Disease Update: Be on Alert for Fire Blight Conditions Later this Week

Posted: May 8, 2014

The apple blossoms are open and conditions are soon to be optimal for fire blight infection. Keep an eye on the weather for favorable temperatures and moisture since forecasts indicate May 9th to 12th (and possibly beyond) to be an infection period. Blossoms need to be protected.
Left - fireblight strike; right - fire blight ooze.

Left - fireblight strike; right - fire blight ooze.

Open Blossoms, Current Forecasts and Bacterial Diseases

It’s taken a long while, but it seems Spring has finally sprung and conditions are turning quite favorable for bacterial diseases.  Combining open blossoms and the current forecasts, the fire blight models are pointing to a very critical infection period later this week.

The minimum requirements for blossom infection and the order in which they must occur are:

  1. Flowers must be open with petals intact (flowers in petal fall are resistant)
  2. An accumulation of at least 198 degree hours above 65ºF
  3. A wetting even as dew or rain
  4. An average daily temperature of 60ºF

In case you’re wondering: for those of you who received rain on May 7, 2014, this wasn’t a fire blight infection event because we had not met 2 of the 4 requirements: enough degree hours hadn’t been accumulated and the average temperature was below 60ºF.  However, with temperatures forecasted to be higher the next several days and rain anticipated, we will achieve the minimum requirements, if not exceed them.  When there are many open flowers, more than 200 cumulative degree hours, extended rains and average temperatures above 65ºF, an epidemic can be more severe.

Management during the bloom period

The antibiotic streptomycin limits the multiplication of bacteria and does not kill large bacterial populations.  Only blossoms open at the time of application are protected against infection.

Apply streptomycin (plus a surfactant) as a complete spray with thorough coverage just before an anticipated infection event when infection risk is moderate to high.  Treat again in 4 days if high risk conditions persist.  Do not exceed four antibiotic sprays a year.  The addition of an activator-type surfactant (Regulaid, LI-700, NIS-80, Activator 90, etc.) will improve coverage and penetration of the flower structure, especially the nectaries where most infections occur.  Once treated, an open blossom is protected until petal fall when it becomes naturally resistant.  Sprays applied too late are not effective in stopping infections already in progress, and those applied too early afford no protection for new flowers opening after treatment. 

In addition, consider applying the plant growth regulator, Apogee (prohexadione calcium), at late bloom when active shoot growth is 1 to 3 inches long to reduce the threat of shoot blight on vigorous trees of susceptible varieties.  Apogee causes shoots to start hardening off approximately 10 to 14 days after application, resulting in reduced susceptibility to shoot blight.  Research has shown that Apogee can be safely tank-mixed with streptomycin plus a surfactant, allowing Apogee to take effect while there is residual protection from streptomycin.  Apogee is not to be considered a replacement for streptomycin sprays for blossom blight control.   

Management during the postbloom through bud set period

Control piercing-sucking insect pests (aphids, leafhoppers, pear psylla) to reduce the incidence of shoot blight.  Monitor orchards closely for early blight symptoms and remove these promptly before extensive necrosis develops.  Do not use streptomycin to control shoot blight after symptoms develop since it is not effective and increases the risk for developing antibiotic resistance.  Avoid extensive cutting that may stimulate shoot growth and lengthen the period of shoot blight susceptibility. 

Severe weather with hail can trigger a trauma blight incident, especially if blight has occurred earlier in the season.  In this case, streptomycin can be applied postbloom when trauma blight is an issue.

Cutting out active fire blight strikes

There is debate over cutting out fire blight strikes while an active fire blight epidemic is in progress.  When fire blight symptoms (blossom blight, oozing, crook at shoot tip) are light to moderate or limited to an area within a larger orchard, prompt removal of all infected tissue can be beneficial.  However, when the epidemic is severe and widespread, extensive cutting should only be done as a salvage effort to limit invasion of the central tree structure.  Cut infected branches 12 to 18 inches below the lowest evidence of disease.  Be sure to destroy (burn) any infected material that was pruned to remove inoculum from the orchard. 

Using MARYBLYT™ to forecast fire blight

SkyBit and NEWA use similar forecasting models to predict fire blight infection; however, if you don’t subscribe to either, but have a weather station collecting daily temperature and rainfall, MARYBLYT™ is an option to consider assisting in timing your antibiotic sprays.  MARYBLYT™ is a free, user-friendly computer program developed to prompt treatment decisions using daily temperature and rainfall to define fire blight risks, predict infection events, as well as predict symptom appearance. 

Conditions for Fire Blight also Good for Apple Scab Infection

In Biglerville, the ascospores are approximately 80% mature and a lot of spores are being released.  This indicates we’re probably going to be peaking spore dispersal soon, too.  The conditions conducive for fire blight will also be great for apple scab infection – a double whammy.  Take care in protecting your trees for apple scab this week, as well.

Monitor Weather and Tree Growth Conditions at Local Level

When controlling for disease, weather and tree growth conditions need to be monitored at a local level within one’s own orchard.
Before chemical products are applied, be sure to be in compliance by obtaining the current usage regulations and examining the product label.  Product information can be easily obtained from CDMS.

Contact Information

Kari A. Peter
  • Assistant Professor
Phone: 717-677-6116