Apple scab infection period April 28-29

At our AREC we recorded split wetting periods totalling 18 hours starting at 54° at 11 AM Apr 28 and continuing at 50° throughout the duration of the wetting events. With expected high ascospore maturity and discharge, this is our most serious scab infection period this year. Hopefully, most blocks have received adequate fungicide protection up to this point and, with only 0.2 inch rain, should have been well protected through this event. Although the temperatures were a bit cooler than the optimum for the rust fungi, the blossoms are still very susceptible to quince rust and a follow-up application of an SI fungicide is advised where disease pressure is high and protection may have been lacking.

The outlook for fire blight infection over the next week remains low to moderate unless temperatures turn out to be significantly warmer than currently predicted. For the longer term, these cool temperatures will probably prolong the bloom period and our greatest threat of blossom blight may come on late bloom.

Since April 11 we have seen 13 dry weather, powdery mildew days, and we can expect secondary infection to be flourishing on new growth soon.

Continued reduced risk of fire blight in the Winchester area

FIRE BLIGHT: Above is an updated graphic from the Maryblyt 7 program. Based on predicted temperatures and wetting, the risk column shows that fire blight infection risk on Apr 29 is high with wetting but infection is not predicted, even with wetting, because the EIP (epiphytic infection potential) is now well below 100. With most cultivars near full bloom and later cultivars with early bloom open, this is a prediction for trees with first bloom open April 16, 2013. The temperature and rainfall data are current through Friday morning, Apr 26. Predicted weather conditions are shown for Apr 26-30. 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). Based on predicted temperatures and wetting, the risk column shows that fire blight infection risk on Apr 29 is high but infection is not predicted, even with wetting, because the EIP (epiphytic infection potential) remains well below 100. For infection to be predicted, wetting must occur after the EIP reaches 100 or higher.  Cooler predicted temperatures result in continued low EIP and reduced risk through May 3 (not shown). The CBS column at the right in the graphic indicates progression toward the appearance of canker blight symptoms on new growth, due to extension of overwintering cankers from last year, 86% of the degree hour requirement by May 3. Canker advancement cannot be prevented by chemical treatment at this time and the presence of symptoms will indicate a build-up of inoculum which could become a factor in the event of a trauma blight situation due to hail injury, etc.


CAUTION: 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. This graphic will be updated April 29. The weather conditions used in the predictive part of this graphic come from the Weather Channel for Winchester, supplemented by site-specific data from SkyBit Inc.

Fire blight and powdery mildew/rusty spot update

FIRE BLIGHT: Below is a cropped graphic from the Maryblyt 7 program. Based on predicted temperatures and wetting, the risk column shows that fire blight infection risk on Apr 24 is high with wetting but infection is not predicted, even with recent wetting, because the EIP (epiphytic infection potential) is now below 100. With most cultivars approaching full bloom open and later bloom opening over the next several days, this is a prediction for trees with first bloom open April 16, 2013. The temperature and rainfall data are current through Wednesday afternoon, Apr 24, with showers predicted this evening. Predicted weather conditions are shown for April 25-29. The components of fire blight risk are indicated in the columns labeled B (blossoms open), H (degree hours for epiphytic bacterial populations), W (wetting by rain or dew), and T (average daily temperature 60 F or above). Based on predicted temperatures and wetting, the risk column shows that fire blight infection risk on Apr 24 is high with wetting but infection is not predicted, even with wetting, because the EIP (epiphytic infection potential) is now well below 100. For infection to be predicted, wetting must occur after the EIP reaches 100 or higher. In high-risk situations, a protective streptomycin application is recommended ahead of predicted infection. Cooler predicted temperatures lead to a reduced risk Apr 25-29 and into next week (not shown). The CBS column at the right in the graphic indicates progression toward the appearance of canker blight symptoms on new growth, due to extension of overwintering cankers from last year, 80% of the degree hour requirement by Apr 29. Canker advancement cannot be prevented by chemical treatment at this time and the presence of symptoms will indicate a build-up of inoculum which could become a factor in the event of a trauma blight situation due to hail injury, etc.
CAUTION: Be aware that predicted risk can change quickly with unanticipated warmer temperatures and wetting. This graphic will be updated April 26. The weather conditions used in the predictive part of this graphic come from the Weather Channel for Winchester, supplemented by site-specific data from SkyBit Inc.

Powdery mildew remains the most active among other apple diseases. Mildew infects during dry weather at temperatures above 53F. Since overwintering mildew spores were first observed on Apr 11, there have been nine day favorable to mildew infection. Rusty spot on peach is caused by the apple powdery mildew fungus, and with this much mildew activity underway, expect increased pressure by rusty spot on susceptible peach cultivars. Protective applications of myclobutanil (Rally) from petal fall through the early cover sprays are suggested for management of rusty spot.

Fire blight and other disease concerns

CAUTION: Based on weather data for our AREC, the fire blight prediction for Friday, Apr 19, remains at the infective level shown in the graphic posted below on Apr 16. If the cooler temperatures prevail as predicted in the ten-day forecast, infection after today is not likely well into next week. Streptomycin, applied Apr 17-19, should effectively cover for this infection event.

On Wednesday, Apr 17, we recorded 12 hours of wetting with slow drying conditions at 56-62° F, conditions favorable for apple scab, quince rust and cedar-apple rust. Blossoms are now very susceptible to quince rust, and for blocks that were not adequately protected for this infection period, it would be prudent to include an SI fungicide with strong after-infection activity in the next fungicide application. 

Powdery mildew has been the most active apple disease so far this year. Mildew spores were observed on new growth from overwintering buds Apr 11, and we have had infection conditions seven of the nine days since then. As early as pink stage, blossoms are highly susceptible to infection which causes the fruit russeting shown below. For mildew-susceptible varieties, include a fungicide with mildew activity in each application from tight cluster stage through to a month after petal fall.

Powdery mildew russeting of fruit can occur as early as pink stage.

Fire blight alert for this week

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

FIRE BLIGHT: Above is a cropped graphic from the Maryblyt 7 program. With early cultivars with bloom open, and more bloom opening over the next several days, here is a prediction for trees with first bloom open today, April 16, 2013. The temperature and rainfall data are current through this evening, April 16. Predicted weather conditions are shown for April 17-20. The components of fire blight risk are indicated in the columns labeled B (blossoms open), H (degree hours for epiphytic bacterial populations), W (wetting by rain or dew), and T (average daily temperature 60 F or above). Based on predicted temperatures and wetting, the risk column shows that fire blight infection will be possible on open bloom if wetting occurs Apr 18-19. For infection to be predicted, wetting must occur after the EIP (epiphytic infection potential) reaches 100 or higher. In high-risk situations, a protective streptomycin application is recommended ahead of predicted infection. Cooler predicted temperatures lead to a reduced risk Apr 20-25. If first bloom opened Apr 17, infection would still be possible on Apr 19, but if first bloom did not open until Apr 18, the EIP would remain below an infective level through the weekend.

This graphic will be updated April 18, or sooner if significant change warrants it. The weather conditions used in the predictive part of this graphic come from the Weather Channel for Winchester, supplemented by site-specific data from SkyBit Inc.

Apple disease status

Today advanced Idared fruit buds were at open cluster stage, and some blossoms are likely to open in the next couple days. The picture below was taken during the wetting period April 12.

Apple scab, cedar-apple rust, and quince rust spores were mature Apr 12, but 12 hours' wetting was probably not quite long enough at 51-55° to cause significant infection by these diseases.

However, powdery mildew spores were also available under favorable dry weather infection conditions Thursday, Apr 11.

Oh yes, speaking of rust conditions, the mystery disease below was caused by one of the rusts, most likely quince rust. This resulted in small mummified fruits and a potential carry-over of bitter rot inoculum for fruit infection this year.

What is it? April 4 meeting reminder

This problem may not be what you think it is...

This is a reminder that our first In-Depth Meeting of the new season will be held at Alson H. Smith Jr. AREC, 595 Laurel Grove Road, Winchester, VA, starting at 7 PM Thursday, April 4. Following horticulture and entomology updates by Drs. Greg Peck and Chris Bergh, we will address this and other pathology questions and issues.

Here's another close-up example:

Scab spores and green tip

Apple scab ascospores were discharged at our AREC with showers starting yesterday morning. The earliest buds on Gala showed some green-tip, but 17 hours of wetting at 39-47° F was not enough for an infection period. It was a “two-minute warning” of sorts. Hopefully now, gradual bud advancement over the next week will allow some opportunity for application of protectant fungicides before another scab infection event. Preventive disease management is the best way to offset development of resistance to the more highly selective fungicides.

Idared apple blossoms - then and now

 On March 24 last year we were concerned about apple scab and rust infection and a threat of fire blight where blossoms had opened March 23. Today there's not much susceptible tissue out there and I'm ok with that! That white stuff on the nearly dormant buds isn't cotton. Can we say climate un-change?


But remember that the apple scab spores are just waiting their turn...

   

March 24, 2012

 

March 25, 2013



Prelude to a new season

The past several days we have seen a blanket of white in the orchards. This is great for holding back the rush of an early season and gets plenty of moisture into the ground. But this moisture on thawed ground will probably bring early scab ascospore maturity, so is it is important to be ready with protective fungicides. After application at green-tip stage, a copper spray will give about a week's protection against scab and also provides a deterrent to the buildup of fire blight bacteria before bloom.

A Late-Season Wrap-up

Fall urea spray to reduce overwintering disease inoculum: A foliar urea spray is recommended as a supplemental application at early leaf drop to reduce overwintering apple scab, Glomerella leaf spot and Alternaria leaf blotch inoculum. The recommended rate is 40 lbs of urea per acre, applied to wet the remaining leaves on the tree and with nozzles also directed to cover leaves already on the orchard floor, under the trees and in the adjacent row middles. This practice has been shown to reduce scab inoculum by as much as 95%, thereby reducing the threat of heavy early season infection next spring. Special consideration should be given to this approach where resistance to strobilurin (QoI) and sterol-inhibiting (SI) fungicides is suspected. Glomerella leaf spot was a new problem in the Winchester area this year and some orchards with the leaf spot also reported developing rots on fruits that were not refrigerated promptly.

Bacterial leaf spot of peach: The leaf spot bacteria overwinter in small twig cankers caused by infections at the leaf abscission scars soon after leaf drop. Where bacterial leaf spot has been a problem, apply a copper spray at early leaf drop to protect those leaf abscission scars thereby reducing the potential for infection next year. A copper spray in the fall (or any time before bud swell in the spring) also serves as the peach leaf curl spray.

Pay attention to abnormal foliage color of trees in a planting: Early, reddish/purplish coloring of foliage often signals a problem with the lower trunk or crown area of that tree. The cause could be voles, dogwood borers, or collar rot, or more than one of these problems. All have specific treatments that improve control and reduce tree loss but need correct diagnosis and treatment to expect effective results. It could also be fire blight in the rootstock of M.9, M.26 or Mark rootstock if there was earlier infection in the planting. There isn't much that can be done for a tree dying from fire blight in the rootstock but the cause should be recognized and noted for improving control measures for next year. This includes thoroughly cutting out overwintering inoculum cankers during the winter and by being ready to protect the planting with copper at early greentip and with streptomycin as needed at bloom.

A final reminder for the year: Make notes, based on harvested crop, of disease or insect problems that affected quality yield in individual orchard blocks this year. Develop management strategies to address these problems at appropriate times next year.

Glomerella leaf spot- New to Frederick County

Yesterday morning I was called to check a block of Golden Delicious trees that are losing one-third or more of their leaves. Before the morning was over I had observed symptoms of Glomerella leaf spot, a rapidly expanding disease, at our AREC and in five other orchards within two miles of our AREC. I also received an e-mailed image of the same problem from Rockingham County. Diagnostic symptoms (shown below), include leaf spots with concentric rings on Gala, Golden Delicious, and Cripps Pink (Pink Lady). The lesions start as purple spots, then growth with concentric rings, and may eventually coalesce. Symptoms may appear on both old and younger leaves and leaves may yellow and fall off.

Glomerella leaf spot. Early lesions are small purple, irregular spots (at bottom) which typically enlarge with concentric growth rings that may coalesce. Lesion enlargement and concentric rings may be due to secondary fungi.

Typical appearance of leaf yellowing caused by Glomerella leaf spot.



Glomerella leaf spot and fruit rot from Nelson County VA, 2011


Glomerella leaf spot was reported in TN in 1998. It built up in NC in 2004 and became more prominent with wet weather in ’05. The following winter I began to receive reports from growers and consultants who thought they may have seen it in southern VA. It became prominent again in southern VA/NC in 2010 and caused some problems in Nelson County, VA in 2010-11.



Fruit rot symptoms caused by Glomerella cingulata, a stage of the bitter rot fungus, tend to be darker than those usually associated with bitter rot. The rot spots often remain small but, like typical bitter rot, the fruit rot caused by the Glomerella strain has a cone-shaped pattern of growth into the fruit.



Recommendations for control as we approach harvest depend on market plan for the affected variety and the days to harvest. Pristine is the single most effective material that may be applied close to harvest. Other possible options include Flint + Captan and Captan + potasssium phosphite.

Leaves are an important overwintering site and overwintering inoculum can be reduced by shredding leaf litter with a flail mower or applying a foliar spray of urea at leaf drop. This can also aid in suppressing other foliar diseases such as apple scab and Alternaria leaf blotch.

WEEKEND WETTING: Split extended wetting periods of 14 hr and 10 hr occurred with a total of 0.75 inch of rain at 70° Aug 25-27. We also had several nights of heavy dews last week, bringing the accumulated wetting hour total from rainfall or dew since Apr 18 to 909 hours.

Extended wetting and summer disease pressure

Nine hours of extended wetting occurred with an 0.06 inch of rain at 69° Aug 14. We also had several nights of heavy dews last week, bringing the accumulated wetting hour total from rainfall or dew since Apr 18 to 835 hours. This total, generally indicative of sooty blotch and flyspeck pressure, is the highest for this time last year since 2003.

Bitter rot and some other rots have appeared on poorly protected apple fruit that were damaged by hail on July 31.

More summer disease weather

Summer disease pressure remained high this week with several nights of heavy dews and with 14 hours of extended wetting with an inch of rain at 69° Aug 9-10.

I was away for a week, and on my return some changes were obvious- lots of apple trees with light green, new growth, and nice red color on ripening (and tasty!) Gala apples.

Where powdery mildew is a problem, the new growth will be susceptible to mildew infection, leading to more overwintering. In such situations, the fungicides Indar and Inspire Super offer some suppression of mildew and other summer diseases such as sooty blotch and flyspeck. Both Indar and Inspire Super have restrictions preventing their application closer than 14 days to harvest. We suggest including a general protectant fungicide such as captan or ziram with the above fungicides to broaden the spectrum and residual activity.

Yesterday I saw striking examples of the effects of fire blight strikes on the prevalence of bitter rot in Nittany apples (shown below). The bitter rot fungus can colonize the dead twigs within six weeks of the initial fire blight blossom infection and become a bitter rot inoculum source for the rest of the year. Any fire blight strikes in a tree should be taken as a warning of a potential rot problem. Notice that the fruit immediately below also has sooty blotch, and that indicates that protective fungicide is lacking which would also permit more rot problems.

Extended wetting August 5-6.

Extended wetting occurred Aug 5-6 with 13 hours wet at 73° and 0.41 in. rain. Accumulated wetting hour total from rainfall or dew since Apr 18 is now at 729 hours, 277 hours ahead the total for this time last year. This is the third highest wetting hour total for this time of the year since 1994. At our AREC, total rainfall for last week was 2.86 inches.

Watch for increased disease pressure from sooty blotch and flyspeck, fruit rots and Alternaria leaf blotch, the fungal disease that causes defoliation and poor fruit quality of Red Delicious apples.

Heavy rainfall and extended wetting event July 31

We had rainfall of 2.5 inches and 17 hours extended wetting at our AREC Jul 31-Aug 1. Most of the heavy rainfall occurred in mid-afternoon and it included scattered hail. But there were additional showers in the evening that extended wetting throughout the night. Mean temperature was 67°.

Most protective fungicides will not weather through 2 inches of rain, and favorable temperatures with some hail injury are likely to increase the potential for fruit rots as well as sooty blotch and flyspeck. 

Extended wetting occurred July 20-21 with 19 hours wet at 66° and 0.34 in. rain. Accumulated wetting hour total from rainfall or dew since Apr 18 is now at 615 hours, 225 hours ahead the total for this time last year.

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.

Some much-needed rainfall July 8-9

Scattered and repeated thunderstorms brought varying amounts of rain and some hail to parts of Frederick County July 8-9. At our AREC, we recorded 1.4 inches of much-needed rain and 16 hr wetting at 72 degrees. We have noticed brown rot on green nectarines, an indication of increasing fungal activity as peaches and nectarines are approaching harvest.

Extended wetting periods and summer disease development

Three extended wetting periods have added to summer disease pressure locally in the past week. At our AREC we recorded: 29-30 June, 10 hr wet at 66° with 0.44 in. rain; 2 July, 8 hr wet at 70° with 0.2 in. rain; 4-5 July, 8 hr wet at 72° with 0.11 in. rain. Some areas to the north in Frederick County probably received more rain than we did. All of these storms had strong winds that could have resulted in some fruit injury, increasing the potential for fruit rots. the accumulated wetting hour total from rainfall or dew since Apr 18 now stands at 470 hours, more than 120 ahead of this date last year.


These warm wetting periods will also increase the threat of brown rot on ripening stone fruits.

Extended wetting and summer diseases

Extended wetting occurred June 18-19 with 15 hours wet at 64° and 0.11 in. rain. With this and other wetting in the past week, the accumulated wetting hour total from rainfall or dew since Apr 18 is now at 413 hours. 

Sooty blotch was first observed in non-treated trees at our AREC June 13, indicative of this year's early summer disease pressure.

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.