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Apple Pest Report:  Friday, April 28, 2006
Vol. 14 No. 2

 Scab
   
    Scab ascospore maturity seems to be running behind apple bud development this year.  I did a scab squash on Friday, April 28 with leaves collected in Manchester, a few miles north of Highmoor Farm.  McIntosh trees at Highmoor Farm reached Green Tip on April 12, were at Half Inch Green on Wednesday, April 26.  Normally,  at least 10% of the ascospores would appear mature by the time apple buds reach Half Inch Green.  But the scab capsules I saw today were all very immature.  Only one capsule has the beginnings of a few defined spores, and those did not appear ready to release. I also did a spore shooting test and found no released ascospores.  This too is unusual when trees are at Half Inch Green.

    In most years, scab is in synchrony with apple tree bud development, with the very first spores beginning to mature and be ready to release around McIntosh Green Tip date.  From that point on, the scab degree day model is a more accurate measure of maturation than microscopic observations of the size and color of scab ascospores.  However, the "scab squash" observation still has some value in gauging the beginning of significant scab spore releases.  Over many years, Dr. David Rosenberger has formulated an informal threshold that significant scab spore releases begin when about 12-15% of the spores look mature.  (Scab ascospores look mature about two weeks before they are actually ready to release).

    The preseason development of scab prior the beginning of spore maturation is not as well understood and spore maturation, release, and the infection process.  Scab ascospore maturity could be delayed this year because of the lack of late winter snow and subsequent lack of rain.  In a typical year, overwintered scabby leaves are kept moist by melting snow and rains prior to bud break in late April.  The dry ground conditions this year could also be delaying normal breakdown of overwintered leaves.  Leaf breakdown is an important factor in curtailing scab infection pressure.  If true, both factors would mean for primary scab infection pressure that starts and ends later than normal relative to apple bud stage development. 

    Heat units come in much faster towards the end of primary scab season (i.e. ascospore releases that normally end shortly after McIntosh Petal Fall) than in the early stage.  Because of this a week delay in the beginning could only represent a two-day extension on the end. 

     Every orchard is unique.  If your orchard had unacceptably high scab last season, you don't need to read these speculations.  In that case, protection should begin at Green Tip and be maintained until two to three weeks after the final ascospore infection period. 

    Conversely, orchards that were under the 5 infected leaves per 100 shoots threshold for a scab index last September can safely delay protection until Tight Cluster. 

    The scab squash method is subject to great variability between samples.  So it is not possible to say with confidence that scab maturation really is running behind this year.  It could have just been an unusual sample of leaves.  It would be unwise to underestimate the infection potential of upcoming rains based on unreliable evidence.  But it does serve as a caution to maintain protection until there is good reason to assume that scab ascospore releases have completed this year.

    I am avoiding the temptation to "adjust" the scab model estimates online at http://pronewengland.org/AllModels/MEmodel/RADARME-Monmouth.htm  Even if the speculation about delayed ascospore maturity is correct, the error range on the model accounts for a wide range of uncertainty.  But it would be prudent to look the high end of the risk ratings for infection periods at the tail end of ascospore releases this year.

    As of Friday night April 28, the next forecast rain for Highmoor Farm is for Monday afternoon, May 1.  If the temperature is below 50F during that rain as predicted, that will inhibit full release of ascospores that are mature by then.  So at Highmoor, the infection period forecast for Monday May 1 looks to be one worth having protection against, but with only about a tenth of the infection pressure of the year's major infection periods that will follow in the coming weeks.  Of course, for growers in southern Maine that are phenologically ahead of Highmoor, a May 1 infection period could deliver a higher portion of the year's primary scab infection pressure.    

 

Insects and Mites
tarnished plant bugTARNISHED PLANT BUG

      If you have traps but haven’t gotten them out yet, they can provide useful information about the need for and timing of tarnished plant bug control if they are put up before tight cluster. If traps are put up late, use a lower threshold than the Silvertip-Pink thresholds as shown below, to account for trap captures missed by late deployment.  You can also inspect flower buds for small beads of oozing sap on warm days.  This is an indication of TPB feeding.

                                   Cumulative TPB per trap thresholds

Type of Market              Silvertip to Tight Cluster                Silvertip to Pink

Wholesale                                     3                                        5
Retail                                           5                                        8

        If control is needed, Imidan is only rated as poor against TPB in the 2006 Cornell Pest Management Guidelines for Commercial Tree Fruit.  But many people apparently find that it provides adequate control.  Synthetic pyrethroids (Ambush, Pounce, Asana, Danitol, Proaxis, Warrior) are more effective against TPB and will also help control first generation leafminer adults, but they come with a potential high cost in detriment to beneficial predators which could lead to reduced natural control of mites, leafminer larvae, and other pests. If you need a maximum TPB control, the newer insecticides Actara, Assail, Calypso, and Avaunt all have higher ratings against TPB than Imidan.  But before spraying anything, consider that TPB damage prior to Tight cluster is probably insignificant as it causes buds to abort.

        A high degree of tarnished plant bug control is hard to achieve. Researchers in New York concluded that spraying for TPB was of debatable value because the differences in damage between blocks that had received insecticide treatment and those that hadn't was not that great, and in their view may not have justified the expense.

        On Wednesday April 26, in three monitored blocks at Highmoor, there were only 4 TPB on 10 traps set at Silver Tip.  There were Maine reports of TPB observations during the warm days earlier in April. 

        TPB aren't likely to be a problem until daytime high temperatures reach the mid 60s, and even then only where they are numerous.  In the past, seasons that started earlier than normal and then had dry sunny weather, resulted in heavier than normal tarnished plant bug feeding damage to apple buds, at least in a few locations. So it is a pest to keep an eye on this year, especially if the warm dry weather prevails in the next two weeks.  If a prebloom insecticide application is needed, an optimum time is at Pink.  That timing should be adequate for TPB, and would provide some early protection against European apple sawfly and Green Fruitworms and other bud-feeding caterpillars.

 

Leafminer moths

    At Highmoor Farm on Wednesday April 26, there were no leafminer moths caught on 10 red trunk traps set at Silver Tip.  These traps can be useful to give early notice of a Leafminer outbreak, but in general the best time to assess the leafminer threat is to look for sap-feeding mines on the undersides of fruit cluster leaves about a week after petal fall.

European Red Mite (ERM)

    ERM egg hatch usually begins shortly after Tight Cluster.  The forecast ERM hatch date for this year is around May 11.  Oil application to smother overwintered mite eggs is much more effective if applied before hatch begins.

 

 McIntosh bud stage dates

The following observed and forecast McIntosh bud stage dates are for Highmoor Farm:

Green tip - April 12
Half Inch Green - April 26
Tight Cluster - May 5
Pink - May 13
King Bloom - May 18
Full Bloom - May 21
95% Petal fall - May 26
Fruit Set - June 2

 

 

    

 Scouting Co-op
 

    The route is about full, so this is the last call for growers with 1 acre or more of apples to sign up for free pest scouting visits.  We will include as many locations as possible, but may not be able to include an orchard if the driving time is more than an hour from the nearest participating orchard. 

 

    TJ Baldwin, a Sustainable Agriculture student at University of Maine has been hired and will begin training on May 30.  Orchard visits will begin June 5.   To sign up, send an email to gkoehler@umext.maine.edu

 

  
 

Sincerely,                                            
Glen

 

Glen Koehler
Pest Management Office
491 College Avenue
Orono, ME  04473
Voice:  207-581-3882
Email:  gkoehler@umext.maine.edu
Web:  PRONewEngland.org
Fax:  207-581-3881

 

What we call the secret of happiness is no more a secret than our willingness to choose life. - Leo Buscaglia