Extended wetting last week will trigger early summer disease development

At the AREC last week an apple scab and cedar-apple rust infection period occurred May 22-23: 24 hr of combined wetting at 60° with 1.14 in. rain. Conditions were similar east of the Blue Ridge in Manassas and Sperryville, but more extreme to the south from Staunton to Roanoke and Floyd and east of the Blue Ridge from the Charlottesville area southward to Lynchburg and Danville. At Winchester, cedar-apple rust galls remain active with more extended wetting periods predicted for this week.

Staunton had two wetting periods of 10 and 42 hr with a total of 0.91 in. rain. In Roanoke, two infection periods totaled 113 hr combined wetting and 10.17 in. rain. The total rainfall in Floyd was 1.92 inches, but with 115 hr of extended wetting. In Albemarle County, combined wetting ranged from 43-55 hr at Crozet, Red Hill and Batesville to 102 hr on Carter Mountain with rainfall totals from 1.3 to 2.6 inches. Lynchburg had 86 hr wetting with 2.95 in rain, Rustburg 114 hr wet with 3.87 in. rain and Danville 89 hr combined wetting with 4.69 in. rain.

In all areas, this extended wetting will contribute to early accumulation of wetting hour totals toward development of sooty blotch and flyspeck. Fortunately, much of the earlier portion of the wetting events was at cooler temperatures not as favorable to rot development, but the latter portion was at warmer temperatures, and occurred after heavy rains would have depleted fungicide residue, making the fruit vulnerable to latent rot infection. This extended wetting was also favorable to secondary scab and late rust infection on foliage, and the extremely long wetting periods can increase the amount of fruit scab.

For purposes of predicting the development of the sooty blotch and flyspeck (SBFS) fungal complex, we record accumulated wetting hours (ACW) from rainfall or dew, starting 10 days after petal fall. The action threshold for SBFS development is 250 ACW. Here is a listing of selected petal fall date and total wetting hour accumulation (ACW) for selected locations: Winchester, May 5, 55 ACW; Staunton, May 5, 133 ACW; Roanoke, Apr 18, 174 ACW; Floyd, Apr 28, 159 ACW; Manassas, Apr 28, 91 ACW; Sperryville Apr 28, 102 ACW; Batesville Apr 18, 164 ACW; Crozet, Apr 18, 138 ACW; Carter Mountain, Apr 18 329 ACW; Red Hill, Apr 18, 150 ACW; Lynchburg, Apr 13, 199 ACW; Rustburg, Apr 13, 201 ACW; Danville, Apr 13, 243 ACW. The action threshold of 250 ACW means that the SBFS fungi would be present on unprotected fruit when the threshold is reached. Lynchburg, Rustburg and Danville are predicted to reach the threshold within the coming week and Carter Mountain has already passed the threshold. ACW is related to wetting from rainfall or dew, and often lower elevations in an orchard accumulate wetting hours more quickly because of wetting from dew, but that obviously is not the case with the higher elevation of Carter Mountain this year.

Apple powdery mildew infection occurs on days without rainfall above 53°, and in the Winchester area we have had 26 days favorable for infection since spores were available on Mar 29.