Extended wetting periods and volumes of rainfall expected to increase sooty blotch/flyspeck and rot pressure

Three extended wetting events at Winchester in the past week favored secondary apple scab  and summer disease development.  These occurred June 10-11: 10 hr of wetting at 73° with 0.23 in. rain, June 17-18: 14 hr of wetting at 65° with 0.69 in. rain and June 21: 16 hr of wetting at 65° with 0.01 in. rain. Cedar-apple rust infection on leaves over most of the length of unprotected shoots (shown below) indicates that infection occurred well into June this year, but galls have now been depleted of inoculum at Winchester AREC.

Cedar-apple rust on York Imperial apple shoot June 20, 2020.
Lesions on all but the youngest leaves show that infection occurred well into June.

Apple powdery mildew infection occurs on days without rainfall above 53°, and in the Winchester area we have had 38 days favorable for infection since spores were available on Mar 29.

Other areas of Virginia also received significant wetting events in the past two weeks, June 10-11, and especially June 14-19. Some of these were extremely long wetting periods with heavier rainfall than Winchester: Staunton June 16-19, 51 hr with 1.88 in. of rain; Roanoke three wetting periods June 14-18, totaling 65 hr and 5.75 in. rain; Batesville June 16-19, 44 hr with 1.31 in. rain; Carter Mt., June 15-18, 64 hr with 0.91 in. rain; Red Hill, three wetting periods June 13-19, totaling 54 hr and 0.9 in. rain; Lynchburg, two periods June 15-19, total 76 hr and 3.79 in. rain; Rustburg, June 14-19, two periods totaling 79 hr and 3.81 in. rain; Danville, two periods, total 67 hr and 1.8 in. rain. These long wetting periods and volumes of rainfall should be expected to increase pressure from sooty blotch and flyspeck and various fungal rot diseases.

To predict the development of the sooty blotch and flyspeck (SBFS) fungal complex, we record accumulated wetting hours (ACW) from rainfall, fog, or dew, starting 10 days after petal fall. The action threshold of 250 ACW predicts that the SBFS fungi are present on unprotected fruit when the threshold is reached, and symptoms will soon appear with further incubation. All of the monitored areas except Winchester and Sperryville have now exceeded the 250-hr threshold, and Winchester and Sperryville will likely pass the threshold in the next 7-10 days. Here is a listing of selected petal fall date and total wetting hour accumulation (ACW) for selected locations as of June 21: Winchester, May 5, 187 ACW; Staunton, May 5, 324 ACW; Roanoke, Apr 18, 316 ACW; Floyd, Apr 28, 291 ACW; Manassas, Apr 28, 257 ACW; Sperryville, Apr 28, 199 ACW; Batesville, Apr 18, 316 ACW; Crozet, Apr 18, 276 ACW; Carter Mountain, Apr 18, 435 ACW; Red Hill, Apr 18, 291 ACW; Lynchburg, Apr 13, 366 ACW; Rustburg, Apr 13, 361 ACW; Danville, Apr 13, 450 ACW. 

Extended wetting events and accumulated wetting hours favor early rot development

Two extended wetting events at Winchester last week favored secondary apple scab and cedar-apple rust infection and early rot development.  These occurred June 4-5: 17 hr of wetting at 69° with 0.83 in. rain and June 5-6: 18 hr of wetting at 69° with 0.42 in. rain. Wetting was variable across other commercial fruit production areas of Virginia. Apple powdery mildew infection occurs on days without rainfall above 53°, and the Winchester area we have had 33 days favorable for infection since spores were available on Mar 29.

To predict the development of the sooty blotch and flyspeck (SBFS) fungal complex, we record accumulated wetting hours (ACW) from rainfall or dew, starting 10 days after petal fall. The action threshold for SBFS development is 250 ACW. Here is a listing of selected petal fall date and total wetting hour accumulation (ACW) for selected locations as of June 9: Winchester, May 5, 112 ACW; Staunton, May 5, 217 ACW; Roanoke, Apr 18, 205 ACW; Floyd, Apr 28, 188 ACW; Manassas, Apr 28, 182 ACW; Sperryville, Apr 28, 146 ACW; Batesville, Apr 18, 208 ACW; Crozet, Apr 18, 178 ACW; Carter Mountain, Apr 18, 314 ACW; Red Hill, Apr 18, 189 ACW; Lynchburg, Apr 13, 243 ACW; Rustburg, Apr 13, 247 ACW; Danville, Apr 13, 331 ACW. 

ACW is related to wetting from rainfall, fog, or dew. The action threshold of 250 ACW signals that the SBFS fungi would be present on unprotected fruit when the threshold is reached and symptoms would appear with further incubation. Carter Mountain and Danville have passed the threshold, and Lynchburg and Rustburg are close to the threshold.  

Extended wetting events and accumulated wetting hours predict the development of sooty blotch and flyspeck

At Winchester last week a secondary apple scab and cedar-apple rust infection period occurred May 28: 7 hr of combined wetting at 69° with 0.09 in. rain. Most of the other commercial fruit production areas of Virginia experienced similar conditions, with relatively warm extended wetting during the period from May 27 to 29. Expect extended wetting events with warmer temperatures to increase the potential for early latent rot infection of fruit. At Winchester,  cedar-apple rust galls remain active. Apple powdery mildew infection occurs on days without rainfall above 53°, and in the Winchester area we have had 29 days favorable for infection since spores were available on Mar 29.

To predict the development of the sooty blotch and flyspeck (SBFS) fungal complex, we record accumulated wetting hours (ACW) from rainfall or dew, starting 10 days after petal fall. The action threshold for SBFS development is 250 ACW. Here is a listing of selected petal fall date and total wetting hour accumulation (ACW) for selected locations: Winchester, May 5, 73 ACW; Staunton, May 5, 167 ACW; Roanoke, Apr 18, 194 ACW; Floyd, Apr 28, 174 ACW; Manassas, Apr 28, 131 ACW; Sperryville, Apr 28, 114 ACW; Batesville, Apr 18, 186 ACW; Crozet, Apr 18, 162 ACW; Carter Mountain, Apr 18 329 ACW; Red Hill, Apr 18, 175 ACW; Lynchburg, Apr 13, 226 ACW; Rustburg, Apr 13, 235 ACW; Danville, Apr 13, 288 ACW. 

The action threshold of 250 ACW signals that the SBFS fungi would be present on unprotected fruit when the threshold is reached and symptoms would appear with further incubation. Carter Mountain and Danville have already passed the threshold, and Lynchburg and Rustburg may reach the threshold within the coming week . ACW is related to wetting from rainfall or dew, and typically, lower elevations in an orchard accumulate wetting hours more quickly because of wetting from dew. So far this year, that was not the case with the higher elevation of Carter Mountain accumulating wetting hours more rapidly from extended wetting from rainfall and fog in the past two weeks.

Extended wetting last week will trigger early summer disease development

At the AREC last week an apple scab and cedar-apple rust infection period occurred May 22-23: 24 hr of combined wetting at 60° with 1.14 in. rain. Conditions were similar east of the Blue Ridge in Manassas and Sperryville, but more extreme to the south from Staunton to Roanoke and Floyd and east of the Blue Ridge from the Charlottesville area southward to Lynchburg and Danville. At Winchester, cedar-apple rust galls remain active with more extended wetting periods predicted for this week.

Staunton had two wetting periods of 10 and 42 hr with a total of 0.91 in. rain. In Roanoke, two infection periods totaled 113 hr combined wetting and 10.17 in. rain. The total rainfall in Floyd was 1.92 inches, but with 115 hr of extended wetting. In Albemarle County, combined wetting ranged from 43-55 hr at Crozet, Red Hill and Batesville to 102 hr on Carter Mountain with rainfall totals from 1.3 to 2.6 inches. Lynchburg had 86 hr wetting with 2.95 in rain, Rustburg 114 hr wet with 3.87 in. rain and Danville 89 hr combined wetting with 4.69 in. rain.

In all areas, this extended wetting will contribute to early accumulation of wetting hour totals toward development of sooty blotch and flyspeck. Fortunately, much of the earlier portion of the wetting events was at cooler temperatures not as favorable to rot development, but the latter portion was at warmer temperatures, and occurred after heavy rains would have depleted fungicide residue, making the fruit vulnerable to latent rot infection. This extended wetting was also favorable to secondary scab and late rust infection on foliage, and the extremely long wetting periods can increase the amount of fruit scab.

For purposes of predicting the development of the sooty blotch and flyspeck (SBFS) fungal complex, we record accumulated wetting hours (ACW) from rainfall or dew, starting 10 days after petal fall. The action threshold for SBFS development is 250 ACW. Here is a listing of selected petal fall date and total wetting hour accumulation (ACW) for selected locations: Winchester, May 5, 55 ACW; Staunton, May 5, 133 ACW; Roanoke, Apr 18, 174 ACW; Floyd, Apr 28, 159 ACW; Manassas, Apr 28, 91 ACW; Sperryville Apr 28, 102 ACW; Batesville Apr 18, 164 ACW; Crozet, Apr 18, 138 ACW; Carter Mountain, Apr 18 329 ACW; Red Hill, Apr 18, 150 ACW; Lynchburg, Apr 13, 199 ACW; Rustburg, Apr 13, 201 ACW; Danville, Apr 13, 243 ACW. The action threshold of 250 ACW means that the SBFS fungi would be present on unprotected fruit when the threshold is reached. Lynchburg, Rustburg and Danville are predicted to reach the threshold within the coming week and Carter Mountain has already passed the threshold. ACW is related to wetting from rainfall or dew, and often lower elevations in an orchard accumulate wetting hours more quickly because of wetting from dew, but that obviously is not the case with the higher elevation of Carter Mountain this year.

Apple powdery mildew infection occurs on days without rainfall above 53°, and in the Winchester area we have had 26 days favorable for infection since spores were available on Mar 29. 

Fire blight infection conditions on late bloom May 15; scab and rust infection May 14-15.

Fire blight infection conditions finally occurred at Winchester on late bloom May 15. Below is a cropped graphic from the Maryblyt 7 program.

Graphic from Maryblyt 7, May 18, 2020. Click to enlarge.

While most apple blocks were past bloom in the Winchester area, a few still had some susceptible bloom. The temperature and rainfall data are current through Monday evening, May 18. Predicted weather conditions are shown for May 19-31. 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 wherever bloom was present May 15. Infection aslo would have been possible with wetting May 16-17. The extended outlook through May 31 is shown primarily to track infection development from May 15, but indicates that infection would be possible wherever there is bloom and wetting May 27-31.

The BBS column tracks the appearance of blossom blight symptoms from infection that occurred May 15, which is now predicted for May 27. The CBS column tracks canker blight symptom appearance and predicted canker margin symptoms (CMS, expansion of overwintered cankers) May 16. Further tracking in the CBS column predicts canker blight symptoms when that value reaches 100 May 28.

Under these conditions a Streptomycin application would have been recommended for May 14-15, to protect any late bloom through the infective period. This would particularly include any young and recently planted trees with flowers. Cooler predicted temperatures the next week indicate a decline in risk, but warmer temperatures will again bring the EIP to an infective level May 26. Note that "wetting" can occur with maintenance and thinning applications, so streptomycin should be included in such applications if there is late bloom. Also note that, while bloom may have escaped infection during this unusually low fire blight pressure year at Winchester, expect canker blight and shoot blight symptoms if cankers were allowed to overwinter in trees that had infection last year.

At the AREC last week an apple scab and cedar-apple rust infection period occurred May 14-15: 8 hr wet at 62° with 0.04 in. rain. Similar conditions also extended south to Staunton and east of the Blue Ridge from Manassas to Sperryville. Staunton and Sperryville to the Charlottesville area also had an infection period May 17. At Winchester, cedar-apple rust galls remain active after this wetting event with only 0.04 inches of rain.

Apple powdery mildew infection occurs on days without rainfall above 53°, and in the Winchester area we have had 16 days favorable for infection since spores were available on Mar 29. Secondary mildew symptoms are now quite common on unprotected trees, as are secondary scab and cedar-apple rust lesions.

Extended wetting is predicted for all major fruit production areas in Virginia May 18-22. This wetting will be highly favorable to secondary scab and late rust infection on foliage and will contribute to recorded accumulated wetting hours for development of sooty blotch and flyspeck (SBFS). For purposes of predicting the development of the SBFS fungal complex, we record accumulated wetting hours from rainfall or dew, starting 10 days after petal fall. This year we will use May 5 as the petal fall date for Winchester, so the start of wetting hour accumulation (ACW) will be from May 15. For the Roanoke area and areas east of the Blue Ridge, we will consider petal fall to have been one week earlier than Winchester, and accumulation of wetting hours will be from May 8. The action threshold for SBFS development is 250 ACW.

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 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.

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.

Apple scab infection April 5-6.

The first general apple scab infection occurred in the Winchester area and east of the Blue Ridge from Rappahannock County and south to Lynchburg April 5-6. At our AREC in Winchester, scab ascospores were released, and we recorded 33 hr of wetting at 40-47° F with 0.5 in. of rainfall. Many apple cultivars such as Cripps Pink, Red Delicious, Gala (shown below), and Idared are in the tight cluster stage, and presented a sizable target of exposed green tissue for scab infection. Hopefully, there was opportunity to apply protective fungicides prior to this infection.

Gala apple buds at tight cluster stage at Winchester 4/6/19.

Cedar-apple rust gall spore horns (shown below)  are expanding and will probably discharge the basidiospores that infect apple with the next wetting period. By tight cluster stage powdery mildew spores are usually available for infection, which occurs on dry days above 53° F

Cedar-apple rust gall at Winchester 4/6/19

Where the bud stage is more advanced to pink or open blossoms, as in central Virginia and the Roanoke area and south, be alert for fire blight infection with warmer weather and shower or thunderstorm activity this week.

Summary of last week's infection events, May 15-19.

At our AREC in Winchester we recorded a very long apple scab and rust infection period May 15-19, with 89 hr of almost continuous wetting at an average of 63° with 5.2 in. rain. We had observed scab lesions prior to this infection event, so potentially this was a very serious secondary infection where there was little fungicide protection to cover the event through five inches of rainfall. Some of the wetting occurred at temperatures near 70° and was favorable for moldy core infection and the start of summer disease activity. 

As a follow-up to this extreme infection period, a suggestion is to apply combinations fungicides with protectant activity and two classes of fungicides to offset the potential for resistance in apple scab; also include an SI fungicide for after-infection activity for cedar-apple rust. A second follow-up application should include another class of activity for scab plus a protectant fungicide.

This weather was also favorable for cherry leaf spot and peach scab, which will remain active for the next month. At our AREC we have had 22 apple powdery mildew infection days since Apr 10. With the long wetting last week, mildew activity was somewhat reduced, but it will be quite active with susceptible shoot growth this week

Other areas in Virginia also experienced long continuous or split wetting periods with amounts of rainfall ranging from two - five inches. We are near the transition from early season to summer disease control, but all areas should be concerned about remaining apple scab, as well as moldy core and early summer disease activity, especially bitter rot. The timing of this event might have also left an opening in protection against for Brooks fruit spot.

Disease update: recent and ongoing scab and rust activity

We have had a lot of apple scab and rust activity this past week, and more on the way. At our AREC we recorded scab and rust infection periods May 10-11 (15 hr wet at 62° with 0.05 in. rain), and May 14-15 (13 hr wet at 66° with 1.11 in. rain). As of the morning of May 16 another infection period is in progress and much rain and wetting is in the forecast for the next three days. We observed scab lesions, probably from infection periods Apr 15-16 or Apr 24-25, on leaves at the base of unprotected shoots. Cedar-apple rust galls are still actively producing spores. 

This weather is also favorable for cherry leaf spot and peach scab, which will remains active for the next month or so.

As indicated in the update May 14, these wetting events at temperatures in the 60s constitute fire blight infection events wherever susceptible flowers are present. Another threat is possible secondary shoot blight with bacteria spreading from blossom blight symptoms now appearing from infections May 5 and 7, and driven by a series of thunderstorms with some scattered hail.

At our AREC we have had 21 apple powdery mildew infection days since Apr 10.

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.