Secondary scab infection May 5-6; fire blight infection conditions on late bloom May 15-18.

At Winchester, most apple varieties are well beyond petal fall, but late bloom susceptible to fire blight persists on some late varieties and some recently planted trees. 

At the AREC last week an apple scab infection period occurred May 5-6: 17 hr wet at 47° with 0.18 in. rain. Similar conditions also extended south to Staunton and east of the Blue Ridge from Loudoun County to the Charlottesville area, where infection conditions resulted from a combined wetting period. At Winchester, cedar-apple rust galls and quince rust cankers remain active after this wetting event.

Apple powdery mildew infection occurs on days without rainfall above 53°, and we have had 14 days favorable for infection since spores were available on Mar 29. Below is an example of primary and secondary powdery mildew on Idared apple.

A secondary powdery mildew lesion (bottom left) next to a primary mildew shoot on Idared apple. 

The Winchester area will likely see its first fire blight infection conditions wherever susceptible bloom remains this weekend, May 16-18. Similar conditions exist for all other major fruit producing areas of Virginia, but with some starting on May 15. Blossom infection has already been reported on apples east of the Blue Ridge south of Charlottesville and in southwest Virginia, and on pears in Clarke County from infection that occurred six weeks ago. Below is an example of late bloom that was observed on Goldrush apple in Nelson County last week.

Late bloom and set fruit on Goldrush apple in Nelson County May 7, 2020.

Just to add a footnote about the unusually cool conditions through April and into mid-May that allowed the Winchester area to escape fire blight infection from Mar 30 until the very latest of bloom: The daily mean high temperature for April this year was 60.0° and, since 1928 only three years had a cooler mean daily high temperature, 1935, 1961, and 1966!  Meanwhile, 17 years had a mean April high greater than 70°

Secondary apple scab and rust infection periods April 28 through May 4; fire blight pressure low to moderate in the Winchester area

At Winchester, most apple trees are at petal fall and only later varieties such as Rome Beauty and some young trees are still in bloom. 

At the AREC in the past week we received three apple scab/rust infection periods.  Apr 28: 9 hr wet at 49° with 0.06 in. rain; Apr 29-30: 19 hr wet at 58° with 1.22 in. rain (heavy scab and rusts) and May 3-4: 7 hr wet at 62° with 0.42 in. rain. Cedar-apple rust galls and quince rust cankers remain active after these wetting events, and unprotected apple blossoms and small fruit remain susceptible to quince rust infection, so a follow-up application including an SI fungicide is suggested. On May 4 cedar rust lesions were evident on flower cluster and shoot leaves, from infection that occurred Apr 7-8.

This past week the length of wetting and amounts of rainfall have been somewhat variable across the major commercial fruit production areas of Virginia, but all areas received at least one extended wetting period favorable to secondary scab infection where control was not achieved during earlier primary infection periods. 

Apple powdery mildew infection occurs on days without rainfall above 53°, and we have had 12 days favorable for infection since spores were available on Mar 29. Expect secondary powdery mildew symptoms to begin appearing in the next week or so.

In the Winchester area, fire blight pressure has been mostly low to moderate on apples that first bloomed Mar 30, and that trend continues for late blooming apples through the coming week, However, fire blight blossom symptoms were reported on early blooming Asian pears Apr 27 in Clarke County. Also blossom infection (shown below) was also evident in young apple trees in central Virginia, apparently from infection that occurred Mar 29.

Blossom blight symptoms in central VA, May 2, 2020. Infection probably occurred Mar 29.

Apple scab and rust infection Apr 23-27; fire blight blossom symptoms reported on early blooming Asian pears in Clarke Co.

At Winchester, Red Delicious apple trees are near petal fall, but there is still much bloom on many apples, and later varieties such as Rome Beauty are now in full bloom. At the AREC we received two recent infection periods.  Apr 23-24: 21 hr wet at 50° with 0.41 in. rain (scab and rusts); Apr 25-27: 31 combined hr wet at 47° with 0.80 in. rain. Cedar-apple rust galls and quince rust cankers were in "full bloom" during these extended wetting events, and unprotected apple blossoms remain very susceptible to quince rust infection, so a follow-up application including an SI fungicide is suggested. On Apr 27 a few cedar rust lesions were evident on flower cluster leaves, from infection that occurred Mar 27-28.

The above conditions for Winchester were generally similar across most of the major commercial fruit production areas of Virginia. The greatest concerns for these events are where earlier infection periods occurred with inadequate fungicide protection, and scab lesions are now sporulating, leading to heavy secondary infection.

Apple powdery mildew infection occurs on days without rainfall above 53°, and we have had ten days favorable for infection since spores were available on Mar 29. Expect secondary powdery mildew symptoms to begin appearing in the next week or so.

Although fire blight pressure has been mostly low to moderate on apples that first bloomed Mar 30 in the Winchester area, fire blight blossom symptoms on early blooming Asian pears were reported Apr 27 in Clarke County. In that case, apparently the pear trees were in bloom by Mar 19 and infection likely occurred on Mar 20, before the first apple blossoms were open at our AREC.

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.

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.

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.

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).

 

Heavy apple scab and rust infection period Apr 12-13

In Winchester, we recorded a heavy apple scab and rust infection period Apr 12-13: 22 hr at 60°, with 0.77 inches of rain. Many cedar-apple rust and quince rust spores were released during this extended wetting period, and flowers are near their peak susceptibility for quince rust infection. In areas where there is a concern for rust infection, include an SI (DMI) fungicide in the next spray application. More wetting at warm temperatures is expected Apr 14-15. 

Apple scab infection conditions were similar to Winchester's southward in the Shenandoah Valley and east of the Blue Ridge to Central Virginia, but with slightly warmer temperatures and up to 1.8 inches of rain in some areas-- enough to erode the protective fungicide residue during the wetting event. 

Powdery mildew conidia have been available for infection at our AREC since Apr 6. Any “dry weather“ day above 53° is suitable for mildew infection. So far, we have had four mildew infection days.

The huge quince rust canker below seen recently in Frederick County has likely been producing rust basidiospores for 25 years or more. Its leaves were infected when it was only a couple years old and now it has a trunk diameter of 5-6 inches.

Sporulating quince rust canker on eastern red cedar. Photo by Mark Sutphin.

Disease update: apple scab, rusts and fire blight

In Winchester, we recorded a heavy apple scab and rust infection period May 5-7 with split wetting totaling 37 hr at 51-60° (mean 53°), with 0.95 in. rain. Also, many cedar-apple rust and quince rust spores were released during this extended wetting period, and flowers remain susceptibility for quince rust infection. In areas where there is a concern for rust infection, include an SI (DMI) fungicide for after-infection control in the next spray application. Rust galls remain active for more spore discharge with additional wetting. Cedar-apple rust lesions have been observed from the infection period Apr 24-25, and scab lesions should be expected this week raising the possibility of secondary infection where scab was not well-controlled at that time. Powdery mildew conidia have been available for infection at our AREC since Apr 10. Any “dry weather “ day above 53° is suitable for mildew infection and we have had 18 mildew infection days.

FIRE BLIGHT CAUTION: While we are past the peak of apple bloom in the Winchester area, susceptible bloom remains on many varieties. Below are the fire blight risk assessments, comparing graphics from Maryblyt and also from the Cougarblight model as shown on our NEWA site, using the same weather data from our NEWA station to make these comparisons. 

Graphic from Maryblyt 7, May 8, 2018. Click to enlarge.

Above is a cropped graphic from the Maryblyt 7 program, with first bloom open Apr 14. The temperature and rainfall data are current through Monday evening, May 7. Predicted weather conditions are shown for May 8-12. 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, dew or a spray application), 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 that infection conditions occurred May 5 and 7, and are predicted for May 10 and 11. High risks are shown for all of the other days between May 2 and May 12 with only wetting lacking for infection May 9 and 12. Because a spray application can serve as the wetting trigger, include streptomycin with spray applications May 8-12. Remember to protect young and recently planted trees, with flowers but considered to be "non-bearing".

The BBS column in the above graphic is tracking predicted symptom development for the first infection May 5, with 64% of the degree hours required for blossom symptom prediction. The CBS column indicates progression toward the appearance of canker margin symptoms due to extension of overwintering cankers from last year, which is predicted for May 8. 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/shoot blight situation due to hail injury, etc. To offset the potential for shoot tip infection in an active fire blight year such as this one, apply the plant growth regulator, prohexadione-calcium (Apogee, Kudos), at petal fall on king bloom. Shoot blight suppression results from hardening off of vegetative shoot growth starting about 10 days after the initial application.

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 4/14/2017. Cougarblight shows color-coded risk assessment as “Cougarblight 4-Day DH” risk is extreme for May 6, 7, and 10-12, and EIP infection conditions are shown for May 7, 10 and 11.

Graphic from Cougarblight, May 8, 2018. Click to enlarge.

Similar fire blight risks prevailed the past several days throughout areas of Virginia wherever susceptible bloom was present and wetting occurred. Although the length of wetting was more variable across the region and some areas did not receive the scab infection period, these areas still may have had enough wetting for fire blight infection.

Heavy scab and rust infection period, Apr 24-25.

In Winchester, we recorded a heavy apple scab and rust infection period Apr 24-25: 30 hr at 47-56° (mean 51°), with 0.6 in. rain. Also, many cedar-apple rust and quince rust spores were released during this extended wetting period, and flowers are near their peak susceptibility for quince rust infection. In areas where there is a concern for rust infection, include an SI (DMI) fungicide in the next spray application. More wetting is expected Apr 26-27. 

Cedar apple rust galls with the spore horns fully expanded Apr 25, 2018.

Powdery mildew conidia have been available for infection at our AREC since Apr 10. Any “dry weather “ day above 53° is suitable for mildew infection. So far, we have had at least ten mildew infection days.

Fire blight: With continued predicted relatively cool temperatures in the, fire blight risk at Winchester should remain low or moderate through the weekend.

Apple scab infection conditions were similar to Winchester's southward in the Shenandoah Valley and east of the Blue Ridge, but with slightly warmer temperatures and with more than two inches of rain in some areas-- enough to erode the protective fungicide residue during the wetting event. 

Early threats by fire blight, apple scab and the rusts

NOTE: This year for fire blight risk assessment, we will be comparing and posting 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.

Maryblyt graphic April 18, 2018. Click to enlarge.

FIRE BLIGHT: Above is a cropped graphic from the Maryblyt 7 program. We will use Saturday, Apr 14 as the date of first bloom open on Idared and Pink Lady cultivars at our AREC. The temperature and rainfall data are current through Wednesday evening, April 18. Predicted weather conditions are shown for April 19-22. 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 15, with an EIP of 120 and wetting from rainfall, but with rapidly dropping temperatures, the daily mean was just 57.5°F. Entering an earlier bloom date raised the EIP for Apr 15, but did not raise the risk of infection if the daily mean temperature remained at 57.5°F. However, raising the daily mean for Apr 15 to 60°F did indicate infection. With cooler predicted temperatures for the coming week or more, fire blight risk should remain low to moderate. However, with warmer than predicted temperatures, expect the risk to increase again.

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 Monday, Apr 23.

Below is the graphic from the Cougarblight model as shown on our NEWA site. Note that we selected orchard blight history option as “Fire blight occurred in your neighborhood last year” and first blossom open date as 4/14/2017. This date should be re-checked each time the site is accessed. Cougarblight shows color-coded risk assessment as “Cougarblight 4-Day DH” which remains low at least through Apr 22. On the highest risk day, the EIP in Cougarblight was 113, comparable to the value of 120 shown for Maryblyt.

Cougarblight graphic April 18, 2018. Click to enlarge.

Scab and rust threats Apr 15-16: Fire blight is not the only disease that was marginally close to infection Apr 15. At our AREC, we recorded a wetting event of 18 hr at 41-47°, with 1.77 in. rain; ten of these hours at 41-42°. This amounted to 88% of the requirement for scab infection. Also, cedar-apple rust and quince rust spores were released during this wetting period, but it is not likely that rust infection occurred at these cool temperatures. However, where there is a concern for rust infection, it is wise to include an SI fungicide in the next spray application.

It was a different story farther south and east of the Blue Ridge, where more advanced bud stages and warmer temperatures Apr 15-16, resulted in likely infection by fire blight, apple scab and the rusts.

Rapid bud development; powdery mildew, scab and rust activity

The past two days we have seen rapid apple bud development in the Winchester area. Friday morning, Apr. 13, we saw heavy powdery mildew sporulation on Idared buds at tight cluster stage. Powdery mildew infects on dry days at temperatures above 53°F, so there were several infection days this week.

Powdery mildew on Idared, Friday morning, Apr. 13, 2018.

By Friday evening, many buds were showing color at the pre-pink stage. With warm temperatures over the weekend, expect open bloom by early next week.

Pre-pink stage of Idared buds Friday evening, Apr. 13, 2018

There is a forecast for thunderstorms and rain with temperatures in the 50s Sunday and into Monday, Apr 15-16, and that would bring a scab infection period as well as our first cedar-apple and quince rust infection period in the Winchester area. 

Cedar-apple rust gall with developing spore horns Apr. 11, 2018

 Generally, the forecast is similar for the areas south to beyond Roanoke and east of the Blue Ridge, with a trend toward earlier and a shorter wetting period, but with warmer temperatures to the south. In short, scab, the rusts, and powdery mildew are now active, and there is a potential for fire blight infection in more advanced areas with open bloom.

These conditions also favor cherry leaf spot and brown rot infection with cherry trees now in bloom, and brown rot and scab on peaches in bloom or petal fall stage.

Apple scab infection period May 11-13. Early summer disease developments.

At Winchester we recorded a heavy secondary apple scab infection period May 11-13, with 46 hr wetting with a mean temperature of 49° and 1.82 inches of rain. This volume of rainfall could have eliminated much of the protective fungicide residue during the extended wetting to allow some scab and rust infection. 

Areas in the Valley, southward to Roanoke, and areas east of the Blue Ridge from Fauquier County south to Lynchburg and Danville also saw extended wetting periods ranging from 32-48 hr, all at temperatures in the low to mid-50s. The more southern areas had split wetting periods or two separate ones starting May 9. These conditions also favored peach scab infection.

Rusts have been active in the Winchester area for six weeks, and some cedar rust galls are still active. Cedar-apple rust lesions (shown below) and some quince rust lesions are now appearing from the Apr 24-26 infection period; others from the extended wetting May 4-6 are expected this week.

May 10, 2017. Early appearance of cedar-apple rust lesions and scab (upper left) from the infection period Apr 24-26.

Apple powdery mildew conidia were available for infection at Winchester as early as Mar 27, and there have been 18 “dry weather“ mildew days suitable for mildew infection from Mar 27 to May 14. The image below illustrates the appearance of a primary mildew shoot and secondary infection on an adjacent shoot. 

Adjacent Golden Delicious shoots infected with primary infection (right) and secondary infection (left). Primary infection emerges with shoot growth from an infected overwintering bud. Secondary infection is usually heavier near a primary inoculum source.

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 Apr 24 as our petal fall date for Winchester, so the start of wetting hour accumulation is from May 4. As of May 14, accumulated wetting hours (ACW) toward the 250 wetting hour threshold for specific treatment against the SBFS fungal complex were: at 909 ft elevation, 98 hr; at 932 ft (the AREC NEWA station), 83 hr; and at the 983 ft elevation, 73 hr ACW. One of the purposes of following three weather stations is to compare wetting hour accumulation at different elevations; however, because many of the early wetting hours this year were a result of rainfall rather than dew, the early accumulation trend this year is a bit different than in some previous years. 


For central Virginia, we have selected Apr 20 as the petal fall date for accumulation of wetting hours by weather stations at Tyro. As of May 14, a sensor placed at 1465 ft. elevation had accumulated only 31 wetting hours from Apr 30, while the one at 1165 ft. had accumulated 72 wetting hours, and the one placed at 941 ft elevation had accumulated 94 hr.

Heavy scab and rust infection period and fire blight outlook Apr 26-May 1.

NOTE: This year for fire blight risk assessment, we are comparing graphics from Maryblyt 7 and from the Cougarblight model as shown on our NEWA site. We are using the same weather data from our NEWA station to make these comparisons in both predictive programs.

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.

SCAB and RUSTS:  We have just seen extended wetting of more than 46 hours. Because apple scab lesions resulting from infection periods during the period March 26 to Apr 7 may have been present, consider this to be a heavy secondary infection period. Also, because of the high susceptibility of developing fruits to quince rust, it is suggested that fungicides with strong after-infection activity against rusts and scab be included in the next fungicide application.

Maryblyt graphic for Winchester, Apr 26, 2017. Click to enlarge.

FIRE BLIGHT: Above is a cropped graphic from the Maryblyt 7 program, using Apr 5 as the date for first bloom open on Idared cultivar. Early-blooming cultivars still have scattered susceptible bloom, and later cultivars are at petal fall but with much susceptible bloom in the Winchester and central Virginia areas. Some recently planted trees still have much susceptible bloom. The temperature and rainfall data are current through Tuesday evening, Apr 25. Predicted weather conditions are shown for April 26-May 1. 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. Based on recorded temperatures and wetting, the risk column shows high risk and possible infection with any wetting Apr 11-12, 15-18, and 21, and if bloom persists, Apr 27-May 1. Infection was indicated for Apr 12, 15-17 and 21. Predicted warming temperatures will again increase risk wherever susceptible bloom remains Apr 26-May 1. Note that risk can change quickly with warmer than predicted temperatures and wetting. In situations where all other requirements for infection have been met except wetting (as for Apr 27-May 1), wetting from any spray application (fungicide, insecticide, or thinning spray) can provide the wetting trigger for infection to occur. In high-risk situations, a protective streptomycin application is recommended ahead of predicted infection. Continue to protect late bloom and open bloom on recently planted trees as needed.

The BBS column is tracking predicted symptom development for the first infection Apr 12 and symptoms are predicted to appear Apr 27. Later infections are tracked with letters b-d, and symptoms for those are predicted to appear Apr 28-30. The CBS column indicates progression toward the appearance of canker margin symptoms due to extension of overwintering cankers from last year, which would be predicted to occur after that value reached 100 Apr 22. 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/shoot blight situation due to hail injury, etc. To offset the potential for shoot tip infection in an active fire blight year such as this one, apply the plant growth regulator, prohexadione-calcium (Apogee, Kudos), at late bloom. Shoot blight suppression results from hardening off of vegetative shoot growth starting about 10 days after the initial application. The fire blight outlook will be updated Friday, Apr 28.

Below is the graphic from the Cougarblight model as shown on our NEWA site. Note that we selected orchard blight history option as “Fire blight occurred in your neighborhood last year” and first blossom open date as 4/5/2017. (This date must be re-set each time this site is accessed). Cougarblight shows color-coded risk as “high” for Apr 27 and “extreme” for Apr 28-30. 

Cougarblight graphic for Winchester, Apr 26, 2017. Click to enlarge.

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.

Scab and rust infection period Apr 6-7; fire blight outlook for Apr 8-12.

SCAB and RUSTS: At our AREC in Winchester, we recorded our fourth apple scab infection period in ten days April 6-7: 14 hr wet at 50 to 55° with 0.53 inches of rain. This was also a possible rust infection period. Red Delicious, Gala, and Idared fruit buds were at pink to early bloom, but Golden Delicious and Rome Beauty were mostly at open cluster. 

FIRE BLIGHT OUTLOOK: This year for fire blight risk assessment, we will be comparing and posting 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.

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.

(Click to enlarge graphic)

FIRE BLIGHT: Above is a cropped graphic from the Maryblyt 7 program. We are using Apr 5 as the date of first bloom open on Idared cultivar. The temperature and rainfall data are current through Friday evening, April 7. Predicted weather conditions are shown for April 8-12. 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. Based on recorded temperatures and wetting, the risk column showed high risk for Apr 5, but EIP was only 18, not yet at the 100 level required for infection. With cooler predicted temperatures through Apr 9, fire blight risk should remain low. However, with warmer predicted temperatures Apr 10-12, the EIP would reach 111 and 126 for Apr 11 and 12, respectively. Based on predicted temperatures, the risk column shows high risk for Apr 11-12, with only wetting lacking as the trigger for fire blight infection these days. Be aware that 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.

Below is the graphic from the Cougarblight model as shown on our NEWA site. Note that we selected orchard blight history option as “Fire blight occurred in your neighborhood last year” and first blossom open date as 4/5/2017. This date must be re-set each time this site is accessed. Cougarblight shows color-coded risk assessment as “Cougarblight 4-Day DH” which remains low through Apr 9, then increases to "Caution" Apr 10, and "High" by Apr 11 and 12 when the EIP reaches 106-114, comparable to the values of 111-126 shown for these dates in Maryblyt. 

(Click to enlarge graphic)

Be aware that risk can change quickly with unpredicted warmer temperatures and wetting. The fire blight outlook will be updated Monday, Apr 10.