Adding Insult to Injury: Scab Warning

In addition to cold injury from our current cold snap, a scab infection period may arrive in the Hudson Valley with the warm rains predicted for Thursday. (No the snow did NOT kill the scab spores, and yes, the spores are certainly mature enough to cause havoc in unsprayed trees at this point if we get a wetting period long enough for infections to occur!) Orchards sprayed last week should still have enough residues to squeak through this potential infection period, but orchards that have not yet been sprayed and those that were sprayed prior to the rains last Monday, March 28th, could be at risk. The situation is further complicated by weather predictions for windy weather through much of the remainder of the week and by concerns that pesticides applied after freezes may exacerbate leaf injury.

Apples in most locations in the Hudson Valley suffered some cold injury last night, with more cold predicted for tonight. It will take some time (probably several weeks) to fully assess the degree of damage. Earlier budding cultivars such as Empire have probably sustained more damage than later-budding cultivars such as Golden Delicious. Additional cold temperatures in the mid-20’s are predicted for Saturday night, although that forecast may still change before Saturday arrives.

If the current forecast for warm rains on Thursday remains unchanged, my recommendation would be to use available spray windows to apply fungicide to the most scab-susceptible cultivars, and perhaps to the entire farm if time allows, either before the Thursday rains or as soon as possible thereafter. However, because cold damage to leaves will increase the potential for phytotoxicity, use the following precautions:

  1. Do not apply anything to leaves that are still frozen. Ideally we would like a day or two for leaves to recover after severe freezes before any pesticides are applied, but that may not be possible in this situation if the scab infection period develops as predicted on Thursday.
  2. If you opt to spray before the Thursday rains, use only mancozeb or Polyram. DO NOT APPLY CAPTAN or SYLLIT or OIL or COPPER to cold injured leaves for at least several days after the last injury event because uptake of these products into the cold injured tissue could make the injury worse. Also, it may be best to apply mancozeb or Polyram without any adjuvants that might enhance penetration.
  3. If you decide to wait until after the rains (which may be the safest bet, given the cold damage and predicted windy conditions), then I suspect that the combination of Inspire Super plus mancozeb will provide the best disease control, especially if it can be applied within 72 hr from the time that rains start on Thursday. It may still provide some benefits if applied within 96 hr because cold weather will prevail again (at least during nights) after the Thursday rain, thereby slowing the fungal infection process. Waiting until after the rain also allows one to see if the predicted rains really do turn into an infection period. If we are lucky, the forecasters will be wrong, the predicted infection period will fizzle, and no fungicide will be required until after leaves have had a chance to heal.

Dr. Warren Stiles was a strong proponent of using a “spring tonic” spray of zinc, boron, and urea to help trees recover from cold injury. His recommendation was to apply 1 lb or 1 qt of boron, 1 qt of zinc chelate, plus 3 lb of urea at tight cluster to pink, with all of those rates being rates per 100 gal of dilute spray. Application of this foliar nutrient mixture was believed to assist with repair of damaged cells and flower parts that might otherwise die or only partially recover from the cold injury.

I really don’t know how beneficial the spring tonic spray might be in the current situation because I’m not sure that anyone has published evidence from controlled trials to validate the benefits. (It definitely will NOT resuscitate flowers that have been totally killed by the cold!) I probably would not apply the spring tonic, and certainly not the urea, until after the potential freeze event predicted for Saturday night because urea could make tissue even more susceptible to freeze injury. I don’t know if boron and zinc might adversely affect leaves just coming out of a severe freeze, so it may not be advisable to add the foliar nutrients to a fungicide spray if that spray will be applied before next week. Nevertheless, this appears to be a season where any surviving buds will need all the help that they can get, and therefore Dr. Stiles spring tonic spray might be worth considered for next week if we still have live buds after all of these cold nights.

A final caution: Very low temperatures are again forecast for early tomorrow morning, but winds may drop after mid-night.  I have heard that some folks are hoping to use wind machines if there is any temperature inversion, and that may make sense if orchard heaters will also be used in the vicinity of the wind machine.  Be aware, however, that running wind machines on very cold nights with low humidity can cause freeze-drying of the foliage. I don’t know the exact conditions that contribute to this phenomenon, but I recall a situation many years ago when I was call to a farm to examine browned leaves that the grower assumed were caused by phytotoxicity from something he had sprayed. We were all scratching our heads until we realized that the damaged was only evident in a ring-pattern around his wind machine!  Having browned foliage is a small price to pay if one can keep the flower buds alive, so wind machines may still be useful tonight. However, deciding how and when to use them may involve choosing the lesser of several evils when degrees of evil cannot be clearly assessed at the time decisions must be made.

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