At Winchester we recorded a heavy secondary apple scab infection period May 11-13, with 46 hr wetting with a mean temperature of 49° and 1.82 inches of rain. This volume of rainfall could have eliminated much of the protective fungicide residue during the extended wetting to allow some scab and rust infection.
Areas in the Valley, southward to Roanoke, and areas east of the Blue Ridge from Fauquier County south to Lynchburg and Danville also saw extended wetting periods ranging from 32-48 hr, all at temperatures in the low to mid-50s. The more southern areas had split wetting periods or two separate ones starting May 9. These conditions also favored peach scab infection.
Rusts have been active in the Winchester area for six weeks, and some cedar rust galls are still active. Cedar-apple rust lesions (shown below) and some quince rust lesions are now appearing from the Apr 24-26 infection period; others from the extended wetting May 4-6 are expected this week.
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| May 10, 2017. Early appearance of cedar-apple rust lesions and scab (upper left) from the infection period Apr 24-26. |
Apple powdery mildew conidia were available for infection at Winchester as early as Mar 27, and there have been 18 “dry weather“ mildew days suitable for mildew infection from Mar 27 to May 14. The image below illustrates the appearance of a primary mildew shoot and secondary infection on an adjacent shoot.
Adjacent Golden Delicious shoots infected with primary infection (right) and secondary infection (left). Primary infection emerges with shoot growth from an infected overwintering bud. Secondary infection is usually heavier near a primary inoculum source.
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 Apr 24 as our petal fall date for Winchester, so the start of wetting hour accumulation is from May 4. As of May 14, accumulated wetting hours (ACW) toward the 250 wetting hour threshold for specific treatment against the SBFS fungal complex were: at 909 ft elevation, 98 hr; at 932 ft (the AREC NEWA station), 83 hr; and at the 983 ft elevation, 73 hr ACW. One of the purposes of following three weather stations is to compare wetting hour accumulation at different elevations; however, because many of the early wetting hours this year were a result of rainfall rather than dew, the early accumulation trend this year is a bit different than in some previous years.
For central Virginia, we have selected Apr 20 as the petal fall date for accumulation of wetting hours by weather stations at Tyro. As of May 14, a sensor placed at 1465 ft. elevation had accumulated only 31 wetting hours from Apr 30, while the one at 1165 ft. had accumulated 72 wetting hours, and the one placed at 941 ft elevation had accumulated 94 hr.
