Thursday, February 13, 2014

Pecan scab: Impacts on harvested nuts

Giles 2013
    Last summer, we had a 3 week long wet period in mid-summer that promoted the rapid spread of pecan scab. By the end of the growing season, scab had a drastic impact on nut size. This week we have been cracking nuts samples from a fungicide trial and found wildly different nut sizes all produced by the same tree (photo at right). It almost looks like the nuts come from different cultivars instead being 100% Giles nuts.
    This variation in nut size is entirely due to the variation in scab lesion coverage over the surface of the shuck. Nuts that were entirely covered by scab early during the nut expansion period are much smaller than nuts infected later in the year.
     Scab infection does not spread evenly across all nut clusters in a tree. Nuts on lower and interior limbs typically have more scab than nuts found at the top of the tree's canopy. Remember, scab spreads most readily under conditions of high humidity. Following rainfall or a heavy dew, nut clusters exposed to the sun and wind at the top of a tree dry off much quicker than nuts in the shade and on low, wind-sheltered limbs.

Giles kernels 2013
    The effect of scab is far more serious on kernel quality. The photo at left shows the kernels we extracted from the nuts in the photo above. The kernels are thin and poorly developed and unattractive. There are even a few wafers. A well managed Giles nut should produce over 52% kernel. This sample had only 45.8% kernel.  When we ran our scab-infected Giles crop though our cleaning machine, we blew also one half the nuts out into the refuse pile.

Chetopa 2013
   Giles was not the only cultivar we had in our fungicide test. We also looked at Chetopa.  Scab infection was typically less of a problem on Chetopa but the disease was still prevalent.  In the photo at right, you can see variation in nut size among our Chetopa nuts but really small pecans were not as numerous.
   When we shelled the Chetopa nuts, the kernels were better than Giles kernels but they still did not have perfect nut quality. These nuts produced 52.8% kernel. A disease free Chetopa nut should produce 55% kernel. Although Chetopa had less scab on the shuck than Giles, we still found the disease had a negative impact on nut size and kernel quality.   
   Our experience with scab in 2013 proves one thing: If you are growing scab susceptible cultivars, using the right fungicide at the right time is essential.