Wednesday, January 15, 2014

Seed source for northern pecan rootstocks

    Its the time of year for stratifying pecan seeds. Most northern pecan growers understand that cold hardy rootstock trees are produced from seeds of northern pecan cultivars. Graft a northern pecan cultivar on a southern pecan rootstock and you may find that the tree below the graft union will freeze during a test winter.
    Over the years, we have used several seed sources to grow trees among them Giles, Kanza, Posey, and Colby (photo above). For years, Giles has been recommended for northern growers but why?  Is it really better than other northern seed sources?  Is there a better northern seed source?
   Whenever you plant pecan seeds, the trees that germinate can be very variable in terms of  vigor and disease resistance. Planting a known cultivar only guarantees you know the female parent--the male parent just flew in on a spring breeze. So in choosing a northern pecan cultivar to plant, its best to collect seed from a northern orchard where all possible male parents have a high level of cold tolerance.
    Now lets talk cultivars. The photo above shows a cross section of four pecan cultivars we have used as seed for nursery trees. Note how the Giles seed is poorly filled as compared to the others. Poorly filled Giles kernels are all too common because this cultivar is susceptible to both over-production and pecan scab (both problems reduce kernel quality).  Even if a pecan is poorly filled, the seed can still germinate and grow into a tree. However, seedlings from poor quality nuts have low seedling energy and struggle to grow in height and girth especially during the first year. Slow seedling growth rate translates into a longer wait to graft and a longer wait until nut production.
   Giles seeds also have a hidden defect not easily recognized. Pollen shed and pistillate flower receptivity overlap in Giles. This overlap leads to partial self-pollination and the production of nuts that, when germinated, demonstrate in-breeding depression.  In the nursery, self-pollinated nuts show up as "runts" and Giles seems to produce more runts than other northern seed sources.  Slow growth rate and an excessive number of runts has made me abandon Giles as a seed source for my nursery trees.

Kanza seedlings
    What seed source do I recommend? Kanza and Posey seeds generate aggressively growing seedlings under commercial nursery conditions. Colby seeds grow well but seedlings seem to have a higher degree of susceptibility to disease, requiring greater reliance on a season long fungicide program.
   Last summer, I grew out a couple hundred Kanza seedlings (photo at left). For the most part I was pleased with the results but still had to cull out the occasional runt. Heartlessly culling out under-performing seedlings will leave you with uniformly-vigorous rootstock trees that preform well when planted in the field.  A genetically strong rootstock increases scion growth rate and decreases the wait until nut production.