Recent extended wetting; early summer disease pressures June 2, 2019

At our AREC in Winchester we recorded two extended wetting events: May 26-27, with 6 hr of wetting at 69° with 0.15 in. rain; and June 2, with 9 hr of wetting at 61° with 0.2 in. rain. Again these extended wetting events at relatively warm temperatures favored secondary apple scab, early summer disease activity on apples, peach scab, cherry leaf spot and brown rot on ripening cherries. 

For purposes of predicting the development of the sooty blotch and flyspeck (SBFS) fungal complex, we record accumulated wetting hours from rainfall or dew, starting 10 days after petal fall. This year we chose May 2 as our petal fall date for Winchester, so the start of wetting hour accumulation was from May 12. As of 8 AM June 2, accumulated wetting hours (ACW) already passed the 250 wetting hour threshold for specific treatment against the SBFS fungal complex: at 909 ft elevation we had 331 hr; at 932 ft (the AREC NEWA station), 93 hr; and at the 983 ft elevation, 90 hr ACW. One of the purposes of following three weather stations is to compare wetting hour accumulation at different elevations. Note that we have already passed the threshold at the lower elevation and the total is more than 100 hr ACW more than last year on this date. At the two higher elevations ACW accumulation is more than 100 less than last year at this time. NOTE: Edited Jun 12: Last week we discovered an error with the sensor at the lower elevation, 909 ft, which had already accumulated 331 hr ACW; that figure was distorted because the wetness sensor had come loose from its stand and dropped into the grass where it remained wet abnormally long.

The sooty blotch/flyspeck model on NEWA indicates accumulated wetting similar to the ACW shown at our NEWA station. To use this model, select a weather station, go to the drop-down menu for diseases and enter the petal fall date (e. g. May 2) to calculate the risk for that location.

Sooty blotch/flyspeck risk summary for Winchester AREC, 932 ft elevation, using May 2 as petal fall date.

For central Virginia, we selected Apr 23 as the petal fall date for accumulation of wetting hours by selected weather stations. As of June 2, Red Hill had 349 ACW (also past the threshold). Lynchburg  had 240  ACW. Also east of the Blue Ridge, the NEWA station at Batesville  has 109 ACW, while the NEWA station at Gadino Cellars near Washington, VA has recorded 150 ACW with Apr 29 as the petal fall date. 

Early summer disease pressure and continued wet weather

In the past two weeks at  our AREC in Winchester we recorded three more extended wetting events: June 19-20, with 17 hr of wetting at 73° with 0.1 in. rain; June 20-21, with 17 hr of wetting at 71° with 1.44 in. rain; June 21-23, with 35 hr of wetting at 64° with 3.05 in. rain. These wetting events favored early summer disease and rot activity on apples, peach scab, and cherry leaf spot and brown rot on ripening cherries and other stone fruits. 

For purposes of predicting the development of the sooty blotch and flyspeck (SBFS) fungal complex, we record accumulated wetting hours from rainfall or dew, starting 10 days after petal fall (from May 17 at Winchester). As of 6 AM June 25, all three of our AREC weather stations accumulated more than the 250 wetting hour threshold for specific treatment against the SBFS fungal complex. These totals were: at 909 ft elevation, 391 hr; at 932 ft (the AREC NEWA station), 333 hr; and at the 983 ft elevation, 343 hr accumulated wetting hours (ACW). This year the wetting hours have been more a result of rainfall rather than wetting from dew, so wetting hour accumulation at different elevations is quite similar, and all have reached the 250-hr threshold. 

For central Virginia, we selected Apr 24 as the petal fall date for accumulation of wetting hours by weather stations at Tyro. As of June 25, a sensor placed at 1465 ft. elevation has now reached the 250-hr threshold with 252 wetting hours, while the one at 1165 ft. had accumulated 259 wetting hours, and the one placed at 941 ft elevation has accumulated 466 hr. Also east of the Blue Ridge, the NEWA station at Batesville has passed the 250-hr threshold with 395 ACW while the NEWA station at Gadino Cellars near Washington, VA has recorded 334 ACW using May 1 as the petal fall date. 

We have now confirmed active bitter rot at our AREC (pictured below), and there have been several reports of bitter rot from central Virginia. 

Bitter rot on Honeycrisp apple (Photo by D. Melby)

Extended wetting events; sooty blotch/flyspeck threshold reached, early bitter rot activity

In the past week at  our AREC in Winchester we recorded two extended wetting events: June 10-11, with 15 hr of wetting at 59° with 0.51 in. rain; June 11-12, with 15 hr of wetting at 59° with 0.01 in. rain. These wetting events favored early summer disease activity on apples, peach scab, cherry leaf spot and brown rot on ripening cherries. 

For purposes of predicting the development of the sooty blotch and flyspeck (SBFS) fungal complex, we record accumulated wetting hours from rainfall or dew, starting 10 days after petal fall. This year we have chosen May 7 as our petal fall date for Winchester, so the start of wetting hour accumulation is from May 17. As of 6 PM June 13, accumulated wetting hours (ACW) toward the 250 wetting hour threshold for specific treatment against the SBFS fungal complex were: at 909 ft elevation, 276 hr; at 932 ft (the AREC NEWA station), 252 hr; and at the 983 ft elevation, 263 hr ACW. This year the wetting hours have been more a result of rainfall rather than wetting from dew, so wetting hour accumulation at different elevations is quite similar, and all have reached the 250-hr threshold. 

For central Virginia, we selected Apr 24 as the petal fall date for accumulation of wetting hours by weather stations at Tyro. As of June 13, a sensor placed at 1465 ft. elevation had accumulated only 172 wetting hours from May 4, while the one at 1165 ft. had accumulated 228 wetting hours, and the one placed at 941 ft elevation had accumulated 390 hr, and passed the 250-hr threshold for specific treatment for sooty blotch and flyspeck on May 29. Also east of the Blue Ridge, the NEWA station at Batesville has passed the 250-hr threshold with 328 ACW while the NEWA station at Gadino Cellars near Washington, VA has recorded 261 ACW using May 1 as the petal fall date. 

Two weeks ago various "rot spots" were reported on several apple varieties from central Virginia. Those have now been confirmed as incubating into bitter rot. 

Another wet week in Winchester

Wet weather continued this past week at Winchester and other parts of Virginia. At our AREC in Winchester we recorded three more extended wetting events: May 31-June 1, with 15 hr of wetting at an average of 70° with 0.65 in. rain; June 1-2, with 11 hr of wetting at 68° with 1.47 in. rain; June 2-3, with 28 hr of wetting at an average of 65° with 1.95 in. rain. Again these heavy rains and extended wetting at relatively warm temperatures favored secondary apple scab, early summer disease activity on apples, peach scab, cherry leaf spot and brown rot on ripening cherries. 

For purposes of predicting the development of the sooty blotch and flyspeck (SBFS) fungal complex, we record accumulated wetting hours from rainfall or dew, starting 10 days after petal fall. This year we have chosen May 7 as our petal fall date for Winchester, so the start of wetting hour accumulation is from May 17. As of 6 PM June 3, accumulated wetting hours (ACW) toward the 250 wetting hour threshold for specific treatment against the SBFS fungal complex were: at 909 ft elevation, 207 hr; at 932 ft (the AREC NEWA station), 196 hr; and at the 983 ft elevation, 209 hr ACW. One of the purposes of following three weather stations is to compare wetting hour accumulation at different elevations. This year the wetting hours have been more a result of rainfall rather than dew, so wetting hour accumulation at different elevations is quite similar compared to previous years. 
The sooty blotch/flyspeck model on NEWA is now functioning and indicates accumulated wetting similar to the ACW shown at our NEWA station. To use this model, go to the drop-down menu for diseases and select the petal fall date to calculate the risk for that location.

Sooty blotch/flyspeck risk summary for Winchester AREC, using May 7 as petal fall date.

For central Virginia, we selected Apr 24 as the petal fall date for accumulation of wetting hours by weather stations at Tyro. As of June 3, a sensor placed at 1465 ft. elevation had accumulated only 127 wetting hours from May 4, while the one at 1165 ft. had accumulated 129 wetting hours, and the one placed at 941 ft elevation had accumulated 319 hr, and passed the 250-hr threshold for specific treatment for sooty blotch and flyspeck on May 29. Also east of the Blue Ridge, the NEWA station at Batesville has passed the 250-hr threshold with 255 ACW while the NEWA station at Gadino Cellars near Washington, VA has recorded 220 ACW with May 1 as the petal fall date. 

Summary of last week's infection events, May 15-19.

At our AREC in Winchester we recorded a very long apple scab and rust infection period May 15-19, with 89 hr of almost continuous wetting at an average of 63° with 5.2 in. rain. We had observed scab lesions prior to this infection event, so potentially this was a very serious secondary infection where there was little fungicide protection to cover the event through five inches of rainfall. Some of the wetting occurred at temperatures near 70° and was favorable for moldy core infection and the start of summer disease activity. 

As a follow-up to this extreme infection period, a suggestion is to apply combinations fungicides with protectant activity and two classes of fungicides to offset the potential for resistance in apple scab; also include an SI fungicide for after-infection activity for cedar-apple rust. A second follow-up application should include another class of activity for scab plus a protectant fungicide.

This weather was also favorable for cherry leaf spot and peach scab, which will remain active for the next month. At our AREC we have had 22 apple powdery mildew infection days since Apr 10. With the long wetting last week, mildew activity was somewhat reduced, but it will be quite active with susceptible shoot growth this week

Other areas in Virginia also experienced long continuous or split wetting periods with amounts of rainfall ranging from two - five inches. We are near the transition from early season to summer disease control, but all areas should be concerned about remaining apple scab, as well as moldy core and early summer disease activity, especially bitter rot. The timing of this event might have also left an opening in protection against for Brooks fruit spot.

Disease update: recent and ongoing scab and rust activity

We have had a lot of apple scab and rust activity this past week, and more on the way. At our AREC we recorded scab and rust infection periods May 10-11 (15 hr wet at 62° with 0.05 in. rain), and May 14-15 (13 hr wet at 66° with 1.11 in. rain). As of the morning of May 16 another infection period is in progress and much rain and wetting is in the forecast for the next three days. We observed scab lesions, probably from infection periods Apr 15-16 or Apr 24-25, on leaves at the base of unprotected shoots. Cedar-apple rust galls are still actively producing spores. 

This weather is also favorable for cherry leaf spot and peach scab, which will remains active for the next month or so.

As indicated in the update May 14, these wetting events at temperatures in the 60s constitute fire blight infection events wherever susceptible flowers are present. Another threat is possible secondary shoot blight with bacteria spreading from blossom blight symptoms now appearing from infections May 5 and 7, and driven by a series of thunderstorms with some scattered hail.

At our AREC we have had 21 apple powdery mildew infection days since Apr 10.

Rapid bud development; powdery mildew, scab and rust activity

The past two days we have seen rapid apple bud development in the Winchester area. Friday morning, Apr. 13, we saw heavy powdery mildew sporulation on Idared buds at tight cluster stage. Powdery mildew infects on dry days at temperatures above 53°F, so there were several infection days this week.

Powdery mildew on Idared, Friday morning, Apr. 13, 2018.

By Friday evening, many buds were showing color at the pre-pink stage. With warm temperatures over the weekend, expect open bloom by early next week.

Pre-pink stage of Idared buds Friday evening, Apr. 13, 2018

There is a forecast for thunderstorms and rain with temperatures in the 50s Sunday and into Monday, Apr 15-16, and that would bring a scab infection period as well as our first cedar-apple and quince rust infection period in the Winchester area. 

Cedar-apple rust gall with developing spore horns Apr. 11, 2018

 Generally, the forecast is similar for the areas south to beyond Roanoke and east of the Blue Ridge, with a trend toward earlier and a shorter wetting period, but with warmer temperatures to the south. In short, scab, the rusts, and powdery mildew are now active, and there is a potential for fire blight infection in more advanced areas with open bloom.

These conditions also favor cherry leaf spot and brown rot infection with cherry trees now in bloom, and brown rot and scab on peaches in bloom or petal fall stage.

Infection periods May 21-23

Scab infection periods occurred May 21-22 (22 hr at 67°) and May 22-23  (12 hr at 64°). Total rainfall for these two events was 0.26 in. at our AREC but some other areas of Frederick County received considerably more rain volume, especially on the evening of May 22. 

These wetting periods at relatively warm temperatures could signal early summer disease pressure. Through this morning we had accumulated 222 wetting hours since Apr 18, rapidly advancing toward the 250 wetting hour threshold for specific treatment against the sooty blotch/flyspeck fungal complex.

Cherry leaf spot symptoms were observed today.

Fungal disease update

Sixteen hours of intermittent wetting May 1-2 favored primary and secondary apple scab infection of leaves and fruit, quince rust fruit infection on cultivars in bloom and cedar-apple rust on foliage of all susceptible cultivars. Because a lot of rust inoculum was still available for infection during this wetting, it would be prudent to include an SI fungicide for after-infection rust control in applications this week. On stone fruits, these weather conditions were also favorable for peach scab and cherry leaf spot infection.

Since primary apple powdery mildew inoculum was first observed March 23, 13 dry-weather days have been favorable for mildew infection. Although this is fewer mildew days than we had to this point last year, secondary mildew lesions will become apparent on new growth in affected blocks.