Summer disease update

 Six extended wetting events since Jul 22 have dramatically increased summer disease pressures in the Winchester area: Jul 22: 6 hr wetting at 73° with 0.9 in. rain, Jul 23-24: 13 hr wetting at 73° with 0.51 in. rain, Jul 30-31: 21 hr wetting at 70° with 1.01 in. rain; Aug 1-2: 15 hr wetting at 72° with 0.45 in. rain, Aug 3-4: 19 hr wetting at 68° with 0.92 in. rain; and Aug 6: 6 hr wetting at 68° with 0.2 in. rain. Suffice it to say that Winchester's extended dry conditions of July have passed! Wetting events at these temperatures have favored development of sooty blotch and flyspeck and the rot fungi. Evidence of early season (scab), mid-season (Brooks spot), and late season diseases (sooty blotch and developing rots) is shown below:

Scab, sooty blotch and rot spots on unprotected Granny Smith fruit at Virginia Tech AREC, Winchester, VA, Aug 10, 2020.

Infection by Brooks spot (8 o'clock and 1 o'clock positions from the calyx), as well as sooty blotch and rot spots on unprotected Idared fruit, Aug 10, 2020. Infection by Brooks spot, one of the earliest "summer diseases", typically occurs about a month after petal fall.

Recent disease pressure has been somewhat heavier than Winchester in other areas of Virginia, especially east of the Blue Ridge. Amounts of rainfall from extended wetting events have varied, especially with the remnants of the recent hurricane. Most were not excessive in areas where rain was needed; some extended wetting occurred with only a few hundredths of an inch of rainfall. All of these have been favorable for rot development where protection was inadequate and volume of rainfall can impact fungicide residual. 

Fire blight-killed shoots harbor the rot fungi and are a common pre-harvest inoculum source. Due to cool temperatures throughout bloom, the Winchester area mostly escaped fire blight this year, but infection was more common east of the Blue Ridge and in the Roanoke area.

Accumulated wetting hours through the summer can be an indicator of summer disease pressure, particularly to predict the development of the sooty blotch and flyspeck (SBFS) fungal complex, but also can be a general indicator of rot pressure. To predict SBFS, 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. Winchester passed the 250-hr threshold on July 21, and symptoms are now present on unprotected fruit, as shown above, with symptoms heavier at lower elevations, which have more wetting hours. Here is a listing of selected petal fall dates and current total wetting hour accumulation (ACW) for selected locations as of Aug 10: Winchester, May 5, 393 ACW; Staunton, May 5, 823 ACW; Roanoke, Apr 18, 447 ACW; Floyd, Apr 28, 470 ACW; Manassas, Apr 28, 694 ACW; Sperryville, Apr 28, 387 ACW; Crozet, Apr 18, 423 ACW; Carter Mountain, Apr 18, 558 ACW; Red Hill, Apr 18, 483 ACW; Lynchburg, Apr 13, 647 ACW; Rustburg, Apr 13, 573 ACW; Danville, Apr 13, 832 ACW. 

Unless unusual fruit disease-related events occur, this will be the final post for this year. With approaching harvest, carefully observe pre-harvest intervals and per acre per year restrictions for all fungicides and all crop protection products.

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. 

Apple summer disease and peach brown rot alert

Recent wetting events totaling more than 3 inches of rain would have depleted any and all fungicide residues available to protect against apple fruit rots and sooty blotch/flyspeck and brown rot on ripening peaches. For the record, July 28-29 we had 17 hr wet at 71-67°, with 1.89 in. rain, and for July 30-31 it was 17 hr wet at 72-67° with 1.43 in. rain.

Sooty blotch/flyspeck thresholds reached at Winchester and Tyro; extended wetting events

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 began wetting hour accumulation at Winchester from May 14. As of Monday, June 6, accumulated wetting hours (ACW) toward the 250 wetting hour threshold for specific treatment against the SBFS fungal complex were: at 909 ft elevation, 257 hr (with the 250-hr threshold reached June 5); at 952 ft elevation, 194 hr; and at the 983 ft elevation, 185 hr ACW.

At Tyro, VA we started wetting hour accumulation May 4, and as of June 6, a sensor placed at the 941 ft elevation had accumulated 245 hr (with the 250-hr threshold reached June 6); the one at 1165 ft. elevation had accumulated 173 wetting hours, and the one 1465 ft. elevation had accumulated 200 wetting hours.

The sensors at the higher elevations at both Winchester and Tyro could reach the threshold ACW within the next week or two, and similar accumulations should be expected for the Carroll-Patrick, Roanoke-Botetourt and Rappahannock-Madison fruit production areas.

At Winchester we had two more extended wetting events at relatively warm temperatures favorable for SBFS and fruit rot development and secondary scab infection last week: June 2-3: 19 hr at 75-64°, with 0.3 in. rain,  and June 4-5: 16 hr at 75-67°.

Summer disease update

Summer disease pressures remained steady last week.  The extended wetting event Aug 10-11, in progress at the time of the last post, amounted to 12 hr wet at 71° with 0.31 in. of rain.

As of the Monday morning, Aug 17, total accumulated wetting hours (ACW) at the AREC were: at 909 ft elevation, 864 hr; at 952 ft elevation, 611 hr; and at the 983 ft elevation, 539 hr ACW. Since accumulation of wetting hours began May 14, this represents an increase of 4.4 wetting hours per foot of elevation drop from 983 to 909 ft. The wetting hour accumulation differential is greater during times when wetting is from dew rather than during wetting from rain, because rain tends to wet both locations similarly. In the past 10 days we have recorded only 0.17 inch of rainfall, but recorded 87 hours of wetting from dew at the 909 ft elevation.  This is why we say to scout for sooty blotch, flyspeck, and fruit rots at lower elevations in an orchard.

At Tyro, in Nelson County, total ACW as of Monday morning, Aug 17, were: at 941 ft elevation, 718 ACW; at 1165 ft, 385 ACW; and at 1465 ft, 433 ACW. An extended wetting event with substantial rainfall was recorded at Tyro Aug 18-19, 16 hr wet and 1.2 in. of rain at 74-69°.

Increasing summer disease and brown rot pressures

Summer disease pressure increased in the past week, with two extended wetting events at our AREC: Aug 6-7, 16 hr wet at 68-65° with 0.11 in. of rain and Aug 9-10, 16 hr wet at 74-72° with 0.52 in. of rain. (Another extended wetting event is in progress at the time of this post).

As of the Monday morning, Aug 10, total accumulated wetting hours (ACW) at the AREC were: at 909 ft elevation, 796 hr; at 952 ft elevation, 567 hr; and at the 983 ft elevation, 518 hr ACW. Since accumulation of wetting hours began May 14, this represents an increase of 3.8 wetting hours per foot of elevation drop from 983 to 909 ft.

At Tyro, in Nelson County, total ACW as of Monday morning, Aug 10, were: at 941 ft elevation, 670 ACW; at 1165 ft, 368 ACW; and at 1465 ft, 409 ACW. 

The recent wetting events will again increase brown rot pressure on ripening peaches, as well as sooty blotch, flyspeck and rots on early ripening apples.

Some late season disease pressures

After enjoying some good harvest weather in September, with only 0.4 inches of rain in the Winchester area, the past two weeks have brought some significant late season disease pressures, but also with much needed rainfall. Extended wetting events occurred last week October 10-11 (27 hours at 55° with 0.4 inches of rain).  This week we had an extended wetting period of more than 60 hours with only intermittent drying Oct 14-17. Much of this was at relatively warm temperatures favorable for rots, including 9 hours at 71°. 

The most recent extended wetting, our longest since late April, also favored fruit scab infection wherever active scab was present in the orchard as a result of poor coverage or missed applications or fungicide resistance early in the season. Rainfall Oct 14-17 totaled more than 2.3 inches and likely depleted fungicide residue, even from applications in the past two weeks. This extended wetting at favorable temperatures was enough to cause "pin-point scab" (storage scab) of late cultivars, and early marketing (rather than long-term storage) is suggested for fruit from fresh-market orchards lacking recent fungicide protection where there was earlier evidence of active scab. 
  

Extended wetting for fruit rots July 8-9; accumulated wetting hours.

July 8-9 we had another extended wetting period: 15 hr with 0.24 in. of rain at 72°. This wetting period was favorable for rots.

As of Thursday morning, July 10, we had accumulated 293 wetting hours since May 18, well beyond the 250-hour threshold for specific treatment against the sooty blotch/flyspeck (SBFS) fungal complex at our usual AREC monitoring site at elevation 950 ft. But at the 910 ft. elevation cumulative wetting hour (CWH) total continues to run about 100 ahead, now standing at 391 CWH. Early sooty blotch symptoms (signs of the fungi) are now visible.  

Orchards at lower elevations in Nelson County (Tyro area) have also exceeded the 250-wetting hour threshold for presence of the SBFS organisms on unprotected fruit (total 345 CWH at 941 ft elevation). CWH total at the higher elevation stood at 200 CWH as of Thursday morning, July 10. Scout lower areas of your orchards regularly for onset of SBFS appearance and adjust your fungicide program accordingly.

Sooty blotch symptoms; extended wetting for fruit rots July 3-4.

Last week July 3-4 we had another extended wetting period: 9 hr with 0.6 in. of rain with a mean 71°. This wetting period was favorable for rots.

As of Monday morning, July 7, we had accumulated 270 wetting hours since May 18, well beyond the 250-hour threshold for specific treatment against the sooty blotch/flyspeck (SBFS) fungal complex at our usual AREC monitoring site at elevation 950 ft. But at the 910 ft. elevation we have recorded 372 CWH and early sooty blotch symptoms (signs of the fungi) are now visible.  

Orchards at lower elevations in Nelson County (Tyro area) have also exceeded the 250-wetting hour threshold for presence of the SBFS organisms on unprotected fruit. Scout your lower orchard areas regularly for onset of SBFS appearance and adjust your fungicide program accordingly.

Heavy scab and rust infection period in progress May 7

Since yesterday morning at 10 AM we have had 24 hr wetting at 54-56°, almost optimum for apple scab, quince rust and cedar-apple rust. Blossoms at petal fall are still very susceptible to quince rust infection so a follow-up including an SI fungicide in the next application is highly recommended (a "no-brainer"). Regarding potential rust inoculum source, winds during the wetting since yesterday have been from the north and east and appear to continue this way through today and tonight, then shift to a more southerly direction with continued shower activity tomorrow.

This will be a pivotal point in this year's scab epidemic. We will soon be nearing the end of predicted primary ascospore discharge, but lesions from our first infection period Apr 17 are expected to be appearing this week. So where control was good for the previous infection periods, we won't have too much to go till we can begin to focus on summer disease control, but where earlier control was inadequate, this infection period will be the source of continued headaches throughout May.

Yesterday was one of those unusual exceptions where there wasn't a forecast for rain early in the day, even with the Weather Channel's hourly predictions while it was raining! Today's SkyBit hourly report for our AREC for yesterday does not show leaf wetness until 4 PM but we had leaf wetness with showers at 10 AM, and leaves have probably remained wet since then.

I usually try to focus on our Winchester situation, where we have a better record of weather conditions, but it appears that those of you east of the Blue Ridge and farther south had similar conditions and substantially more rain than we did yesterday. Hopefully apples a little more advanced, with fruit at thinning time will be less susceptible to quince rust, but any trees with some blossoms in recent petal fall should be considered susceptible, and scatterred quince rust-infected fruits can hang on the tree and become a nuisance as a rot inoculum source as shown in the what-is-it blog below:  http://treefruitdisease.blogspot.com/2013/04/what-is-it-april-4-meeting-reminder.html

Recent extended wetting periods

This past week we had three extended wetting periods and more than 2.5 in. of rain (as of 4 PM today, but we are really not complaining about the rainfall): Jul 15-16, 10 hr wet at 70°; Jul 18-19, 8 hr wet at 71°; Jul 19-20, 17 hr wet at 72°. In the past two weeks we have gotten more than 4 inches of rain, which would have eroded most protective fungicide residues, adding considerable rot pressure on apples and other fruits. Cumulative wetting hours now stand at 572, the highest total for this time of the year since 2003, and the fourth highest total since 1994.


There have been several reports of rots appearing where was earlier fire blight infection. This should be expected because any dead twigs can be quickly colonized by several rot fungi, leading to a build-up of inoculum and subsequent infection under the warm and wet conditions such as have occurred recently.


Scout for sooty blotch and flyspeck in lower elevation areas where you typically see these problems first, and make appropriate management adjustments as needed.

Extended wetting and validation of the sooty blotch flyspeck wetting hour threshold

Extended wetting June 11-13 involved two combined wetting periods of 14 hours at 69° and 12 hours at 66° with a total of 1.4 in. rain. These wetting periods favored secondary scab and all summer diseases on apples and brown rot on ripening stone fruits. Accumulated wetting hour total from rainfall or dew since Apr 18 is now at 367 hours. 


Because of early bloom and petal fall this year, there had been some questions about using the Apr 8 petal fall date as the trigger for accumulating wetting hours, starting 10 days later. The 250 accumulated wetting hour threshold is used to predict the presence of sooty blotch/flyspeck (SBFS) fungi on unprotected fruit. Using the Apr 18 date we had passed the 250-wetting hour threshold by May 25, four weeks ahead of last year and the second earliest date for reaching this predictive threshold on record since 1994. We are now seeing SBFS on fruit samples collected at our AREC three weeks ago and incubated under high humidity since then. This indicates that fungi were present May 25 and confirms the validity of the Apr 8 petal fall date and Apr 18 as the accumulation date in spite of this year's early bloom and petal fall dates.

Lee's left-overs

Tropical storm Lee brought us rainfall that totaled 2.72 inches over several days, Sept. 4-7. The ground seemed to soak up this volume of rain surprisingly well, and it will be beneficial for sizing the apple crop. Of course this amount of rain and the extended wetting should be expected to have some consequences for disease development, particularly if fungicide residues were low. Keep watching for sooty blotch and flyspeck, as well for as fruit rots such as bitter rot.

For the record, we had extended wetting periods of 16 hr Sept 4-5 at 69º F, 47 hr Sept 5-7 at 69º F, 12 hr at 64º F Sept 7-8 with Lee, and a more recent one of 22 hr at 68º degrees. Generally, wetting in the higher temperature ranges favor more rapid rot infection and development, and ripening fruit are more susceptible to infection. Typically, residues of a protectant fungicide such as captan do not weather through more than two inches of rainfall in a one week period.

Summer diseases

Extended wetting last night, July 7-8:  8 hr at 70º with 0.15 in. rain. Consider wetting periods 70º and above to be more favorable for rot diseases, especially bitter rot.

Sooty blotch and flyspeck

Sooty blotch symptoms were observed on unprotected fruit at our AREC July 5, with symptoms probably appearing in lower areas of the farm late last week.

Last week we accumulated 26 hours of wetting from rain and dew, bringing us to a total of 288 hours, 38 hr past the 250-hour action threshold for development of sooty blotch and flyspeck on unprotected fruit. There were two extended wetting periods from rain in the last week: June 27 (11 hr at 67º with 0.05 in. rain) and July 4-5 (14 hr at 84-66º with only 0.01 in. rain at our AREC, but with lightning to the north of Winchester as a backdrop for the fireworks display).  The last two nights brought an additional accumulation of 16 wetting hours from dew.

Wetting hour accumulation from dew may be more likely in lower areas, while wetting from rain will probably affect higher and lower elevations in an orchard more uniformly. Typically, symptoms appear earlier in lower areas but it is prudent to begin scouting for symptoms in those areas most likely to show symptoms, whether because of low elevation or lack of recent fungicide protection.

Appearance of sooty blotch/flyspeck symptoms probably also indicate a lack of fungicide residue to effectively control rot development. Where present, fire blight-killed shoots should be viewed as a potential fruit rot inoculum source.