Flyspeck fungicide respray estimates - Background

       Apple scab takes precedence in fungicide respray decisions until primary apple scab ascospore releases are complete and orchard scouting verifies that apple scab infections were adequately controlled.  (Fewer than 5 infected leaves per 100 fruit clusters and vegetative shoots is the recommended threshold for this determination).  Fungicide protection against apple scab will be more than adequate to prevent flyspeck and sooty blotch. 

       Reliable confirmation of successful apple scab control is not possible until about four weeks after petal fall.  This is because the final primary scab infection period is usually not until the first rain after petal fall, after which it takes about 10 days for any infections that began to develop into lesions.  1st generation lesions can be difficult to find in upper tree canopies.  It takes a rain after all 1st generation lesions have appeared plus another 10 days to allow all 2nd generation apple scab lesions to appear.  Checking the orchard after that date is the best evidence that apple scab control was successful enough that subsequent fungicide applications can be based on the need to prevent flyspeck. 

          After scab control is confirmed, less frequent fungicide applications designed to maintain protection against flyspeck are adequate in most Maine orchards.  Sufficient fungicide protection to prevent flyspeck should also prevent problems with sooty blotch, summer rots, or with a very low number of active apple scab infections.

       The date of the final fungicide application is an important factor in preventing flyspeck from appearing before harvest.  The intent of the flyspeck fungicide interval tables is to help growers identify the earliest final spray date that is likely to prevent flyspeck infections from appearing on fruit before harvest or in storage.  Comparing weather-based rules to observed rain and LW values in table form provides an objective overview.  Other factors such as block history and marketing concerns should be considered in choosing fungicide application dates to prevent flyspeck. 

       Overwintered flyspeck colonies on brambles, oaks, maple, and other orchard border host plants begin to release ascospores as early as the Pink stage of apple.  But flyspeck does not overwinter to a significant degree inside the orchard.  Therefore, plant pathologists currently feel that protection against flyspeck is not needed until border row host plants begin producing 2nd generation spores (conidia).  This begins roughly at 270 cumulative leaf wet (LW) hours after apple petal fall.  It usually takes 4 - 6 weeks for 270 LW hours to accumulate after petal fall.  With dry weather, there can be a gap of days to weeks between the end of residual protection from the final scab spray, and the beginning of need for fungicide protection against flyspeck.  With frequent rain in the weeks after petal fall there can be no gap at all.

       The risk of flyspeck infection might increase to a higher level when another "generation" of flyspeck is completed at around 540 cumulative LW hours after petal fall.  But a flyspeck spore collection study did not show a trend for higher spore counts as the summer progressed.

     Flyspeck colony growth rate varies with temperature.  The temperature effect on growth is used in the Orchard Radar flyspeck model to adjust leaf wetness hours for relative flyspeck growth during those wet hours.

       The flyspeck fungicide interval tables estimate
1) the date when protection wears off,
2) the latest safe date for a postinfection spray after that date, and
3) the latest safe harvest date in order to minimize the risk of flyspeck symptoms appearing before harvest. 

     These estimates account for type of fungicide, amount of rain, cumulative leaf wetness (LW) hours, temperature, and number of days since the previous fungicide application.  For dates beyond the range of the current weather forecast, estimates are based on climatic average daily rain and LW values.

       The 'Protection End Date' for each spray date is the estimated date when the fungicide is no longer protecting fruit against new flyspeck infections.  If fungicide protection is renewed on or before the end of protection, then there should be no days on which new flyspeck infections can begin and therefore no problem with flyspeck becoming visible later if the final spray is applied close enough to harvest.

       The 'Deadline date' for postinfection control is defined as the date when 38 temperature adjusted LW hours have accumulated since the 'Protection End Date'.  Application of Topsin M, or a strobilurin fungicide (Flint, Sovran, Pristine) within the 38 LW hour postinfection window is expected to stop further development of flyspeck infections that may have begun on days when protection had lapsed.  However, postinfection control of flyspeck is poorly understood.  Captan applied alone is not known to provide postinfection control.

        For each fungicide application date, three dates are given for when flyspeck infections that began after protection from that application is expected to have been depleted by time and sunlight or cumulative rainfall. 

       The early date on the left in parentheses and dark blue font is a worst case scenario early date for possible flyspeck appearance if the weather beyond forecast range is in the highest 20% of the climatic records for both elevated rainfall and temperature.

       The main date in the center column in bold black font, is when 212 temperature adjusted LW hours have accumulated since fungicide protection is expected to have been depleted.  The 212 hour figure is based on the 270 hour threshold for non-adjusted LW hours and analysis of the typical relation between temperature-adjusted and non-adjusted LW hour accumulation at a number of New England orchard locations over different years.

      The low risk date on the right side in parentheses and brown font is an estimate adjusted for low-risk trees that require 50% more cumulative LW hours before flyspeck colonies appear.  Low risk trees are those on a site with
      -  no history of problems with flyspeck, sooty blotch, or summer rot fungi;
     - good air drainage (i.e. exposed to wind or on a slope, not in an orchard low spot where morning fog persists);
     - at least 25 meters from dense growth of brambles and other alternate host vegetation; (ideally, inspection of border vegetation should find visible flyspeck colonies on less than 10% of bramble and other alternate host plant stems);
     - trees are well pruned with canopies that are open to air and sunlight

        The estimate flyspeck appearance dates are for apples sold as Pick Your Own or picked just prior to retail sale without extended storage.  For apples going into storage, remember that apples may sit moist in bins before being placed into storage.  Once in storage, it can take several days for apples to reach low enough temperature to halt further flyspeck development, and during this time condensation may keep apple cuticle (where flyspeck grows) moist enough to support flyspeck growth even if the apples were dry when picked.  Thus, if flyspeck was very close to becoming visible at harvest and then the apples are moist and warm enough in storage, flyspeck could become visible after harvest.

        Three fungicide groups are represented in the flyspeck respray date tables.  The fungicide groupings, estimated depletion dates, and estimated potential flyspeck appearance dates employ rules of thumb based on fungicide field trials by Dr. David A. Rosenberger at the Cornell University Hudson Valley Lab.  The fungicide groupings and their associated protection depletion rules are listed at the bottom of this page.  A summary of Dr. Rosenberger's flyspeck observations is discussed in the article "Timing Sprays for Flyspeck and Sooty blotch", by D. A. Rosenberger; Scaffolds Fruit Journal 14:14; June 25, 2005; http://www.nysaes.cornell.edu/ent/scaffolds/2005/050620.html#disease
with an update at http://www.nysaes.cornell.edu/ent/scaffolds/2006/060626.html#diseases

      Those guidelines were updated based on field observations of fungicide depletion after exposure to high amount of rain in 2008.  The depletion criteria for different types of fungicide are shown below.

       Respray guidelines and latest safe harvest dates are also based on research led by Dr. Turner B. Sutton, North Carolina State University.  This research is summarized in "Sooty Blotch and Flyspeck of Apple:  Etiology, Biology, and Control"; by S.M. Williamson and T.B. Sutton; Plant Disease 84:714-724, 2000.

       While the rules used for the fungicide interval tables are based on researcher observations over many years, those rules have not been specifically tested in research trials.  The association of flyspeck development with cumulative LW hours has been validated in New England, but has only been studied in the spring.  However, field observations in New England and New York indicate that the 270 hour guideline for non-temperature-adjusted LW hours also applies to flyspeck development in September. 

      The temperature adjustment was defined in laboratory studies, and appears to conform to field observations and to improve the ability of the model to adjust to varying temperature conditions, but no formal validation study has been conducted.  The temperature adjustment doesn't make much difference in estimates during the warm summer months, but it does represent slowing growth rate of flyspeck as temperatures cool in late September and October.  This is also the period when flyspeck is most likely to become a problem.

       A weakness in the flyspeck fungicide interval estimates is that there is no accounting for inoculum level and site characteristics.  The "Typical flyspeck risk" estimates are appropriate for orchards with moderate risk of flyspeck infection.  Flyspeck risk varies between orchards, and even between different locations and cultivars within the same orchard.  Two key factors are canopy density and proximity to a wooded border that blocks air drainage and that contains brambles and other host plants.  Well-pruned trees with open canopies, and blocks with good air drainage, wind exposure, and regular mowing are much less susceptible to flyspeck infection than orchards with the opposite characteristics.  Yellow-skinned apples such as Golden Delicious are much more likely to be downgraded because of flyspeck than red cultivars for which background color makes flyspeck colonies less visible. 

       Shorter respray intervals and more frequent fungicide applications than estimated in the tables may be needed for sites with high risk or past history of flyspeck infection.  Conversely, longer intervals between fungicide applications may be adequate for sites with lower risk.

* Pristine 5 ozs. / 100 gals. dilute, with minimum 14.5 ozs./Acre regardless of tree size as per label.

      The depletion rule for Pristine is 21 days or 2.5 inches rain since application, whichever comes first.

Group A fungicides are any of following dosages:

* Flint 50% WDG 0.67 - 0.8 ozs. / 100 gals. dilute

* Sovran 50% WG 1.0 - 1.6 ozs. / 100 gals. dilute

* Topsin M 70WP 3 - 5 ozs. / 100 gals. dilute+ reduced dose of Captan 50WP or another protectant fungicide.  (An equivalent rate of alternative thiophanate methyl formulation can be substituted.)

* A phosphite or phoshorous acid fungicide (e.g. Fosphite, Topaz, Agri-Fos, Fungi-Phite, Phostrol, ProPhyt) at full dose + captan at no less than  half of the full dose.

     The depletion rule for Group A is 21 days or 2.0 inches rain since application, whichever comes first.

     Application of an EBDC fungicide in June would also count as a Group A material, but for most Northeastern orchards in most years, there is little need for flyspeck prevention before July.  By the time need for flyspeck prevention become most significant in July, the 77 day preharvest interval for EBDC fungicides prevents their use.  The EBDC fungicides are: 

* mancozeb (Dithane, Manzate, Penncozeb 75DF, 80WP, 80DF) at 1 lb. / 100 gals. dilute

* mancozeb (Manex 4F) at 0.8 quart / 100 gals. dilute

* metiram (Polyram 80DF) at 1 lb. / 100 gals. dilute

 

Group B fungicides are any of following dosages:

* Captan 80WDG 1.25 lbs. / 100 gals. dilute
 (or equivalent full-dose of alternative captan formulation)

* Ziram 76WP 1.5 lbs. / 100 gals. dilute

     The protection depletion rule for Group B is 14 days or 1.5 inches rain since application, whichever comes first.

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     There is no flyspeck table for sulfur fungicide.  Sulfur is relatively weak at protecting against flyspeck so more frequent and later applications than indicated for Group B are recommended.  Also, with a sulfur fungicide program the need for continued protection against secondary apple scab often supersedes flyspeck risk in determining respray intervals.