Saturday, January 9, 2016

Pecan cultivar yields: The impact of pecan scab

    During an average year, we receive about 40 inches of rain at the Pecan Experiment Field. During 2015, more than 65 inches fell on our pecan groves. With all that rain, we experienced a severe outbreak of pecan scab. During the summer, we made three applications of fungicides to slow the spread of scab but found that some cultivars still became covered by the disease (photo at right).
  In previous posts made during the 2015 growing season, I addressed the impact of scab on nut size and kernel fill and even photographed a couple of cultivars completely destroyed by scab.
    This week we finally finished cleaning and weighing the nuts harvested from one of our pecan cultivar trials. After reviewing the 2015 data (table below), I was struck by the observation that the lowest yielding cultivars in the trial were also the cultivars with the least amount of scab resistance.

    In the table, I've listed yield as yield per acre. In the field there are 16 trees of each cultivar in this trial representing 1/3 of an acre of trees. The scab rating listed is a multi-year average for each cultivar that can be used to judge the relative susceptibility of a cultivar to scab infection. A rating of "1" means total scab resistance while nuts completed covered by scab receive a rating of "5".
    Scab resistant Kanza trees produced almost 3 times as many nuts as scab susceptible Chetopa and Giles trees this past growing season. Our fungicide program reduced but did not eliminate scab infections on Chetopa and Giles in 2015. After looking at the nuts blown out of the cleaner, I can report that scab definitely reduced kernel fill on these scab susceptible cultivars. More nuts in the cleaning pile means less marketable nuts to report as total yield.
    Not all scab resistant cultivars produced outstanding yields. Norton, an old northern pecan cultivar, is scab resistant but is not a prolific nut producer. Stuart has only moderate scab resistance but produced an excellent crop in 2015. All in all, it looks like I'll be grafting more Kanza next spring.