Tuesday, May 12, 2020

Secondary scab infection May 5-6; fire blight infection conditions on late bloom May 15-18.

At Winchester, most apple varieties are well beyond petal fall, but late bloom susceptible to fire blight persists on some late varieties and some recently planted trees

At the AREC last week an apple scab infection period occurred May 5-6: 17 hr wet at 47° with 0.18 in. rain. Similar conditions also extended south to Staunton and east of the Blue Ridge from Loudoun County to the Charlottesville area, where infection conditions resulted from a combined wetting period. At Winchester, cedar-apple rust galls and quince rust cankers remain active after this wetting event.

Apple powdery mildew infection occurs on days without rainfall above 53°, and we have had 14 days favorable for infection since spores were available on Mar 29. Below is an example of primary and secondary powdery mildew on Idared apple.

A secondary powdery mildew lesion (bottom left) next to a primary mildew shoot on Idared apple. 

The Winchester area will likely see its first fire blight infection conditions wherever susceptible bloom remains this weekend, May 16-18. Similar conditions exist for all other major fruit producing areas of Virginia, but with some starting on May 15. Blossom infection has already been reported on apples east of the Blue Ridge south of Charlottesville and in southwest Virginia, and on pears in Clarke County from infection that occurred six weeks ago. Below is an example of late bloom that was observed on Goldrush apple in Nelson County last week.

Late bloom and set fruit on Goldrush apple in Nelson County May 7, 2020.
Just to add a footnote about the unusually cool conditions through April and into mid-May that allowed the Winchester area to escape fire blight infection from Mar 30 until the very latest of bloom: The daily mean high temperature for April this year was 60.0° and, since 1928 only three years had a cooler mean daily high temperature, 1935, 1961, and 1966!  Meanwhile, 17 years had a mean April high greater than 70°