Apple Pest Report:  Tuesday, June 17, 2003 
Vol. 11 No. 7

Scab   

    Scab lesions have been hard to find in sprayed blocks so far.  Unsprayed trees are hammered with scab at this point.

     The following article was written by Dr. Dave Rosenberger of Cornell University's Hudson Valley Lab for the June 16 Scaffolds newsletter.

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"What is the best approach for keeping apple scab off of fruit in orchards with a moderate level of scab on terminal leaves?  Unfortunately, there is no simple answer to this question.  Below are four options to consider.

   Option 1: 
     Make at least two applications of captan alone at the maximum label rate per acre.  Applications at this time of year can be 10-14 days apart unless rainfall (>1.5 inches) removes captan residues before 10 days have elapsed.  Captan is very effective for protecting fruit, especially when combined with high temperatures of 80-85F.  However, if cool wet weather persists into July, then continued applications (more than two sprays) using high rates of captan may be essential.  If weather becomes more normal (hotter and drier), then the risk of fruit infection will subside until September when scab might become active again.

Option 2: 
     Apply captan at maximum label rates as noted above, but tank-mix the high rate of captan with an SI fungicide (Nova, Rubigan, or Procure).  The full rate of captan is needed because the SI's will shut down scab on leaves but will do little to protect the fruit.  This option is considerably more expensive than Option 1.  It will prove more effective than Option 1 IF orchards do not contain SI-resistant populations of scab and IF weather stays cool and wet for another month.  Option 2 also provides extended control of mildew.  However, if weather become hotter (days > 80-85F) and drier, then Option 1 will probably work just as well as the more expensive Option 2 for controlling scab.
 
   Option 3:  Apply captan at 50% of maximum label rates in a tank-mix with Flint or Sovran.  Flint and Sovran will provide better protection of fruit than the SI fungicides so a half-rate of captan should be adequate.  The fact that Sovran and Flint bind to cuticular waxes should make them more resistant to wash-off than is captan.  However, Sovran and Flint will be less effective than the SI fungicides (in the absence of resistance) for arresting incubating but still invisible infections in leaves.  Thus, the trade-off here is potentially better control of fruit scab with Sovran or Flint compared with greater reduction of total inoculum when SI's are applied to leaves with incubating lesions.

      It is very difficult to predict which option (2 or 3) will result in the least fruit scab.  In two years of trials where I tested these products on trees with visible scab, I got better control with the SI's in one year and with the strobilurins (Sovran, Flint) in the other.  The variation is largely due to the details of exactly when the products are applied within the scab incubation period.  Be aware, however, that where Sovran and Flint have been used alone (without any contact fungicide) to stop previous scab epidemics in Michigan and Western NY, the results have generally been less than satisfactory.  Thus, I personally would gamble on options 1 or 2 rather than option 3.

 


  

 

 

  

Option 4: 
     Apply captan at 50% of maximum label rate in combination with a full rate of dodine (Syllit).  This is an extremely risky approach because we cannot accurately predict which orchards have dodine resistance.  However, if you have not used ANY dodine, not even in the first spray of the season, for at least 10 years, then this approach might be very effective.  Only one spray of dodine should be applied and the follow-up spray should be the full rate of captan because a single spray of dodine may be enough to re-select for dodine-resistant strains that survive at low levels in most orchards.

     The specifics of the orchards involved should be considered when choosing among the options noted above.  There is significantly more risk of getting scab on fruit of susceptible cultivars such as McIntosh and Ginger Gold and than on more scab-resistant cultivars such as Empire and Delicious.  Therefore, captan alone might suffice for the more resistant cultivars, whereas an SI-plus-captan might be warranted for McIntosh blocks.  Similarly, vigorous young trees that are still actively growing have the potential for several more cycles of leaf scab, whereas older orchards with a heavy crop will soon set terminal buds.  The SI-captan combination is more likely to pay for itself in orchards with vigorous terminal growth because the SI's can quickly reduce the amount of inoculum available for infecting new leaves (unless, of course, the scab is SI-resistant).

     Unfortunately, some orchards may already have so much scab that spending more money for fungicides will only increase costs for a crop that is already lost.  In cases where a significant number of fruit already have visible scab, one must assume that additional fruit may soon develop symptoms from infections that occurred last week unless heroic measures have already been employed to save the crop.  There is no point in spending big dollars for SI or strobilurin fungicides for a crop that will not pay for itself."
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 Fire blight (the lack thereof)
 
     Leaves damage from foliar nutrient salts or captan could be confused with fire blight.  But actual fire blight at this time would be limited to blossom blight infections. Shoot blight would not start showing until around June 30.  Fire blight kills leaves by blocking the vascular system that feeds them, so as infection proceeds you will see whole leaves and whole clusters affected, not spots on leaves.  When shoot blight has had time to show up, then whole shoots wilt and die.  Fire blight damage caught early may have gummy exudate.

     Spray burn usually appears as discrete spots on leaves and margins of leaves where the spray was most concentrated. 
The damaged tissue is dry with no oozing.

     I don't expect to hear reports of fire blight in Maine this year given the cool weather during bloom  If you ever do find fire blight infections, remember that by the time you see damage it is too late to apply streptomycin.  Streptomycin is an antibiotic to prevent blossom infections.  If used against a running fire blight outbreak it is not likely to provide any benefit and will increase the chance of resistance. 

     Rapid and comprehensive sanitation is the way to address a fire blight outbreak.  Of course, that is expensive and time consuming, and the treatment itself hurts the tree.  That is why we focus on preventing fire blight infections during bloom.
       

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 INSECT and MITE PESTS

 Plum Curculio  

     Plum curculio weren't too busy laying eggs between Friday and Monday, June 14-16 in the Monmouth area, probably due to the cool temperatures.  Activity is expected to increase now that the initial 100 degree day lag period is over and with rising temperatures.

    Here is the current plan to address the spray situation at Highmoor Farm.  Avaunt and Captec were applied on Friday June 13.  This was the "First Cover" spray.  In many years, a Petal fall and First Cover spray are all that is needed to control plum curculio and codling moth.  Then a break is taken until protection against apple maggot flies is needed, which usually reach threshold on traps at Highmoor Farm around July 20.

      Based on rain and number of days since application, insecticide protection from the June 13 application is expected to run out around Monday June 23. The plum curculio protection window does not end until June 30.  Thus, a June 23 depletion date leaves a third of the plum curculio protection window left unprotected.  Therefore, insecticide coverage will be renewed with a Second Cover spray on Monday June 23.  In cool wet years like this one a Second Cover spray is needed to get through plum curculio season without damage. 

     While June 23 is a bit early relative to the ideal timing against codling moth, experience has shown that even a First Cover application is adequate to prevent noticeable damage by leafrollers and first generation codling moth.

      Renewing fungicide coverage on June 23 will provide solid protection through the end of June when second generation lesions from the season's final scab ascospore infection period will have had time to appear.  That way, if we do find any scab outbreaks, the fruit and new foliage will have been protected from spreading infections prior to discovery of the problem.

     If scouting over the next two weeks finds few scabby leaves, then the interval until the next spray will be extended into mid-late July when trap catches indicate need for protection against apple maggot flies.  That will also be time to renew fungicide coverage to protect against fly speck and sooty blotch disease, and to suppress what level of scab there is (and there is probably a little even when scouting doesn't find it).    

   

 

 European apple sawfly  on unsprayed trees checked Monday, June 16 appear to have begun moving from their first fruit to drill into a second fruit in the cluster.  EAS is not a problem where a conventional insecticide is used at Petal Fall.  But in years with a prolonged petal fall, EAS damage can occur in mixed plantings on early cultivars left unsprayed for several days after petal fall because later cultivars were still in bloom. 

     For organic growers and others dealing with an existing EAS problem, protection in the coming week should help to keep EAS from spreading their destruction to additional fruit in the cluster.  Organic options were discussed in the previous issue.

 


  European red mites  are at their seasonal low point when first generation adults are dying off and second generation eggs have not hatched.  Second generation nymphs are expected to start appearing around June 21-24 in the Sanford and Monmouth areas respectively.  The second generation will start laying eggs for a third generation starting around June 29 in Sanford, and July 2 in Monmouth.

 

 Codling moth  control in conventional orchards gets taken care of by plum curculio protection as mentioned in the PC section.  But for organic growers codling moth requires specific attention.  Surround coverage is supposed to reduce first generation codling moth damage.  But that would require maintaining coverage until first generation codling moth egg hatch is complete.  The expected date for 95% egg hatch at Highmoor this year isn't until July 20.  Thus, a Surround program that cuts off earlier than that is leaving opportunity for first generation codling moth damage.

     The estimated best date to begin a series of 3 Bt applications to control first generation codling moth is when eggs start hatching.  That date is upon as, with egg hatch expected to start June 17.
       

Sincerely,                                            
Glen

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