Early season apple disease outlook, March 27.

Overnight wetting March 26-27 at our AREC, Winchester, ranged from 11 to 15 hr at 46-55 degrees and resulted in a possible apple scab infection period at the longer wetting interval. There was a rather wide range in bud stages during this wetting event, from tight cluster in the more advanced Granny Smith and Red Delicious to only 1/4-inch green tip in Golden Delicious, York, and Rome Beauty. 

Tight cluster stage on Granny Smith apple, Winchester, VA, March 27, 2017.

Mildew conidiospores were observed on Idared apple at early tight cluster stage (below) and conditions Monday afternoon, March 27 were favorable for infection. 

Mildew-infected Idared bud (center), compared to healthy fruit buds, upper left.

This extended wetting also brought maturity to the cedar apple rust teliospores on overwintering galls (below), as well as to quince rust spores in cankers on the eastern red cedar. It is not too likely that spores were available in time for rust infection to occur in the Winchester area during this wetting event, but consider additional wetting events this week as probable rust infection periods wherever rusts are an annual problem.

Cedar apple rust gall with mature telial horns.

A check of weather stations east of the Blue Ridge Mountains in Rappahannock County and central Virginia indicates similar length of wetting, and some at warmer temperatures. Generally, expect similar or more advanced stages of bud development and mature disease inoculum in these areas to the south and east. 

Transition to summer diseases

We have had extremely cool conditions the first three weeks of May, with temperatures reaching the 80s only one day (May 2) and barely nudging the 70s only five other days. We are now seeing the development of scab and cedar-apple rust lesions from the infection periods outlined in previous posts: Apr 22-23 and Apr 28-May 12. Mildew-susceptible cultivars are showing considerable secondary infection, a result of the earlier 24 dry weather mildew infection days through Apr 25. Two more extended wetting events last week favored secondary scab and moldy core infection and kicked off the early summer disease pressures. These were: May 21, 14 hr wet at 54° with 0.62 in. of rain, and May 21-23, 33 hr wet at 54° with 0.34 in. of rain. (Actually these two could be lumped together, increasing the threat of fruit infection where primary scab was not well-controlled Apr 22-May 5).

Fire blight infection symptoms appeared locally as early as May 9, from infection that occurred Apr 21-26; symptoms from infection May 2-3 are expected later this week. After we reach late bloom in most years, temperatures are warm enough that protection of late bloom is routinely needed. However this year we have gone 19 consecutive days without infection conditions. But this week conditions return to "normal", with temperatures in the 80s increasing the risk (EIP, effective inoculum potential) to an infective level wherever wetting occurs on late bloom remaining into next week. Continue to protect prolonged bloom, especially on newly planted trees, as needed. With the prolonged cool conditions, we are still seeing late bloom on varieties such as Rome three weeks after we would like to be finished thinking about fire blight!

For purposes of predicting the development of the sooty blotch and flyspeck (SBFS) fungal complex, we record accumulated wetting hours from rainfall or dew, starting 10 days after petal fall. This year we have again chosen May 4 as our petal fall date for Winchester, so the start of wetting hour accumulation is from May 14. As of Monday, May 23, accumulated wetting hours (ACW) toward the 250 wetting hour threshold for specific treatment against the SBFS fungal complex were: at 909 ft elevation, 94 hr; at 952 ft elevation, 76 hr; and at the 983 ft elevation, 73 hr ACW.
To those farther south and east of the Blue Ridge, your normal petal fall date would have been a week or more ahead of ours at Winchester, so we have selected Apr 27 as the petal fall date for accumulation of wetting hours by weather stations at Tyro, VA. One of the purposes of having three weather stations at this location was to compare wetting hour accumulation at different elevations, but because many of the early wetting hours were a result of rainfall rather then dew, the early accumulation trend this year is a bit different than in previous years. As of May 23, a sensor placed at 1465 ft. elevation had accumulated 130 wetting hours from May 4, the one at 1165 ft. elevation had accumulated 107 wetting hours, and the one placed at 941 ft elevation had accumulated 133 hr.

Scab and rust infection period

As predicted Saturday, extended wetting from March 24-26 resulted in scab, quince rust and cedar-apple rust infection periods at our AREC. A split wetting period was recorded with 8 hours of wetting at 61 degrees, followed by four hours of marginal wetting/drying, then 13 more hours of wetting from 55 to 48 degrees and 3 hours of intermittent wetting. An inch of rainfall, early in the wetting period, likely depleted the residual effects of any protective fungicide residues applied before these events. Cedar-apple and quince rust spores were released within four hours of the start of the wetting period Saturday. For the record, wind direction from potential cedar tree rust spore inoculum sources during the spore discharge period, was mostly from the east or northeast.

In areas where rusts are a common problem, it is advised that an SI fungicide be included in the mix to be applied this week for after-infection control of rusts; in areas where scab is resistant to the SIs, other fungicides are advised to be included for after-infection and protective control of scab and suppressive effects on powdery mildew which infected last week and will continue this week.

The threat of fire blight on apples did not materialize at our AREC, with temperatures about 3 degrees cooler than would have been required, based on bloom first open Friday, March 23. For pears which were in bloom earlier in the week, or apples in areas with bloom open Thursday, infection was possible with wetting Saturday. Cool forecasted temperatures should minimize any threat of fire blight for the rest of this week.

Scab infection period

Wetting last night at our AREC gave us less than 0.01 inch of rainfall, but it stayed wet 14 hr at 57-67 degrees, and resulted in our first apple scab infection period of 2012. As noted in yesterday's post below, the spore target was big for many varieties. Other areas of Frederick County had more rainfall. With the relatively warm temperatures, cedar-apple rust and quince rust spores matured quickly, but probably did not infect last night, at least in our situation. Blossoms beyond tight cluster with their bases exposed would be susceptible to quince rust. Also as noted yesterday, mildew spores were available and infection has already occurred. So these constitute the present apple disease control spectrum concerns. But with the way it's going this year, we may be concerned about fire blight on early bloom before we know it.

Accumulated wetting hours

Last week we had 11 hours of wetting, bringing us to a total of 154 accumulated hours at our AREC.

This year we had 28 dry weather "mildew infection days" in April-May compared to 45 last year, but secondary mildew symptoms are common where control was not adequate. Control is still advised where shoot growth is continuing. Over the 18-year period that we have recorded this, the number of "mildew days in April-May has ranged from 24 to 49.

Fungal disease update

Sixteen hours of intermittent wetting May 1-2 favored primary and secondary apple scab infection of leaves and fruit, quince rust fruit infection on cultivars in bloom and cedar-apple rust on foliage of all susceptible cultivars. Because a lot of rust inoculum was still available for infection during this wetting, it would be prudent to include an SI fungicide for after-infection rust control in applications this week. On stone fruits, these weather conditions were also favorable for peach scab and cherry leaf spot infection.

Since primary apple powdery mildew inoculum was first observed March 23, 13 dry-weather days have been favorable for mildew infection. Although this is fewer mildew days than we had to this point last year, secondary mildew lesions will become apparent on new growth in affected blocks.

Fungal disease and fire blight update

Primary apple scab lesions were observed today on unprotected trees at our AREC. Size and positions of infected leaves and appearance of lesions suggest that some could have been from infection Mar 30-Apr 1 and some from Apr 8-9. It is likely that they were present during infection periods Apr 22-23 and Apr 24-25 and, as such, could have provided secondary inoculum for heavy fruit infection in poorly protected orchards. Last night's wetting with 1.7 inches of rain was not quite long enough for scab or rust infection, but the 11-hr wetting event at 60-69° F Apr 24-25 favored quince rust of blossoms and cedar-apple rust infection of leaves and fruit, as well as scab. Quince and rust gall inoculum is still available on the cedar trees in our area.

Since primary powdery mildew inoculum was first observed Mar 23, eleven dry weather days have been favorable for mildew infection.

FIRE BLIGHT 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. Some apple trees have reached petal fall but others still have much bloom still open and susceptible. Late cultivars such as Rome and York may be near full bloom. The temperature and wetting data are current through Thursday morning, April 28. Predicted weather conditions are shown for April 28-May 2. 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 is possible with rainfall today, Apr 28 (note red color) and would again be possible if wetting were to occur Apr 29-30. For infection to be predicted, wetting must occur after the EIP (epiphytic infection potential) reaches 100 or higher. This year’s bloom season has seen considerable sustained fire blight pressure and some infection will be expected in commercial orchards. In high-risk situations, a protective streptomycin application is recommended ahead of predicted infection. The BBS and CBS columns are tracking predicted blossom blight symptom (from infection Apr 11 and later ones as a, c and d) and canker blight symptom appearance, respectively. Numbers in these columns represent percent of required degree hours for early blossom symptom appearance and canker margin symptoms Apr 26. Today, early symptoms are evident from experimental inoculations done Apr 23. Today’s update was a result of changes in the weather forecast from yesterday. RISKS CAN INCREASE QUICKLY WITH WETTING FOLLOWING WARMER THAN PREDICTED TEMPERATURES. Growers are advised to be alert to changes in forecasts on a day to day basis and protect high-risk blocks through late bloom as needed. This graphic will not be updated until May 2 unless some 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.

Wetting March 23-24

At our AREC we recorded wetting from 7:15 PM Mar 23 - 10:30 AM Mar 24, about 85% of the duration of wetting required for scab infection at temperatures that ranged from 49F down to 37F. But temperatures and area showers were variable earlier on March 23 and some local areas may have had their first scab infection. Wetting occurred north of Winchester by 6 PM while the temperature was at 55F; other areas had showers near noon March 23 and may have remained wet longer. Scab protectants applied before this wetting probably weathered through the rainfall which totaled about half an inch. At our AREC, apple bud stage varies from 1/2"- green for York (shown above) and Rome to tight cluster for several cultivars including Gala, Red Delicious and Idared. Where no fungicides have been applied and length of wetting is marginal or in doubt, it would be prudent to include one with after-infection control in the first application. Also, consider that powdery mildew spores are now available for infection during dry weather 53F and higher, and cedar-apple rust gall spore horns are extended to more than a half-inch, indicating potential for rust infection with the next rain in the mid-50s and higher.

Mildew days and wetting hours

As an indicator of relative annual mildew pressure, we note the number of days above 53º without rain as potential "mildew days". From March 30, when powdery mildew spores were first were available on infected emerging buds, until June 1, we had 45 mildew days this year. Since 1993, the number of such days from tight cluster to six weeks after petal fall (about third cover) has ranged from 24 (in '96 and '03) to 49 in '99. This year ranks as the second most frequent mildew weather in 18 years.

Accumulated wetting hours (starting April 29, 10 days after petal fall, Apr 19) now stand at 185 hr. The wetting hour threshold for presence of sooty blotch and flyspeck fungi on unprotected fruit is 250 hr. Rappahannock county (Washington, VA) has logged 237 wetting hours, also as of June 7, using the Apr 19 petal fall and Apr 29 start accumulation date. Rappahannock's total will be updated every two weeks.

Early season apple disease summary

Apple scab:
Scab ascospores were first trapped with rains March 22.
First primary scab infection period: Mar 28-29 (20 hr at 42-44º mean with 1.62 in. rain);
Scab lesions were observed on Golden Delicious Apr 12.
Primary or secondary scab infection periods:
Apr 13-14 (wet 22 hours at 44º with 0.07 inches of rain); Consider this likely to be an early secondary infection period where lesions were sporulating if 1/2" greentip-tight cluster growth was not protected for the Mar 28-29 infection period. This wetting period was too cold for rust infection.
Apr 25-27 (intermittent wetting approximately 28 hr at 53-63º with 1.3-in. rain total for event).
May 2-3 (11 hr at 72º with 0.4 in. rain).
All overwintering ascospores should be discharged by now, but it is important now to be sure that there are not lesions in the tree which would indicate potential for secondary infection of fruit.

Rusts:
Cedar-apple rust infection periods Apr 25-27 (28 hr at 53-63º with 1.3-in. rain total for event) and May 2-3 (11 hr at 72º with 0.4 in. rain); infection is not yet visible from either of these infection periods. Quince rust infection this year not too likely because most fruit would have become resistant by the time they reached thinning size and warmer wetting periods occurred.
Much cedar-apple rust inoculum remains to be discharged and inoculum may persist late into spring as it did several years ago. Heavy cedar rust infection can reduce return bloom. If the orchard is in a rust-prone location and residual protection is in doubt, it is best to supplement the next application with an SI fungicide for after-infection rust control.

Powdery mildew:
Mildew has been our most active disease this year. Spores were available on infected emerging buds by Mar 30.
Infection pressure has been almost ongoing with 29 dry weather “mildew days” since Mar 30.

Fire Blight:
Earliest bloom open at our AREC on Idareds Apr 5.
Infection events occurred at our AREC Apr 8 and Apr 16 (some local areas may also have had wetting of early bloom Apr 6). Blossom blight symptoms were predicted to start appearing Apr 30 from Apr 8 and May 2 from Apr 16. (Blossom blight symptoms have been reported from Frederick County). Canker blight symptoms were predicted for May 1, and trauma blight symptoms were predicted to appear May 4 from blight due to hail damage Apr 25. Temperatures favored infection with wetting where late bloom was still Apr 30-May 5. Wetting from a spray application is adequate for infection on an otherwise dry day.

Scab and rust infection period

A scab and rust infection period ended Monday morning, May 3: 11 hr, 72 deg., 0.4 in. rain. Heavy secondary/primary apple scab from the April 14 infection period is now evident on unprotected shoot leaves. Consider options for stronger "catch-up" treatments if your orchard was not properly covered for the earlier infection periods March 28 and Apr 14 or the more recent ones which could result in damaging secondary fruit infection, Apr 24-25, Apr 25-27, and May 2-3.

Secondary mildew infection is also apparent from ongoing infections which occurred on at least 21 days in April.

Apple Blossom Festival weekend temperatures were actually slightly warmer than those indicated as predicted below, resulting in possible fire blight infection on late bloom wherever wetting occurred. The warmer temperatures should also bring earlier than predicted blossom blight symptom development this week.

For purposes of monitoring cumulative wetting hours for sooty blotch and flyspeck development, we are using Apr 19 as petal fall and wetting hour accumulation of wetting hours begins 10 days later, Apr 29. So far we have accumulated 11 hours toward the 250-hr threshold. 

Summary of apple disease conditions to date

Scab
Scab spores were first trapped with rains March 22.
The first scab infection period occurred Mar 28-29.
Lesions were observed Apr 12.
Apr 14- apple scab infection period; wet 22 hours with 0.1 inches of rain.

Powdery mildew
Mildew spores were available on infected emerging buds by Mar 30.
There have already been 17 dry weather “mildew days” in April

Fire blight
Infection events occurred at our AREC Apr 8 and Apr 16 (some local areas may also have had wetting of early bloom Apr 6).
Possible infection yet on late bloom with warmer temperatures this weekend.

Rusts
Not much rust activity so far. 
Cedar-apple rust spores ready to be discharged.
Quince rust infection not likely as fruit will lose susceptibility by thinning time.
Cedar-apple rust inoculum may persist late into spring as it did several years ago.

Disease situation

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 most cultivars near full bloom and all with some bloom open, here is a prediction for trees with first bloom open Monday, April 5, 2010. The temperature and rainfall data are current through Friday morning, April 9. Predicted weather conditions are shown for April 9-12. The components of fire blight risk are indicated in the columns labeled B (blossoms open), H (degree hours for epiphytic bacterial populations), W (wetting by rain or dew), and T (average daily temperature 60 F or above). The risk column shows that fire blight infection occurred as predicted with storms with strong winds yesterday afternoon. The wind-driven rains made it likely that bacteria would have spread throughout an orchard from blossoms that were open earlier to ones that had opened just before the rain. For infection to be predicted, wetting has to occur after the EIP (epiphytic infection potential) reaches 100 or higher, and this level had reached 4X that threshold.  Cooler temperatures will reduce the fire blight threat through the weekend. The Maryblyt graphic will be updated April 12 or earlier if it changes dramatically. 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.

Other diseases:
Yesterday's 10-hr wetting from 4 PM to 2 AM this morning was not quite long for apple scab infection, and cedar-apple rust spores were not released. Predicted dry weather through most of next week will again favor powdery mildew infection.

Another mildew year??

In spite of the early scab infection period, it is starting out as a powdery mildew year. Powdery mildew is our only "dry weather" fungal apple disease.  The past 3 days and 8 of the next 10 days are predicted mildew infection days. The image above shows heavy sporulation covering an infected blossom cluster 4/2/10. The image above right shows a healthy bud on the left compared to a mildew-infected bud (right) 3/31/10. Expect secondary infection symptoms to start appearing during bloom to petal fall. Fruit infection occurs about pink stage and causes fruit russet (right).

Update 3/29/10

Apple scab:

Bud stage on different cultivars now ranges from green tip to tight cluster. Wetting from 2 PM March 28-1 PM March 29 resulted in our first scab infection. Assuming that few orchards have had any protectant fungicide (copper, EBDC, etc.) applied before the recent rains, it is advised that a fungicide mixture including some back action on scab be applied as soon as it is suitable for spraying. Where the scab fungus has become resistant to SI fungicides (Rally, Rubigan, etc.), after-infection fungicide options include a strobilurin (Flint, Sovran) or an AP (Vangard, Scala) and these should be combined with an EBDC (mancozeb). For processing varieties, where there is less concern about fruit russet problems, there may be interest in applying copper as a protectant for fire blight and scab. We have tested the tank-mix combination of Vangard + copper and found that it is suitable to add Vangard to copper for post-infection control of scab. Be aware that copper does not give post-infection control of scab, and the Vangard + copper tank-mix will not control rusts or mildew.

Apple powdery mildew:

Predicted warm and dry weather later this week will bring rapid bud advancement and early secondary powdery mildew infection of leaves and blossoms.  The image at the right shows chains of  powdery mildew conidia emerging from an infected Idared flower sepal 3/31/10.

Rusts:
Check the rust gall inoculum in your area; cedar-apple rust and quince rust spores will likely be released with the next warm rain.