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Disease Development Status (as of May 25, 2012)

Posted: May 29, 2012

From May 1 through May 24 we had spotty rain that accumulated 5.18 inches. These wet conditions favor apple scab, rust, bacterial spot and cherry leaf spot development. And when rain stops, powdery mildew takes off, which is difficult to manage once it gets established in the orchard.
Powdery mildew infection periods

Powdery mildew infection periods

Dr. Noemi O. Halbrendt, Penn State Fruit Research and Extension Center Plant Pathology Senior Research Associate

Apple Scab and Wet Weather

We observed the peak (100%) of mature ascospores trapped at FREC orchards on April 17.  This was followed by some continuous wetting events that led to several scab infection periods totaling 28 scab infection events during the primary period from March 16 to date (May 25). Weather data were recorded with electronic monitoring systems (Spectrum Weather System), and scab infection periods were calculated using a modified Mills apple scab infection model. Ascospore release is now down to 0.5% from that of 100% spore release on April 17.  Although mature spores trapped at FREC are now only 75 compared to 28,000 spores in April, it is always safest to consider primary scab to end by June 15.  This means primary scab is not quite over yet and it is imperative to use cover spray programs to manage scab and summer diseases.

Powdery Mildew and Dry Weather

Powdery mildew pressure remains high this season, with 20 days favorable for infection from April 1 through May 25.  Powdery mildew infection develops at 50 to 77 degrees F and does not require moisture to germinate and multiply.  Recent warm temperatures and vigorously growing shoots are perfect conditions for powdery mildew to explode. Sterol inhibitor (SI) fungicides are very effective in controlling powdery mildew. If you have already used your limit of 3 SI applications and still have high powdery mildew pressure, consider using 5-6 lb Sulfur + 0.5 % Lime Sulfur or 2 qt Sulforix.  This program is being evaluated in a FREC research orchard and showing good powdery mildew control compared to nontreated trees.  Sulforix is a lighter version of Lime Sulfur with surfactant. CAUTION:  Do not use sulfur when oil was applied within 14 days and when the temperature is 85 degrees F and above.

Bacterial Spot on Stone Fruit

Warm (70 to 85 degrees F) and wet weather favor bacterial spot infection. We have observed nine infection periods from April 15 to date. We saw the first symptom on leaves 3 weeks ago and are now seeing development on fruit.  It is recommended to continue spraying with effective bactericides during wet and warm conditions.


Information on new materials that have been registered this year may be found in the previous Fruit Times.  Infection periods are posted at the FREC website