop a moving train!

Here is a game plan for fire blight management as we move through this very insane period right now:

  • If you sprayed streptomycin on May 4 or 5 (full bloom), consider another streptomycin spray later this week (May 10 – 12) if you still are experiencing substantial bloom in your orchards. A typical rule of thumb is not stretch strep sprays any more than seven days during bloom. Tank mixing Actigard (2 oz/A), Vacciplant (20 fl oz/A) or Regalia (2 qt/A) with your streptomycin is recommended to give extra protection. When streptomycin is applied, you will kill any bacteria on the stigma, thereby resetting the clock for disease potential. When using streptomycin, you have 48 hours for strep protection when there is a wetting event: 24 hours before and 24 hours after. Remember not to exceed three complete streptomycin sprays during bloom.
  • If you have semi-dwarf trees that have had a history of fire blight and potentially leftover cankers in the tree, begin prohexadione calcium applications (Apogee or Kudos; 6 – 12 oz/A). You can guarantee canker blight will be occurring in these trees this year based on the weather we have been experiencing. Taking this action will limit the incidence of shoot blight, which can be problematic bacterial sources for nearby orchards.
  • If you have young dwarf trees and do not want to use the low ProCa rate (2 oz/A), apply Cueva (2 qt/A) in those susceptible orchards beginning later this week. Use Cueva on a regular basis (at least every other cover spray) through the end of June. Cueva is a copper, so please be mindful of your tank mixtures when spraying, particularly the pH and adjuvants, and slow drying conditions. Also, do not mix copper and streptomycin. We have observed a decrease in efficacy for fire blight control.

Apple Scab: May 10 is a predicted infection period

We continue to detect very high mature scab spore availability from the overwintering leaves (averaging 15,000+ spores this week). This is on track for when the highest scab disease pressure occurs, which is about two weeks covering bloom and petal fall. According to NEWA, many apple-growing regions in Pennsylvania are showing a potential scab infection event for May 10. Here is a game plan for apple scab management as we move through this tricky period:

  • If you haven’t already done so, use the FRAC Group 7 fungicides during the bloom through petal fall sprays. These products include Aprovia, Fontelis, Sercadis, Luna Tranquility, Luna Sensation, Merivon, and Pristine. Tank mix with a broad spectrum, such as mancozeb (3 lb/A), for fungicide resistance management.
  • If frequent wetting events occur during this time, consider rotating with FRAC Group 3 and/or 9 fungicides. Some of these products include Inspire Super, Vangard, Indar, Rally, Procure/Trionic, or Rhyme. These would be a good choice for powdery mildew during the dry periods.
  • If you are using half sprays, please keep your intervals short in order to not have a lapse in protection.
  • Monitor conditions closely in your orchard and region for apple scab disease conditions. The best management plan is to have your fungicides applied prior to the wetting event.
  • Remember 2 inches of rain washes off protection.