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°

Fire blight update; scab and rust infection

The observations, conditions, and recommendations reported for Winchester, VA are provided as a guide to fire blight risk assessment for the area of the Virginia Tech AREC located six miles southwest of Winchester. At our AREC there is still a lot of susceptible bloom on late blooming cultivars such as Rome. 

FIRE BLIGHT ALERT: The risk column of Maryblyt 7 graphic below, based on predicted temperatures and wetting for May 14-18, shows that fire blight risk will remain extremely high through Friday, May 16 wherever bloom is present. In high-risk situations, a protective streptomycin application is recommended ahead of predicted infection conditions. In the event of hail injury or other trauma blight situation, apply streptomycin as soon as possible after the event. Routine spray applications can provide enough wetting to meet the wetting requirement for infection if all other infection conditions have been met and this would have been true for May 11-12. Under the warm temperature conditions shown for the this week, streptomycin remains residually effective for only about 3 days. 

With this fire blight pressure, you still might want to consider applying the plant growth regulator, Apogee to suppress shoot blight if hail or another trauma blight event were to occur. Optimum timing of the first Apogee application for this purpose is at petal fall on king bloom, and a suppressive effect should not be expected until 7-10 after Apogee application.

The temperature and rainfall data are current through Wednesday morning, May 14, for the graphic above from the Maryblyt 7 program. Predicted weather conditions are shown for May 12-16. The components of fire blight risk are indicated in the columns labeled B (blossoms open), H (degree hours for epiphytic bacterial population buildup, EIP >100), W (wetting by rain or dew), and T (average daily temperature 60 F or above).  

The BBS column is tracking predicted appearance of blossom blight symptoms from infection May 8 to predicted symptom appearance (108a) May 14. The CBS column at the right in the graphic indicates progression toward the appearance of canker blight symptoms on new growth, due to extension of overwintering cankers from last year, with canker margin symptoms (CMS) by May 8 and canker blight symptoms May 14. Canker advancement cannot be prevented by a chemical treatment, and the presence of symptoms will indicate a build-up of inoculum which could become a factor in the event of a trauma blight situation due to hail injury, etc. The weather conditions used in the predictive part of this graphic come from the Weather Channel for Winchester, supplemented by site-specific data from SkyBit Inc.

SCAB and RUSTS: An apple scab and rust infection period is in progress, with more than 14 hr of wetting and temperatures in the mid-60s. Much cedar-apple and quince rust inoculum is still available and many small unprotected fruits are still susceptible. We are now into a heavy secondary scab situation where scab was not adequately controlled on Apr 11-12, Apr 14-15, or Apr 28-May 1. This wetting period also favors infection by peach scab.

Wetting period, hail and trauma blight prediction, and sooty blotch/flyspeck wetting hours

Last night we recorded an apple scab infection period (8 hr wet, mean 67°F).

Last Tuesday afternoon, May 17, scattered hail occurred with 1.6” of rain in some areas of Frederick County. Tracking Maryblyt prediction for this single event with this week's highs in the mid-80s and lows in the mid-60s, indicates that trauma blight symptoms should begin to appear May 25. Typically trauma blight is more common in affected areas that also had some blossom blight from infection during bloom. The plant growth regulator, Apogee, applied at petal fall on king bloom, helps to reduce the amount of shoot tip infection (shoot blight) in these situations.

Since May 8 we now have accumulated 109 hr toward the 250 wetting hour threshold for specific treatment against the sooty blotch and flyspeck fungal complex.