Extended wetting periods May 27-30 and accumulated wetting hours

We had extended wetting May 27-28: 11 hours with 65° and 0.33 in. rain. Last night's extended wetting, May 29-30, was 10 hours at 71° and 0.12 in. rain. These wetting periods favored secondary scab and all summer diseases on apples and brown rot on ripening cherries and other stone fruits.


On May 25 we passed the 250-wetting hour threshold for specific treatment against the sooty blotch/flyspeck fungal complex. Accumulated wetting hour total from rainfall or dew since Apr 18 is now at 285 hours. This is four weeks ahead of last year and the second earliest week for reaching this predictive threshold since we began recording this in 1994. In seven of these 19 years we reached this threshold one week later.

Extended wetting May 24-25

Last night's extended wetting came to 15 hours with 60-70° temperatures and 0.06 in. rain. This wetting favored scab and all summer diseases on apples and brown rot on ripening cherries and other stone fruits.

Infection period May 23-24

Last night's wetting (12 hr at 63° with 0.04 in. rain) qualified as a scab infection period. 

This wetting brought the accumulated wetting hours since Apr 18 to 234 hours. In the next couple days it is likely that we will reach the 250 wetting hour threshold for specific treatment against the sooty blotch/flyspeck fungal complex. This would be the second earliest week for reaching this predictive threshold since we began recording this in 1994. Only 2003 was earlier. This signals early summer disease pressure from the sooty blotch/flyspeck fungal complex as well as other "summer diseases" and Alternaria leaf blotch.

As expected, there are early indications of heavy brown rot pressure on ripening cherries.

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.

Early season disease update; sooty blotch and flyspeck wetting hour accumulation

A scab and cedar-apple rust infection period occurred May 13-15 (26 hours wet at 61°, 0.4 in. rain). There was still cedar rust inoculum available for foliar infection but galls are now mostly depleted at our AREC. If you have a recurrent cedar-apple rust problem in your orchard, check the cedar galls in your area for remaining inoculum potential with wetting today.

Through May 20 there have been 38 days favorable for powdery mildew infection since spores were first available Mar 19. Secondary scab and mildew infection are heavy on non-treated trees.

Some shoot blight was reported last week, resulting from scattered hail in some areas of Frederick County May 2. These "trauma blight" symptoms appeared near the time predicted by Maryblyt for May 15. Numerous early reports of fire blight have come from areas east of the Blue Ridge and the Roanoke area south. These likely occurred as blossom infection where bloom was open earlier than in the Winchester area.

For purposes of predicting the development of the sooty blotch and flyspeck (SBFS) fungal complex, we record accumulated wetting hours, starting 10 days after petal fall. With the extended bloom period this year, the petal fall date was somewhat subjective, but we have settled on Apr 8 for petal fall. So the start of wetting hour accumulation will be from Apr 18, 20 days ahead of last year and 11 days ahead of 2010. Through this morning we have accumulated 189 wetting hours toward the 250 wetting hour threshold for specific treatment against the SBFS fungal complex.

Scab and rust infection periods May 5 & 8

Wetting periods favorable for scab and cedar-apple rust infection occurred May 5-6 (13 hours wet at 63°,< 0.01 in. rain) and May 8-9 (split wetting, 18 hours at 64°, 0.4 in. rain). There are still plenty of active rust galls giving off inoculum for foliar infection. Fruit are no longer susceptible to rust infection.

Through May 10 there have been 30 days favorable for powdery mildew infection since spores were first available Mar 19.

There was scattered hail in some areas of Frederick County May 2 and fire blight shoot symptoms related to trauma blight infection are predicted to appear May 15.

The above wetting periods were also favorable for scab infection on peaches and nectarines.

Disease update for the weekend

The recent fire blight scenario unfolded not too far off of that shown in the graphic below for Apr 28. Briefly put, based on recorded and predicted temperatures, any day with wetting from May 1 through May 11 is a potential fire blight infection day where flowers persist. There is still susceptible bloom on Romes, York and Nittany, and some other varieties such as Golden Delicious and Gala, some of which may have been initiated in response to frost injury March 27. Some of these days will have wetting from rain or dew; others will have wetting from thinning or maintenance sprays. On May 2 there was scattered hail that may have caused a trauma blight situation in some areas of Frederick County

Powdery mildew pressure remains high this season, with 26 days favorable for infection through Apr 30. With recent warmer temperatures and more rapid shoot growth, secondary infection is evident and foliage susceptibility very high. Shoots showing secondary infection are likely to have infected lateral buds that will become overwintering inoculum sources for next year's mildew epidemic.

Wetting periods at our AREC this week have been close for secondary scab but not quite long enough for cedar-apple rust infection of foliage. Fruit at thinning size are no longer susceptible to cedar or quince rust infection. There is still a lot of inoculum present on cedar rust gall spore horns.