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.

Extended wetting events this week

It has been a wet week at our AREC this past week, with significant infection events five consecutive days, including the one noted in the previous post for June 8-9. These were as follows: June 9-10 (8 hr, mean 70° with 0.02 in. rain); June 10-11 (16 hr, mean 76°, with 0.19 in. rain); June 11-12 (14 hr, mean 70° with 0.11 in. rain); June 12-13 (16 hr, mean 70° with 0.44 in rain). Total rainfall for June 9-13 at our AREC was 1.36 inches but volumes with the several storms varied widely throughout the local area and there was also scattered hail in some locations.

Temperatures and length of wetting during these events favored secondary apple scab and early summer disease development. Cedar rust galls were producing spores through all of these events, but are nearly depleted now. Secondary spread of fire blight was possible where growing shoot tips are still susceptible.

In addition to developing activity by the usual summer diseases, with these recent warmer wetting periods, be alert for possible early indications of Alternaria leaf blotch on Red Delicious and Glomerella leaf spot on Gala, Golden Delicious and Cripps Pink (Pink Lady) apples. This is also a critical time for protection against Brooks fruit spot.

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 10 days after petal fall. As of Friday afternoon, June 13, we had accumulated 136 wetting hours toward the 250-hour threshold for specific treatment against the SBFS fungal complex. 

Infection conditions for June 9-13 were similar at Tyro, VA. As of June 12, a sensor placed in an apple tree at 1165 ft. elevation had accumulated 90 wetting hours from May 11, while one placed at 941 ft elevation had accumulated 172 hr. The station at the highest elevation (1465 ft) apparently spent some time in the clouds this week and continues to register more wetting hours than the intermediate one, 125 CWH.

Spring and summer disease update

In the past five days we have had two more apple scab and cedar-apple rust infection periods at our AREC June 4-5: 23 hours wetting at 73-60°, and June 8-9 at 74-66°. Note that cedar rust galls are still actively producing spores.

In addition to developing activity by the usual summer diseases, with the recent warmer wetting periods, be alert for possible early indications of Alternaria leaf blotch on Red Delicious and Glomerella leaf spot on Gala, Golden Delicious and Cripps Pink (Pink Lady) apples. This is also a critical time for protection against Brooks fruit spot.

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 10 days after petal fall. As of this morning, June 8, we had accumulated 80 wetting hours toward the 250-hour threshold for specific treatment against the SBFS fungal complex. 

At Tyro, VA we selected May 11 as the start date for accumulation of wetting hours by weather stations at Tyro, VA. As of June 5, a sensor placed in an apple tree at 1165 ft. elevation had accumulated 62 wetting hours from May 11, while one placed at 941 ft elevation had accumulated 128 hr. It is interesting that the station at the highest elevation (1465 ft) registered more wetting hours than the intermediate one, 81 CWH.

Scab and rust infection period; fire blight; tracking sooty blotch and flyspeck development

We recorded another apple scab and cedar-apple rust infection period at our AREC May 29-30: 22 hours wetting with 0.6 inches of rain at 55-58°.

Last week we had several reports and observations of fire blight outbreaks. The onset of symptoms and local temperature history over the past several weeks indicate that these probably dated back to secondary infection that occurred about May 13, when there were some strong storms in the area.

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 May 8 as our petal fall start date for Winchester, so the start of wetting hour accumulation is from May 18. This is 5 days later than 2013 and 30 days later than 2012. By the end of the wetting period Friday, May 30, we had accumulated 33 wetting hours toward the 250 wetting hour threshold for specific treatment against the SBFS fungal complex. 

A caution to those further south and east of the Blue Ridge: Your petal fall date may have been a week or more ahead of ours and we have arbitrarily selected May 11 as the start 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. As of May 30, a sensor placed in an apple tree at 1165 ft. elevation had accumulated 50 wetting hours from May 11, while one placed at 941 ft elevation had accumulated 109 hr!