Transition to summer diseases

We have had extremely cool conditions the first three weeks of May, with temperatures reaching the 80s only one day (May 2) and barely nudging the 70s only five other days. We are now seeing the development of scab and cedar-apple rust lesions from the infection periods outlined in previous posts: Apr 22-23 and Apr 28-May 12. Mildew-susceptible cultivars are showing considerable secondary infection, a result of the earlier 24 dry weather mildew infection days through Apr 25. Two more extended wetting events last week favored secondary scab and moldy core infection and kicked off the early summer disease pressures. These were: May 21, 14 hr wet at 54° with 0.62 in. of rain, and May 21-23, 33 hr wet at 54° with 0.34 in. of rain. (Actually these two could be lumped together, increasing the threat of fruit infection where primary scab was not well-controlled Apr 22-May 5).

Fire blight infection symptoms appeared locally as early as May 9, from infection that occurred Apr 21-26; symptoms from infection May 2-3 are expected later this week. After we reach late bloom in most years, temperatures are warm enough that protection of late bloom is routinely needed. However this year we have gone 19 consecutive days without infection conditions. But this week conditions return to "normal", with temperatures in the 80s increasing the risk (EIP, effective inoculum potential) to an infective level wherever wetting occurs on late bloom remaining into next week. Continue to protect prolonged bloom, especially on newly planted trees, as needed. With the prolonged cool conditions, we are still seeing late bloom on varieties such as Rome three weeks after we would like to be finished thinking about fire blight!

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 again chosen May 4 as our petal fall date for Winchester, so the start of wetting hour accumulation is from May 14. As of Monday, May 23, accumulated wetting hours (ACW) toward the 250 wetting hour threshold for specific treatment against the SBFS fungal complex were: at 909 ft elevation, 94 hr; at 952 ft elevation, 76 hr; and at the 983 ft elevation, 73 hr ACW.
To those farther south and east of the Blue Ridge, your normal petal fall date would have been a week or more ahead of ours at Winchester, so we have selected Apr 27 as the petal fall date for accumulation of wetting hours by weather stations at Tyro, VA. One of the purposes of having three weather stations at this location was to compare wetting hour accumulation at different elevations, but because many of the early wetting hours were a result of rainfall rather then dew, the early accumulation trend this year is a bit different than in previous years. As of May 23, a sensor placed at 1465 ft. elevation had accumulated 130 wetting hours from May 4, the one at 1165 ft. elevation had accumulated 107 wetting hours, and the one placed at 941 ft elevation had accumulated 133 hr.

Sooty blotch and flyspeck and other summer diseases

Early signs of sooty blotch and flyspeck were observed in lower elevation areas at our AREC Monday, June 29. At this elevation (909 ft) we had reached the 250-hour total accumulated wetting hour (ACW) action threshold June 10. At higher elevations, the dates that the 250-hr threshold was reached and ACW totals June 29 were: 952 ft elevation- June 13, 332 hr ACW; 983 ft elevation- June 21, 297 ACW. Note that the threshold had been reached by June 21 at all AREC locations ranging from 909 to 983 ft elevations.

At our AREC we had two more extended wetting periods that favored summer disease development: June 25-26, 14 hr wetting at 72-64° with 0.02 in. of rain; June 26-28, 32 hr wetting at 72-59° with 1.62 in. of rain. This latter volume of rainfall would have depleted most fungicide residue available for sooty blotch, flyspeck and rot control.

At Tyro in Nelson County, a sensor placed at at 941 ft elevation had already recorded the 250-hr ACW threshold by June 3 for a total of 384 hr by June 29. By June 29, the ones at higher elevations, 1465 ft and 1165 ft, had not yet reached the 250-hr threshold with only 216 and 212 wetting hours, respectively.

Sooty blotch and flyspeck

Sooty blotch and flyspeck were observed in a commercial orchard in Nelson county yesterday, July 1. The pinkish area may indicate early rot infection. I do not have recorded wetting hours for this location.