Flyspeck fungicide respray estimates - Background

      Until scab control is confirmed however, fungicide respray decisions should be made according to the estimated risk of secondary scab infection.  Apple scab takes precedence 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). 

      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, and it takes about 10 days for infections from that develop into lesions.  1st generation lesions can be difficult to find in upper tree canopies, so checking the orchard to confirm successful scab control before 2nd generation lesions have had time to develop can be misleading.  

       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.  If fungicide protection is adequate to prevent flyspeck, then there should be not significant problem with sooty blotch or with a very low number of apple scab infections.

       Overwintered flyspeck colonies on brambles, oaks, maple and other orchard border host plants begin to release ascospores as early as the Pink and Bloom stage of apple.  But flyspeck does not overwinter to a significant degree inside the orchard.  Therefore, there is not significant risk of fruit infections until border row host plants begin producing 2nd generation spores (conidia).  The estimated date for this is 270 LW hours after petal fall. 

       Unless there is exceptionally severe flyspeck risk, plant pathologists currently feel that protection against flyspeck is not needed until there have been 270 cumulative LW hours after petal fall.  It usually takes 4 - 6 weeks for 270 LW hours to accumulate after petal fall.  There can be a gap of days to weeks between the end of need for fungicide protection against secondary apple scab and the beginning of need for fungicide protection against flyspeck.

       Presumably, the risk of flyspeck infection increases to a higher level when another "generation" of flyspeck is completed at around 540 cumulative LW hours after petal fall.

       The flyspeck fungicide interval tables estimate the date when protection wears off, allowable postinfection respray date, and latest safe harvest date with low risk of flyspeck symptoms.  These estimates account for type of fungicide, amount of rain, accumulation of leaf wetness (LW) hours, and number of days since the previous application.  For dates beyond the range of the current weather forecast, estimates are based on climatic average daily rain and leaf wetness values.

       Three fungicide groups are represented in the flyspeck respray date tables.  The groupings, estimated depletion dates, and estimated 'latest safe harvest dates' employ rules of thumb based on fungicide field trials by Dr. David A. Rosenberger at the Cornell University Hudson Valley Lab.  The groupings and their associated protection depletion rules are shown at the bottom of this page.  A summary of Dr. Rosenberger's flyspeck observations is discussed in Timing Sprays for Flyspeck and Sooty blotch, 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

       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; S.M. Williamson and T.B. Sutton; Plant Disease 84:714-724, 2000.

       There no research validated fungicide depletion rules for flyspeck protection.  The guidelines used for the fungicide interval tables are based on researcher observations over many years.  The tables provide objective interpretation of the influence of weather on the need for fungicide reapplication to maintain flyspeck suppression. 

       The association of flyspeck development with cumulative LW hours has only been studied in the spring.  However, field observations in recent years indicate that the 270 LW hour guideline also applies to flyspeck development in September.

       The main weakness in the flyspeck fungicide interval table estimates is that there is no accounting for inoculum level and site characteristics.  The "Typical flyspeck risk" estimates are appropriate for orchards with moderately high risk of flyspeck infection.  Shorter respray intervals and more frequent fungicide applications may be needed for sites with extremely high risk or past history of flyspeck infection.  Conversely, longer intervals between fungicide applications may be adequate for sites with lower risk.

       The degree of flyspeck infection risk varies greatly between locations and tree characteristics.  Proximity to orchard borders with alternate host plants, especially brambles, is a key factor.  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 by flyspeck than red cultivars for which background color makes flyspeck colonies less visible.  These estimates are best used in combination with site and market specific factors in making fungicide application decisions. 

       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 days on which new flyspeck infections can begin and therefore no problem with flyspeck becoming visible later (assuming that the final spray is applied close enough to harvest).

       The 'Deadline date' for postinfection control is defined as the date when 50 LW hours have accumulated since the 'Protection End Date'.  Application of Topsin M (thiophanate methyl), which is the preferred option for postinfection flyspeck control, or a strobilurin fungicide (Flint, Sovran, Pristine) within the 50 LW 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.

        The 'Latest Safe Harvest Date with Low Flyspeck Risk' for fruit going into storage estimates allow 220 LW hours from the end of fungicide protection until harvest.  Only 220 LW hours are allowed (50 less than the 270 required for symptoms to develop) as a safety buffer to account for measurement error in accumulating leaf wetness hours, and because 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.

       For Pick Your Own and apples picked just prior to retail sale without extended storage, the full 270 hours of wetness are allowed in the estimated latest safe harvest date. 

       The date of the final fungicide application seems to be the single most important factor distinguishing orchards that have flyspeck appear before harvest from those that do not.  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.      

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

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

* 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.)

     The estimated 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 50WP 2 lbs. / 100 gals. dilute
 (equivalent rate of alternative captan formulation can be substituted for 2 lb. 50WP)

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

     The estimated depletion rule for this Group B is 14 days or 2.0 inches rain since application, whichever comes first.

Group C fungicides
 

* Captan 50WP 1 lb. / 100 gals. dilute
(equivalent rate of alternative captan formulation can be substituted for 1 lb. 50WP)

* Ziram 76WP 1 lb. / 100 gals. dilute

    The estimated depletion rule for this Group C is 10 days or 1.5 inches rain since application, whichever comes first.

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     There is no flyspeck table for sulfur fungicide.  The decision rules for Group C are a best guess for sulfur fungicides applied at full label dose.  Application of these rules for use of sulfur fungicide against flyspeck is purely speculative and not based on the same research observations used to define respray rules for the other fungicides listed.  Also, with a sulfur fungicide program the need for continued protection against secondary apple scab is likely to supersede flyspeck in determining respray intervals.

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       To provide more realistic fungicide interval guidelines for low-risk sites, a second set of tables is provided.  It is important to note that these tables are only applicable for trees that meet the following conditions:
     - no history of problems with flyspeck, sooty blotch, or summer rot fungi;
     - trees are located where there is good air drainage (i.e. exposed to wind or on a slope, not in an orchard low spot where morning fog persists);
     - trees are 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 extended fungicide interval guidelines for low risk trees have been reviewed by a plant pathologist who has done extensive research on flyspeck control, however users must note that these relaxed guidelines have not been validated by field research, and are simply a best guess at spray intervals more applicable for low risk trees.
    
       Suggested spray intervals for low risk trees are shown below.  The dosages for each group are same as listed above.

Group A fungicides - 25 days or 3.0 inches rain
Group B fungicides - 21 days or 2.5 inches rain
Group C fungicides - Not enough information to estimate extended interval for low risk trees, use same interval as shown for normal risk trees.