Thursday, September 19, 2013

How much scab is too much scab?

   Growers are seeing more pecan scab this year than they have in several years. Many were caught by surprise by blackened shucks that appeared after four weeks of unusually rainy weather in late-July through early-August. However, once the shuck becomes covered in scab lesions, there is literally nothing that can be done to "cure" this disease.
   The other day I was looking over some nut samples collected from 'Chetopa' pecan trees. The amount of scab I found on the shucks varied considerably (photo above). This made me wonder, "How much scab is too much scab?"
    In a previous post, I mentioned than scab infection can lead to a dramatic decrease in nut size and you can see that same effect in the photo above. But scab also impacts kernel fill and shuck opening.
    The three small nuts on the right side of the photo are completely covered by scab. These nuts may have some kernel inside but the shucks will never open at harvest. These 'stick-tights' will end up in the cleaner's trash  pile.
    The three nuts in the middle have more than 50% of the shuck's surface covered by scab but you can still see a little green. For these nuts, scab has reduced nut size somewhat but not as much as nuts that were completely covered by the disease. At harvest, the shucks of these heavily infected nuts will open but separation of shuck from nut might be incomplete. This can lead to a partial stick-tight  where part of the shuck remains firmly attached to the shell. Like full stick-tights, partial stick-tights are unmarketable. If the nut releases fully from the shuck, the kernel inside will be edible but percent kernel will be reduced.
    The three nuts on the left of the photo have scattered scab lesions on the shucks. This level of scab infection did not effect nut size and will have no effect on shuck opening or kernel fill. It seems pecans can tolerate a little bit of scab but once lesions start to cover more than 50% of the shuck's surface, yield losses from scab will be significant.