Thursday, August 11, 2016

Pecan scab causes nut drop

   Over the past few years, we have set out nut drop cages to monitor the causes of nut abortion over the summer months.  The trees in this trial are not treated with insecticides or fungicides and it's been amazing to discover how many pests can cause nut loss. The tree that has suffered the greatest nut loss this year is a native pecan trees susceptible to pecan scab. The photo at right are just a few of the pecans that fell of the tree in early August. Every nut is covered with scab lesions.

    After finding numerous scab infected nuts in the drop cages, I decided use our hydraulic lift to find out what was going on up in the tree. The photos above show two nut clusters produced by the same native pecan tree. The photos were taken with exactly the same camera settings to allow for direct nut size comparisons. The heavily scab covered nuts (left photo) are much smaller than the still green nuts spotted with scab lesions (right photo). In a previous post, I explained how scab impacts nut grown and kernel fill. But in the photo above I was able to document how scab can cause nut abortion. The orange arrow points to a small scab-infected nut that is just about to fall from the tree. In fact, after taking the photo, the nut fell in a gust of wind.
   By cutting open the nuts that had fallen from the tree, I came to understand that scab causes nuts to abort when the infection starts early during the fruit expansion period. All the nuts I've found on the ground ceased growing before kernels reached the 1/2 water stage. This tells me that the scab infection on these nuts started right after nut fertilization and advanced to full shuck coverage by early July. Nuts that become covered with scab lesions later in the season usually remain attached to the tree and fall during harvest as blackened stick-tight.