Monday, March 17, 2014

Cold injury on Pawnee: Temperature and fruiting stress

   Pawnee is a large, early-ripening pecan cultivar grown by many northern pecan producers (photo at right). In previous years we have noted cold injury on Pawnee when winter temperatures drop below -10 F. So I thought it would be interesting to see how Pawnee fared following a colder than normal winter in the Mid-West. Recently, I asked some friends to provide me with some Pawnee shoots cut from their orchards growing near Columbia, MO. The lowest temperature they recorded at their location was -12 F. For comparison, I cut some shoots from our trees growing near Chetopa, KS where we recorded a winter low of -3 F.  
    In looking for cold injury, I selected both shoots that produced pecans last fall (i.e. fruiting shoots) and shoots that produced only leaves in 2013 (i.e. vegetative shoots). Using my knife, I peeled back an area of bark on each shoot to check for internal browning in the bark and cambium (indications of cold injury).

   The photo at left shows a vegetative shoot I cut from a Pawnee tree growing at the Pecan Field (KS). Inside the oval is a close-up of the tissues inside this twig. Note that everything inside the bark looks green and healthy. No cold damage here. A temperature of -3 F was not cold enough to cause injury to this Pawnee stem.

    Now let's look at a fruiting shoot cut from the same tree (photo at right). Again, a close-up of internal bark tissues is shown inside the oval. Note the internal browning of both the bark and the cambium. In this case, the stress of producing pecans caused this shoot to suffer cold injury at -3 F.

    In Columbia MO the winter low was -12 F. In the photo at right, I've placed two Pawnee shoots collected from an orchard just outside of Columbia. A vegetative shoot is on the left and a fruiting shoot on the right. A close-up of the internal bark for these two shoots is shown within the oval (in same left to right order).
    At -12 F both Pawnee shoots showed signs of internal browning. The vegetative shoot had only very slight discoloration while the browning observed within the fruiting shoot was intense.
    From this year's observations and observations we've made following previous winters, it also looks like Pawnee usually develops cold injury problems when temperatures drop below -10 F. In addition, it is clear that the stress of nut production can reduce the cold hardiness of Pawnee shoots helping to intensify alternate bearing .