Summer diseases and powdery mildew; Pre-harvest apple disease outlook in the Winchester area

In the past month we recorded only two major extended wetting events at our AREC in Winchester: Jul 22-23 (18 hr wet at 68° with 2.74 in. rain) and yesterday, Aug 13-14 (14 hr wet at 72° with 0.5 in. rain). However, there were also nine other wetting events less than 5 hr in length, with relatively small amounts of rainfall, and some of those may have had greater amounts of rainfall and stayed wet longer in surrounding areas. 

Due to comparatively delayed wetting hour accumulation this year, sooty blotch was slower to make its appearance, and was first observed at our AREC last week (shown below).

Sooty blotch (and scab) observed on unprotected Fuji apples August 8, 2019.

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 chose May 2 as the petal fall date for Winchester, so the start of wetting hour accumulation (ACW) was from May 12. As of Aug 14, total wetting hour accumulation by our AREC NEWA weather station was 342 hr and the 250-hr action threshold was reached Jul 11. In 2017 by Aug 14 we had recorded 462 ACW, the threshold was reached July 5 and SBFS was observed July 7. By contrast, during the extremely wet year last year (2018), we saw 630 ACW by Aug 14, the 250-hr threshold was reached June 12, and SBFS was observed by June 27.

In trees not protected by fungicides this year, bitter rot is more common than SBFS, indicative of early summer disease pressure in May and June.

Powdery mildew on late shoot growth: As frequently occurs following heavy rains after a dry period, the 2.7 inches of rain Jul 22-23, promoted a flush of renewed late season growth that was very susceptible to mildew (see below). 

Powdery mildew on late season growth, Granny Smith apple, July 31, 2019

Late infection often leads to a lot of mildew carryover in buds, and a recurrent problem and a significant reduction in yield the next year. This is particularly true with very susceptible varieties like Granny Smith, Idared, Ginger Gold and Honeycrisp, and in situations like this, it is prudent to select late-season fungicides with mildew activity as well as rot and SBFS activity. Examples include products such as Merivon, Luna Sensation, Pristine, Inspire Super and Indar. Caution: Remember to observe allowed pre-harvest intervals for all fungicides as well as insecticides and other products.

Increasing summer disease pressures

Summer disease pressures have increased in the past two weeks. At our AREC in Winchester, we recorded three extended wetting events since the last post Jul 6: Jul 5-6 (13 hr wet at 73° with 0.06 in. rain; Jul 8 (7 hr wet at 73° with 0.12 in. rain); and Jul 11 (8 hr wet at 79° with 0.63 in. rain). Infection by the bitter rot and white rot fungi will occur quickly with wetting and temperatures in the 70s.

The image below, sent to me from the Winchester area, suggests increasing rot pressures.

Rots with mummies. (Photo by W. Mackintosh).

The proximity and distribution of the spots to the mummies suggests that they are developing rots. Some spots were described as having droplets on them. The explanation is that it is probably droplets splashed off the mummies, carrying spores and other dark juicy material. Some droplets carrying spores may incite rot infection and others may not, or some might stay latent for a period of time. Some rots might just be growing slower than others so they get over run by the faster growing organism.

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 chose May 2 as the petal fall date for Winchester, so the start of wetting hour accumulation (ACW) was from May 12. Wetting hour accumulation in July has brought the total ACW to 273 hr, (past the 250-hr action threshold). ACW at Roanoke remains similar to Winchester at 269 hr, but with only 0.36 in. rainfall in July.

Other areas of Virginia with (ACW total) and rainfall in July are: Staunton (452 hr, 1.40 in.); Rappahannock County (Gadino Cellars, 362 hr, 3.06 in. rain); Red Hill (878 hr, 0.24 in.); Lynchburg (552 hr, 1.54 in,); Floyd (362 hr, 1.66 in.); Danville (649 hr, 2.53 in. rainfall). 

Variable summer disease pressures

Summer disease pressures the past month have been generally lighter at Winchester than in other parts of Virginia. 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 chose May 2 as the petal fall date for Winchester, so the start of wetting hour accumulation (ACW) was from May 12. Wetting hours accumulated quite rapidly in May, but more slowly in late June and early July; however, rains the past two days have brought the total ACW (234 hrs) approaching the 250 hr threshold with more rain expected through the weekend. ACW is similar to Winchester at Roanoke (239 hr).

Other areas of Virginia generally had more rainfall than Winchester and more wetting hours in June. Examples, with ACW totals, are: Staunton (377 hr); Rappahannock County (Gadino Cellars) 319; Red Hill (733 hr); Lynchburg (480 hr); Floyd (331 hr); Danville (548 hr). 

At our AREC in Winchester we recorded only three extended wetting events since the posts early last month: Jun 13 (10 hr wet at 59° with 0.19 in. rain; Jun 24 (9 hr wet at 74° with 0.54 in. rain); and Jul 4-5 (16 hr wet at 70° with 0.74 in. rain). Other local areas experienced thunderstorms which resulted in more total rainfall and, especially where those occurred in the evening, more ACW.

Summer disease activity

At our AREC in Winchester we recorded an extended wetting event: Jun 9-10, with 30 hr of wetting at 63° with 0.62 in. rain. This wetting favored development of secondary apple scab, early summer disease activity on apples, peach scab, cherry leaf spot and brown rot on ripening cherries. 

Other areas of Virginia generally had more rainfall and longer wetting in the past week with these examples: Rappahannock County (Gadino Cellars) 30 hr wet at 63° with 1.84 in. rain; Red Hill (Jun 7-11, 67 hr wet at 62-70° with 3.5 in. rain); Lynchburg (Jun 7-11, 53 hr wet at 63° with 2.35 in. rain); Roanoke (Jun 6-10, 63 hr wet at 68° with 4.39 in. rain); Floyd (Jun 6-11, 90 hr wet at 63° with 3.25 in. rain); Danville (Jun 6-9, 55 hr wet at 70° with 2.47 in. rain). Heavy amounts of rainfall and lengthy wetting periods at warm temperatures are favorable for Glomerella leaf spot and bitter rot and other fruit rots as well as sooty blotch and flyspeck.

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 chose May 2 as our petal fall date for Winchester, so the start of wetting hour accumulation was from May 12. As of Jun 11, accumulated 147 wetting hours (ACW) toward the 250 wetting hour threshold for specific treatment against the SBFS fungal complex at our AREC NEWA station, 932 ft elevation. (Last week I had noted that at a lower elevation, 909 ft, we had already accumulated 331 hr ACW, but that figure was distorted because the wetness sensor had come loose from its stand and dropped into the grass where it remained wet abnormally long). 

Most other areas of Virginia have accumulated more wetting hours than Winchester, based on approximate petal fall dates and increased length of wetting in these areas: Rappahannock County (Gadino Cellars) 199 hr; Red Hill 472 hr; Lynchburg, 319 hr; Roanoke, 172 hr; Floyd 220 hr; Danville, 364 hr. Note that the values shown in bold font have passed the 250 wetting hour threshold. This means that the SBFS fungi are now present on unprotected fruit, and would develop symptoms if samples were taken and incubated under moist/humid conditions. Specific protection against SBFS and the rots is recommended at this time.

Peach brown rot: The 3-week period leading up to harvest for individual peach varieties and other stone fruits is a critical time for protection from brown rot. Weather conditions, especially rainfall, will affect how much rot appears on varieties as they ripen. During this period, step up the program to include fungicides specifically active against brown rot. Include Captan with those classes of chemistry that are at risk for development of resistance and rotate chemical classes in the final applications. Application interval should be about 3 weeks and 1 week to harvest for ‘normal’ conditions, but might need to be shortened to offset frequent, heavy rainfall.

Recent extended wetting; early summer disease pressures June 2, 2019

At our AREC in Winchester we recorded two extended wetting events: May 26-27, with 6 hr of wetting at 69° with 0.15 in. rain; and June 2, with 9 hr of wetting at 61° with 0.2 in. rain. Again these extended wetting events at relatively warm temperatures favored secondary apple scab, early summer disease activity on apples, peach scab, cherry leaf spot and brown rot on ripening cherries. 

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 chose May 2 as our petal fall date for Winchester, so the start of wetting hour accumulation was from May 12. As of 8 AM June 2, accumulated wetting hours (ACW) already passed the 250 wetting hour threshold for specific treatment against the SBFS fungal complex: at 909 ft elevation we had 331 hr; at 932 ft (the AREC NEWA station), 93 hr; and at the 983 ft elevation, 90 hr ACW. One of the purposes of following three weather stations is to compare wetting hour accumulation at different elevations. Note that we have already passed the threshold at the lower elevation and the total is more than 100 hr ACW more than last year on this date. At the two higher elevations ACW accumulation is more than 100 less than last year at this time. NOTE: Edited Jun 12: Last week we discovered an error with the sensor at the lower elevation, 909 ft, which had already accumulated 331 hr ACW; that figure was distorted because the wetness sensor had come loose from its stand and dropped into the grass where it remained wet abnormally long.

The sooty blotch/flyspeck model on NEWA indicates accumulated wetting similar to the ACW shown at our NEWA station. To use this model, select a weather station, go to the drop-down menu for diseases and enter the petal fall date (e. g. May 2) to calculate the risk for that location.

Sooty blotch/flyspeck risk summary for Winchester AREC, 932 ft elevation, using May 2 as petal fall date.

For central Virginia, we selected Apr 23 as the petal fall date for accumulation of wetting hours by selected weather stations. As of June 2, Red Hill had 349 ACW (also past the threshold). Lynchburg  had 240  ACW. Also east of the Blue Ridge, the NEWA station at Batesville  has 109 ACW, while the NEWA station at Gadino Cellars near Washington, VA has recorded 150 ACW with Apr 29 as the petal fall date. 

Heavy scab infection period May 10-12.

We recorded another heavy combined apple scab infection period at Winchester over this past weekend: May 10-13 (47 combined hr wet at 53° with 1.44 inches of rain). Scab lesions have been observed on unprotected trees in the Winchester area. A follow-up fungicide application with after-infection scab activity is suggested. Cedar-apple rust gall inoculum is now mostly depleted.

Also, we have had 20 days favorable for powdery mildew infection since spores were available at Winchester Apr 6. 

Heavy scab and rust infection, May 3-6

We recorded two heavy apple scab and rust infection periods at Winchester over the weekend: May 3-4 (19 hr wet at 64° with 0.4 inches of rain) and May 4-5 (more than 21 hr wet at 62° with 2.18 inches of rain). The last wetting period, with heavy rainfall, is still in progress. This amount of rainfall depleted any fungicide residue applied last week, resulting in potentially heavy rust and scab infection to foliage and fruit. A follow-up fungicide application with after-infection activity is suggested.

Also, we have had 18 days favorable for powdery mildew infection since spores were available at Winchester Apr 6. The fire blight outlook for Winchester remains much as indicated in the post on May 2: the risk of infection remains high wherever there is late bloom present.

Fire blight outlook into next 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. Fruit producers in other areas are encouraged to consult their state extension specialists for information similar to that provided here.

Graphic from Maryblyt 7, May 1, 2019. Click to enlarge.

FIRE BLIGHT: Above is a cropped graphic from the Maryblyt 7 program. We have moved through bloom in a timely fashion in the Winchester area, but many apple blocks still have some susceptible bloom. The temperature and rainfall data are current through Wednesday evening, May 1. Predicted weather conditions are shown for May 2-6. 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, and this must coincide with an average daily temperature of 60°F or more. Based on recorded temperatures and wetting, the risk column shows the infection was possible Apr 19 and 25, and was possible with wetting Apr 18, 23 and 24, and will again be possible with wetting May 2-4. The temperature is marginally close for infection May 5. The extended outlook May 6 through May 10 (not shown) indicates infection is possible wherever there is bloom and wetting through May 10.

Streptomycin application is recommended for tomorrow, May 2, to protect any late bloom into the weekend. Also, remember to protect any young and recently planted trees that have flowers. Predicted temperatures into next week (not shown) indicate that infection will remain possible wherever there is late bloom and wetting every day next week. Note that "wetting" can occur with maintenance and thinning applications, so streptomycin should be included in such applications where there is late bloom. 
The BBS column tracks the appearance of blossom blight symptoms from infection that occurred Apr 19, which is now predicted for May 3. The CBS column is tracking canker blight symptom appearance and predicts canker margin symptoms (CMS, expansion of overwintered cankers) is predicted May 1. Further tracking in the CBS column will predict canker blight symptoms when that value reaches 100.

Below is the graphic from the Cougarblight model as shown on our NEWA site. with “Fire blight occurred in your neighborhood last year” selected as the orchard blight history option and first blossom open date as 4/13/2019. Cougarblight shows color-coded risk assessments as “Cougarblight 4-Day DH” and infection potential EIP value. This risk is "extreme" for May 2-6, with infection possible if wetting occurs May 2-4. The average temperature of 60 F is lacking for May 5.

Graphic from the NEWA/Cougarblight model, May 1, 2019. Click to enlarge.

Fire blight risks can change quickly with warmer than predicted temperatures and wetting; in high-risk situations, a streptomycin application is more effective when applied ahead of predicted infection. Unless unexpected weather conditions arise, this will be the last fire blight update for this season.

Fire blight outlook Apr 29-May 4; secondary scab infection Apr 27-28.

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. Fruit producers in other areas are encouraged to consult their state extension specialists for information similar to that provided here.

Graphic from Maryblyt 7, April 29, 2019. Click to enlarge.

FIRE BLIGHT: Above is a cropped graphic from the Maryblyt 7 program. Most apple trees still have susceptible bloom in the Winchester area. The temperature and rainfall data are current through Sunday evening, April 28. Predicted weather conditions are shown for April 29-May 4. 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, and this must coincide with an average daily temperature of 60°F or more. Based on recorded temperatures and wetting, the risk column shows the infection was possible Apr 19 and 25, and would be possible with wetting Apr 18, 23 and 24, and will again be possible with wetting May 1-4. 
Streptomycin application is recommended for tomorrow, Apr 30 or Wednesday, May 1 to protect late bloom into the weekend. Also, remember to protect any young and recently planted trees that have flowers. Predicted temperatures into next week (not shown) indicate that infection will remain possible wherever there is late bloom and wetting every day next week. Note that "wetting" can occur with maintenance and thinning applications, so streptomycin should be included in such applications while late bloom is present. The BBS column tracks the appearance of blossom blight symptoms from infection that occurred Apr 19, which is predicted for May 2. The CBS column is tracking canker blight symptom appearance and predicts canker margin symptoms (CMS, expansion of overwintered cankers) is predicted Apr 30.

Below is the graphic from the Cougarblight model as shown on our NEWA site. with “Fire blight occurred in your neighborhood last year” selected as the orchard blight history option and first blossom open date as 4/13/2019. Cougarblight shows color-coded risk assessments as “Cougarblight 4-Day DH” and infection potential EIP value. This risk is "extreme" for May 1-4 with infection possible if wetting occurs. 

Graphic from the NEWA/Cougarblight model, April 29, 2019. Click to enlarge.

Note that real fire blight risks can change quickly with warmer than predicted temperatures and wetting and, in high-risk situations, a streptomycin application is more effective when applied ahead of predicted infection.

We recorded a secondary apple scab infection period at Winchester Apr 27-28: 13 hr wet at 50° with 0.16 inches of rain. This would have been a secondary infection period where scab was not well controlled during the Apr 5-6 infection, resulting in scab lesions with spores now available for secondary infection. 

Scab and rust infection Apr 25-26

We recorded a apple scab and rust infection period at Winchester Apr 25-26: 14 hr wet at 57° with 0.13 inches of rain. This was also an infection period in central Virginia and could have been a secondary infection period where scab was not well controlled during the Apr 5-6 infection, resulting in scab lesions with spores now available for secondary infection. Cedar-apple rust lesions are now visible from the infection period Apr 7-8.

Also, we have had 14 days favorable for powdery mildew infection since mildew spores were available at Winchester Apr 6.

Fire blight infection possible April 25

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.

Graphic from Maryblyt 7, April 24, 2019. Click to enlarge.

FIRE BLIGHT: Above is a cropped graphic from the Maryblyt 7 program. Apple trees are near full bloom in the Winchester area. The temperature and rainfall data are current through Tuesday night, April 23. Predicted weather conditions are shown for April 24-28. 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, and this must coincide with an average daily temperature of 60°F or more. Based on recorded temperatures and wetting, the risk column shows the infection was possible Apr 19 and 25, and would be possible with wetting Apr 18, 23 and 24. Streptomycin is recommended to be applied today or tomorrow, Apr 24 or 25, and for any regular spray application when wetting is the only factor lacking for infection. Also, remember to protect young and recently planted trees, which have flowers but might be considered to be "non-bearing". Fire blight risk should subside with cooler predicted temperatures Apr 26-28. The BBS column is tracking the appearance of blossom blight symptoms from infection that occurred Apr 19. Symptom appearance is predicted when this value reaches 100.

Below is the graphic from the Cougarblight model as shown on our NEWA site. We selected orchard blight history option as “Fire blight occurred in your neighborhood last year” and first blossom open date as 4/13/2019. Cougarblight shows color-coded risk assessments as “Cougarblight 4-Day DH” and infection potential EIP value. This risk is "high" for Apr 23, 24 and 26 and "extreme" or infection tomorrow, Apr 25, and also with infection possible with a trace of wetting from rainfall today, Apr 24, so the current predictions are similar for the Cougarblight and Maryblyt models. 

Graphic from the NEWA/Cougarblight model, April 24, 2019. Click to enlarge.

Note that real fire blight risks can change quickly with warmer than predicted temperatures and wetting. In high-risk situations, a streptomycin application is more effective when applied ahead of predicted infection.

Apple scab and rust infection Apr 19-20; fire blight outlook this week

We recorded another apple scab and rust infection period at Winchester Apr 19-20: 13 hr wet at 62° with 1.01 inches of rain. Considering the timing of this infection period while flowers are highly susceptible to quince rust infection, it is prudent to include an SI  fungicide for after-infection control of rusts in the next spray application. Similar wetting and temperature conditions occurred east of the Blue Ridge and southward to Roanoke and beyond. Such conditions were also favorable for infection by peach scab on small fruits which are now vulnerable after the "shuck-split" stage.
Also, we have had 9 days favorable for powdery mildew infection since mildew spores were available Apr 6.

Fire blight outlook for Winchester: Below is a cropped graphic from the Maryblyt 7 program. Apple trees are now near full bloom in the Winchester area. The temperature and rainfall data are current through Sunday evening, April 21. Predicted weather conditions are shown for April 22-26. 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, and this must coincide with an average daily temperature of 60°F or more. 

Graphic from Maryblyt 7, April 22, 2019. Click to enlarge.

Based on recorded temperatures and wetting, the risk column shows that infection was possible Apr 19. and risk remained high high for Apr 20, but subsided Apr 21-22. Note that the EIP is near 100 or above for Apr 23-26, so a slight increase in temperature or wetting this week could change the risk factors that are lacking (EIP of 100, average temperature of 60,wetting). As it stands, a special caution is in order for Apr 25 when only wetting is lacking for infection. Because any spray application can serve as the wetting trigger, include streptomycin with any spray applied under such conditions through late bloom. Also, remember to protect young and recently planted trees, which may have flowers but might be considered to be "non-bearing". The BBS column is tracking the appearance of blossom blight symptoms from infection that occurred Apr 19. Symptom appearance is predicted when this value reaches 100.

Below is the graphic from the Cougarblight model as shown on our NEWA site. We selected orchard blight history option as “Fire blight occurred in your neighborhood last year” and first blossom open date as 4/13/2019. Cougarblight shows color-coded risk assessment as “Cougarblight 4-Day DH”. This risk is "caution" for Apr 22 and "high" for Apr 23-26. Note that if we were to select "Fire blight is now active in your neighborhood" as the orchard blight history, "caution" becomes "high" and "high" becomes "extreme".  The difference in EIP level shown for Apr 20 in this graphic compared to the Maryblyt graphic above is related to the temperatures that were entered on Apr 18-19 (not shown in the Cougarblight graphic). 

Graphic from Cougarblight, April 22, 2019. Click to enlarge.

Note that real fire blight risks can change quickly with warmer than predicted temperatures and wetting. In high-risk situations, a protective streptomycin application is recommended ahead of predicted infection.

Possible fire blight, scab and rust infection today

NOTE: This year for fire blight risk assessment, we are again comparing posted graphics from Maryblyt 7, as in previous years, with those from the Cougarblight model as shown on our NEWA site. We will be using the same recorded weather data from our NEWA station to make these comparisons. 

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.

Graphic from Maryblyt, April 19, 2019. Click to enlarge.

FIRE BLIGHT: Above is a cropped graphic from the Maryblyt 7 program. Most cultivars are now have bloom with some near full bloom in the Winchester area. The temperature and rainfall data are current through Friday morning, April 19. Predicted weather conditions are shown for April 19-23. 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, and this must coincide with an average daily temperature of 60°F or more. Based on recorded temperatures and wetting, the risk column shows high risk for Apr 13 and 16, but with an EIP less than 100; by Apr 18 the EIP reaches 100, but wetting is lacking. Apr 19 shows a significant risk of infection with all infection criteria met. For such situations a spray of streptomycin is recommended by Apr 19. With cooler temperatures Apr 20-22 risk should subside, but will again begin to increase with warmer temperatures Apr 23. The BBS column in the Maryblyt graphic is tracking the appearance of blossom blight symptoms from infection that occurs today, Apr 19. Symptoms appearance is predicted when this value reaches 100.

Below is the graphic from the Cougarblight model as shown on our NEWA site. We selected orchard blight history option as “Fire blight occurred in your neighborhood last year” and first blossom open date as 4/13/2019. Cougarblight shows color-coded risk assessment as “Cougarblight 4-Day DH” risk is "caution" for Apr 18 and "high" for Apr 19-22. This is a rather important difference between the two predictive models. Note that this graphic would indicate "extreme" risk if today's EIP were 100, but it is 99, not quite triggering the "extreme" risk. Caution: Both of these models are a guide to fire blight risk, and the EIP shown in the graphic below is very close to infection conditions with rain today Apr 19. 

Graphic from Cougarblight 7, April 19, 2019. Click to enlarge.

Risk can change quickly with warmer than predicted temperatures and wetting. In high-risk situations, a protective streptomycin application is recommended ahead of predicted infection. The fire blight outlook will be updated Apr 22.

Also note that apple scab, cedar-apple rust and quince rust infection are likely today, with locally heavy rainfall expected during the day and continued wetting through tonight.

Powdery mildew is now active. Mildew conidia have been available for infection at our AREC since Apr 6. Any “dry weather“ day above 53° is suitable for mildew infection, and we have had seven mildew infection days since Apr 6.

Increasing fire blight threat Apr 18-19!

NOTE: This year for fire blight risk assessment, we are again comparing posted graphics from Maryblyt 7, as in previous years, with those from the Cougarblight model as shown on our NEWA site. We will be using the same recorded weather data from our NEWA station to make these comparisons. We thank Dr. Mizuho Nita for hosting the Maryblyt 7.1 download site at: http://grapepathology.org/maryblyt 

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.

Graphic from Maryblyt 7, April 17, 2019. Click to enlarge.

FIRE BLIGHT: Above is a cropped graphic from the Maryblyt 7 program. Most cultivars now have some bloom open in the Winchester area. The temperature and rainfall data are current through Wednesday evening, April 17. Predicted weather conditions are shown for April 18-22. 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, and this must coincide with an average daily temperature of 60°F or more. Based on recorded temperatures and wetting, the risk column shows high risk for Apr 13 and 16, but with an EIP less than 100; by Apr 18 the EIP reaches 100, but wetting is lacking. Apr 19 shows a significant risk of infection with all infection criteria met. For such situations a spray of streptomycin is recommended on Apr 18 or 19. Cooler temperatures Apr 20-22 should reduce fire blight risk.

Below is the graphic from the Cougarblight model as shown on our NEWA site. We selected orchard blight history option as “Fire blight occurred in your neighborhood last year” and first blossom open date as 4/13/2019. Cougarblight shows color-coded risk assessment as “Cougarblight 4-Day DH” risk is "caution" for Apr 18 and "high" for Apr 19-22. This is a rather important difference between the two predictive models. The apparent unusual difference in amount of risk predicted by Maryblyt and Cougarblight is because the EIP does not reach 100 (only 85) in Cougarblight but in Maryblyt it is 103 on Apr 18 and 116 on Apr 19 due to higher daily average temperatures, especially for Apr 18.

Graphic from Cougarblight, April 17, 2019. Click to enlarge.

Risk can change quickly with warmer than predicted temperatures and wetting. In high-risk situations, a protective streptomycin application is recommended ahead of predicted infection. The fire blight outlook will be updated Apr 19.

Apple scab and rust infection period Apr 14-15

We recorded another apple scab and rust infection period at Winchester Apr 14-15: 14 hr wet at 62° with 0.53 inches of rain. This was another critical cedar-apple rust and quince rust infection period with many spores released during the extended wetting period, and flowers highly susceptible to quince rust infection. Considering this infection period following the recent one Apr 12-13, it is prudent to include an SI (DMI) fungicide for after-infection control of rusts in the next spray application. More rainfall and infection is expected Apr 19-20. 

Infection conditions were similar southward to Roanoke and east of the Blue Ridge from Manassas to Central Virginia. Rainfall amounts since Apr 12 ranged from 1.5 to nearly 2 inches. This was enough to deplete protective fungicide residue, and fungicides with after-infection control are suggested.

Previously, I posted a picture of a large quince rust canker, which illustrates the perennial nature of quince rust inoculum levels. Thus, serious quince rust epidemics are dependent on the release of spores while the blossoms are susceptible, the condition that has just occurred over the past several days. Below is a contrasting picture that shows a small quince rust canker on third year growth and tiny cedar-apple rust galls (upper left) with a single spore horn protruding. These were obviously produced on last years new green foliage on the cedar tree. However, both contribute to overall rust inoculum levels.

Sporulating quince rust canker (lower right), and small cedar-apple rust galls (upper left).

 

Fire blight outlook at Winchester this week

King bloom on Idared, Apr 14, 2019.

NOTE: This year for fire blight risk assessment, we will again compare posted graphics from Maryblyt 7 as in previous years, and also from the Cougarblight model as shown on our NEWA site. We will be using the same weather data from our NEWA station to make these comparisons. We thank Dr. Mizuho Nita for hosting the Maryblyt 7.1 download site at: http://grapepathology.org/maryblyt 

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.

 

Graphic from Maryblyt 7, April 14, 2019. Click to enlarge.

FIRE BLIGHT: Above is a cropped graphic from the Maryblyt 7 program. We will use Saturday, Apr 13 as the date of first bloom open on early cultivars such as Idared and Pink Lady. The temperature and rainfall data are current through Sunday evening, April 14. Predicted weather conditions are shown for April 15-20. 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, and this must coincide with an average daily temperature of 60°F or more. Based on recorded temperatures and wetting, the risk column shows high risk for Apr 13, 15, and 16, but lacks the required EIP of 100. Entering an earlier bloom date did not raise the EIP for Apr 13, and did not raise the risk of infection. With cooler predicted temperatures mid- week, fire blight risk should be low to moderate Apr 17-18. However, with warmer than predicted temperatures, expect the risk to increase again.

Below is the graphic from the Cougarblight model as shown on our NEWA site. We selected orchard blight history option as “Fire blight occurred in your neighborhood last year” and first blossom open date as 4/13/2019. Cougarblight shows color-coded risk assessment as “Cougarblight 4-Day DH” risk is low for Apr 14-16.

Graphic from Cougarblight, April 14, 2019. Click to enlarge.

Be aware that risk can change quickly with unpredicted warmer temperatures and wetting. In high-risk situations, a protective streptomycin application is recommended ahead of predicted infection. The fire blight outlook will be updated Apr 17.