Extended wetting events, mildew still active; wetting hour accumulation related to elevation

We had two more extended wetting events at our AREC this past week: June 16-17 (12 hr, mean 71° with 0.6 in. rain) and June 19-20 (14 hr, mean 71°, with 0.4 in. rain).

The temperatures and length of wetting during these events favored secondary apple scab and early summer disease development. Secondary spread of fire blight was possible where growing shoot tips are still susceptible. Cedar rust galls are mostly depleted now. 

Powdery mildew remains active with continued shoot growth. Since mildew spores were first observed Apr 9, we have had 42 dry weather “mildew infection days”: 15 in April, 20 in May and 7 in June. So it is not hard to find secondary infection in susceptible unprotected orchards.

For purposes of predicting development of the sooty blotch and flyspeck (SBFS) fungal complex, we record accumulated wetting hours (CWH) from rainfall or dew, starting May 18, ten days after petal fall. This week there was about twice as much wetting hour accumulation at Winchester than at the central Virginia location. 

By Friday morning, June 20, at our usual AREC monitoring site at elevation 950 ft, we had accumulated 176 wetting hours toward the 250-hour threshold for specific treatment against the SBFS fungal complex. But at a lower AREC elevation (910 ft) we have already surpassed this threshold with 289 CWH.  At Tyro, VA as of June 20, the sensor at 1165 ft. elevation had accumulated 99 wetting hours from May 11, while one placed at 941 ft had accumulated 193 hr. Both of these locations illustrate the importance of elevation for wetting hour accumulation. Scout your lower orchard areas for early onset of SBFS appearance and adjust you fungicide program accordingly.