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.