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.

Disease conditions May 8-12

Wet weather disease conditions persisted this week with infection periods each day May 8-12: May 8: 8 hours wet at 52-56° with 0.1 in. of rain; May 9: 15 hr wet with 0.1 in. of rain; May 10: 8 hr at 53° with 0.03 in. of rain; May 10-11: 19 hr at 55° with 0.25 in. of rain; May 11-12: 16 hr at 55° with 0.16 in. of rain. This now totals 13 infection periods in 15 days! Several of these infection periods again involved both scab and rusts.

Hopefully, the fruits of most varieties are now big enough that they have become resistant to quince rust . However, Romes and some other varieties with late bloom where there was frost damage might be the exception. There still is some cedar-apple rust inoculum left that could infect the youngest leaves, but the long wetting periods are now depleting much of the rust inoculum. This week we saw cedar-apple rust lesions developing from infection that occurred Apr 22-23. (Photo by Allen Cochran II below).

Early cedar-apple rust symptoms on York apple leaf. May 10, 2016. 

With this amount of wetting during the bloom and immediate post-bloom growth stages, moldy core becomes an unusual concern on varieties such as Red Delicious, Fuji, and Cameo. Moldy core is caused by as many as 14 different fungi that invade during the six-week period that the calyx tube stays open, so broad-spectrum residual fungicide protection is the most effective approach to avoiding this problem. Although the recent volume of rain has not been extreme given the number of wetting periods and hours of wetting that we’ve had, but the conditions have made it difficult to maintain adequate protection the past two weeks.

Fire blight: This week we saw fire blight blossom infection symptoms (shown below) on unprotected York apple trees, resulting from infection that occurred about Apr 20-21. Continue to protect prolonged late bloom as needed. 

Advanced blossom blight symptoms May 10, 2016

Heavy apple scab pressure April 28 -May 7

Last week, following four scab/rust infection periods during April 28 to May 2, we had four more infection periods: May 2-3: 15 hours wet at 65-58° with 0.1 in. of rain; May 3-4: 16 hr wet at 66-46° with 0.08 in. of rain; May 4-5: 20 hr at 57-48° with 0.4 in. of rain; May 5-7: 38 hr at 54-48° with 0.97 in. of rain. This totals eight infection periods in ten days! Several of these infection periods involved both scab and rusts.

Remaining apple blossoms and small fruit may still be susceptible to quince rust so it is prudent to include a sterol-inhibiting fungicide for after-infection rust control in the next fungicide application. Scab and rust lesions from the Apr 22-23 infection period are expected to start appearing this week.

Fire blight: Continue to protect prolonged bloom as needed. 

Powdery mildew: We have had 21 dry weather “mildew infection days” since mildew spores were first available Mar 16.

Recent scab and rust infection periods and fire blight outlook, May 2, 2016

At our AREC in Winchester, we have had four scab or rust infection periods within the past four days. April 28-29: a heavy apple scab and rust infection period: 31 hours at 53-46° with 0.67 in. of rain; Apr 30: 10 hours at 51° with 0.01 in. of rain; Apr 30-May 1: 18 hours at 53° with 0.53 in. of rain; May 1-2: 12 hours at 59° with 0.16 in. of rain. Locally, some of these wetting periods may have been extended by showers or foggy conditions. Apple blossoms and small fruit are still very susceptible to quince rust so it would be prudent to include a sterol-inhibiting fungicide for after-infection rust control in the next fungicide application.

FIRE BLIGHT: Below is a cropped graphic from the Maryblyt 7 program. Susceptible blossoms are still present on some cultivars. The temperature and rainfall data are current through Monday morning, May 2. Predicted weather conditions are shown for May 2-7. The components of fire blight risk are indicated in the columns labeled B (blossoms open), H (degree hours for epiphytic bacterial populations), W (wetting by rain or dew), and T (average daily temperature 60 F or above). Based on predicted temperatures, the risk column shows infection conditions for Apr 21, 22 and 24-26. The fire blight risk remains high for May 2-4, and slightly warmer temperatures with wetting could result in infection. For optimum control, a protective streptomycin application is recommended ahead of predicted infection, but it can also provide about one day after infection suppression.

The weather conditions used in the predictive part of this graphic come from the Weather Channel for Winchester, supplemented by site-specific data from SkyBit Inc. Be aware that risks on late bloom can change quickly with unpredicted warmer temperatures and wetting.