Summary of early season fungal disease activity on apple and peach at Winchester.

Apple scab ascospores were first trapped Mar 10. Scab infection periods occurred on Mar 26-27: wet 15 hr at 46-60°, 0.23 in. rain. (also rust infection); Mar 27-28: wet 23 hr at 53-62°, 0.36 in. rain. (also rust infection); Mar 31-Apr 1: wet 23 hr at 41-48° with 0.95 in rain; Apr 3-4: wet 15 hr at 52-63° with 0.03 in rain. (also rust infection); Apr 6-7: wet 14 hr at 50-55° with 0.53 in rain. (also rust infection).

Apple scab pressure has been significant, with five infection periods in the past three weeks. Lesions from the March infection periods will be expected to appear in the next week and conidia will then be produced for heavy secondary infection. These five infection periods from Mar 26 to Apr 7 came with enough total rainfall to challenge weathering capabilities of protective fungicides.

Cedar-apple rust and quince rust have been active for about two weeks. Pink to petal fall is the peak time for susceptibility to quince rust. We suggest EBDC fungicides as routine protectants for rusts and scab, but always include an SI fungicide (Rally, Rhyme, Inspire Super, Indar) for after-infection rust control if there is any doubt about lack of protective residue.

Apple powdery mildew conidia were available Mar 27. The image of mildew below illustrates heavy sporulation of a primary infection source of inoculum. Any mild “dry weather “ day above 53° is suitable for mildew infection. From Mar 27 to Apr 14 we have had at least nine mildew infection days.

Primary powdery mildew infection on Ginger Gold apple.
Note heavy sporulation on Apr 14, 2017.

Early peach leaf curl symptoms were observed at our AREC Apr 14 (below). This infection occurs with extended wetting soon after bud swell. In our test plots, a protective fungicide applied Feb 28 gave excellent control, but the same fungicide applied Mar 9 was less effective.

Leaf curl symptoms on Redhaven peach Apr 14, 2017.

Peach scab overwinters in lesions on peach shoots (below). Lesions are now producing spores, and it is important to maintain a protective fungicide residue to prevent infection during extended wetting periods over the next 4-6 weeks. Infection can require as much as 7 weeks to appear on fruit.

Peach scab lesion on a Redhaven twig Apr 14,2017.
Close proximity to the young fruit makes it a challenge to protect against infection.

Apple scab outlook!

Red Delicious bud stage March 18, 2017

Apple scab outlook: At our AREC in Winchester we had some early green tip on Red Delicious and other apples in early March, but they were still only about 1/4-inch green by March 18 (shown above). Apple scab ascospore maturation was initially delayed this year because of lack of snow cover and not much rain-wetting of leaves on the ground until March 6. We caught the first discharge of apple scab ascospores with five hours of wetting March 10. Snow cover March 13-17 helped to get the ascospore maturity up to date and more spores were discharged with melting snow and rain Friday night and Saturday, March 18. Expect abundant ascospore discharge with any additional wetting from here on out.  

Scab infection could have been possible in other areas such as Rappahannock County and central Virginia as early March 6-8 if buds and spore maturity were more advanced and where wetting may have been more continuous through that period. At Winchester we had several periods that could be lumped together as a split-wetting infection period.

It looks like bud development will continue to be slow over the next 10 days. We often suggest a copper or other protective fungicide spray before the first scab infection period of the season. Copper should not be applied to fresh market fruit after 1/4-inch green tip stage because of potential for fruit russetting. It looks like there will still be opportunity to apply copper in the Winchester area this week. Where there is concern that scab infection may have occurred in the past day or two, it would be prudent to add Vangard (cyprodinil) to improve post-infection control. Dodine (Syllit) is another early season option where scab may be the only apple disease of concern up to pink stage.

We applied our bud swell spray for peach leaf curl February 24 and the wetting March 6-8 was probably enough to cause leaf curl infection.

Remarkably, peach and apple buds at our AREC do not show much commercial freeze damage considering the many freezing nights that we have had with wind since the earliest green tip on apples and bud swell-pink on peaches.

As a reminder, we have installed a publicly accessible weather station at our AREC in Winchester. The station, which updates hourly, is located near the laboratory building at an elevation of 933 feet. This station is part of the NEWA system, so to access it one can either click on the  NEWA weather station locator map, or go directly to this URL: 

       http://newa.cornell.edu/index.php?page=weather-station-page&WeatherStation=va_win

Explore the web site to view the numerous predictive programs that are offered there. We will refer to these more as the season progresses.

Fire blight and other disease threats this week

CAUTION:  The observations, conditions, and recommendations reported for Winchester, VA are provided as a guide to fire blight risk assessment only for the immediate area of the Virginia Tech AREC located six miles southwest of Winchester. Use of the information reported here for making orchard management decisions outside of that area is not our intent. Fruit producers outside of that area are encouraged to consult their state extension specialists for information similar to that provided here.

  

Maryblyt outlook April 25, 2016

FIRE BLIGHT:  Above is a cropped graphic from the Maryblyt 7 program. Susceptible flowers are still open on most cultivars. Here is the prediction for trees with first bloom open Monday, April 11, 2016. The temperature and rainfall data are current through Monday morning, April 25. Predicted weather conditions are shown for April 25-29. 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). For infection to be predicted, wetting must occur after the EIP (epiphytic infection potential) reaches 100 or higher, which has been true since Apr 18. Based on predicted temperatures, the risk column shows infection conditions for Apr 21, 22 and 23 and only wetting is lacking for Apr 25-27. In situations where all other requirements for infection have been met except wetting, wetting from a maintenance spray application can provide the wetting trigger for infection to occur. In high-risk situations, a protective streptomycin application is recommended ahead of predicted infection. A streptomycin spray remains effective about 3 days in warmer conditions when it is needed most and can provide about one day after-infection suppression.

The BBS column is tracking predicted symptom development for the first infection Apr 21. The CBS column indicates progression toward the appearance of canker margin symptoms due to extension of overwintering cankers from last year, which are predicted to occur when that value reaches 100 Apr 27. Canker advancement cannot be prevented by chemical treatment at this time and the presence of symptoms will signal build-up of inoculum which could become a factor in the event of a trauma blight situation due to hail injury, etc.

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 risk can change quickly with unpredicted warmer temperatures and wetting. The fire blight outlook will be updated Wednesday, Apr 27.

OTHER DISEASES:  We recorded a 15-hr scab, cedar-apple rust and quince rust infection period Apr 22-23. The infection period was as long as 23 hr in Tyro, VA. Where fungicides were not applied recently enough to cover through this infection event, we suggest ones with after-infection activity for scab and rusts, applied as soon as possible. 

Cedar-apple rust galls with expanded spore horns April 22, 2016.

Meanwhile, powdery mildew has been by far our most active apple disease so far this year. Mildew is our only "dry weather" disease and we have had 24 infection days since spores were first available March 16. Secondary infection, shown below, has been evident for at least two weeks. 

Secondary powdery mildew infection, Ginger Gold apple.

Peach leaf curl infection occurred at our AREC as early as March 13. Complete control can be achieved with a single application of an effective fungicide in the dormant season any time from leaf drop in the fall until just before infection occurs with wetting after first bud swell in the spring.

Peach leaf curl, Turk Mt. Overlook, Skyline Drive, Virginia

Finally, after the early season we have had this year, we are happy to see peach and apple trees making little fruits at our AREC!

Shuck-split stage on Redhaven peach, April 22, 2016, 

It's a new season!

At our AREC we had green tip on Red Delicious (shown above) and other apples March 16 and about 1/4-inch green by March 17. 55 hours of wetting, with discharge of apple scab ascospores, had occurred from March 13-15. This was more than three times the length of wetting required for scab infection where green tissue was exposed, but our buds had not advanced to that stage by that time. 

Scab infection was likely in areas where buds were more advanced to the green-tip stage. In Tyro, Virginia, 51 hours of wetting was recorded at temperatures that were about 5 degrees warmer than at Winchester, resulting in more than four times the length of wetting required for scab infection. Apple scab ascospore maturity at green-tip stage is normal and should be expected except in years when buds advance rapidly without any moisture to mature the spores. 

Powdery mildew spore production is advanced compared to most years. Typically, buds infected with overwintering mildew are somewhat delayed compared to their healthy counterparts. However, I was surprised to see mildew-infected buds as advanced as healthy ones (upper right, below), and they had an abundance of conidia (below). In contrast to wetting conditions needed for scab infection, powdery mildew is able to infect on dry days with temperatures above 53 degrees F.

Mildew infected Idared bud, upper right.

Mildew conidia on Idared. Winchester, March 17, 2016

Cedar-apple rust galls are also ready to produce infective basidiospores with the next warm wetting period, so protective apple fungicides should now be included for mildew and rusts as well as for scab. 

Cedar-apple rust gall ready to produce spores March 16. Virginia Tech AREC, Winchester.

Redhaven peach bud development at Virginia Tech AREC, Winchester, March 17, 2016.

Peach buds (above) were more advanced than apples, and unprotected buds would have been readily susceptible to leaf curl infection with wetting March 13-15. Because spores of the leaf curl fungus are carried on the surface of the tree from the fall to spring, fungicides applied in the fall or during the winter give good protection from leaf curl. Fall applications help to avoid the spring rush to spray before pruning brush is removed from the orchard.