Monday, June 15, 2015

Post-pollination nut drop

       Every spring we scout for pecan nut casebearer damage starting shortly after the end of the pollination season. While I'm up in the canopy searching for casebearer, I have noticed that a certain percentage of  flowers simply drop from the tree. I even found what looks like a perfectly normal pistillate flower dropped off and resting on a leaf (photo at left).

      During the the month of May (pollination season in SE Kansas), we experienced several extended periods of rainy weather. Several growers have been concerned that the wet weather inhibited pollination and nuts are dropping due to a lack of proper pollination. The nut clusters pictured above were from the same tree. The photo on the left has two pistillate flowers that are drying up and are ready to drop off. Only one healthy nut remains. In contrast, the nut cluster on the right has five healthy nuts. If rain was a factor in nut drop this year, all pistillate flowers clusters on a single tree would be showing signs of poor pollination. 

   There are actually two types of pollination season nut abortions. The first occurs early, during pollination. A pecan shoot that has insufficient resources can create a entire cluster of small, ill-formed pistillate flowers. These flowers often fall off right in the middle of pollination season. Later, the dried up peduncle will also fall off (photo above left). The second period of nut loss occurs after pollination has been completed.  In this case, what looks like healthy flowers drop from the tree sometimes leaving the entire peduncle bare (photo above right).

   One of the most common types of nut abortion I have seen is the dropping off of terminal flowers in the cluster (photo at right). As a tree creates a new pistillate flower cluster, nutlets are formed from base of the peduncle to the terminal. As pistillate flowers are formed, a flowering shoot can simply "run out of gas" and create small or ill-formed flowers at the cluster's terminal.  What ends up looking like a lack of pollination, is actually nut abortion caused by a weak female flower. Although weak female flowers usually occur at the terminal of a cluster, poorly formed flowers can occur anywhere in the cluster (photo above right)  

    We've recorded post-pollination nut drop for a period of several years and noticed some definite trends. In the graph at right, I've plotted the number of nuts per cluster over time (the month of June). Notice that during odd-numbered years, cluster counts start off high but dropped off sharply. During even-numbered years, trees produced fewer
nuts/cluster but suffered less nut drop. Switching over to yield data for those same 6 years (table at right), we find that odd-numbered years were high crop load years while even-number year produced lesser crops.
    Taken together, these two data sets tell me that pecan trees may have a way of regulating crop load that has nothing to do with pollination. If a tree produces an excess of pistillate flowers ("on" years) post-pollination nut drop will appear severe. In contrast, trees during "off" years seem to want to hang on to every flower they can. 
    This year, our groves are experiencing an "on" year. That might be why we are noticing more post-pollination nut drop.