The simplest approach to measuring productivity is to assess yield on a per-tree basis; it is straightforward and easy to understand. The problem comes when comparing trees of different sizes. For instance, comparing the productivity of apple trees on M.9 NAKBT337 and those on G.30 on a per-tree basis would be meaningless. The size of trees on M.9 NAKBT337 would allow much greater planting densities than would that of trees on G.30. Therefore, trees on M.9 NAKBT337 could have a much greater number of trees per acre producing a smaller quantity of fruit per tree and at the same time have similar or greater per-acre yield than trees on G.30. So, the most accurate assessment might be to plant several trees on a particular rootstock cultivar at an appropriate density and compare block yield to that of trees on different rootstock planted at appropriate densities. Two problems arise from this approach: it would take a large land area to make simple comparisons, and we do not know the appropriate density for new plant material.
W. Autio, D. Greene, and W. Lord presented a comparison of several apple rootstocks with 'McIntosh' as the scion cultivar. They also evaluated and compared different approaches to measuring apple tree productivity. See this article titled "Performance of `McIntosh' Apple Trees on Seven Rootstocks and a Comparison of Methods of Productivity Assessment" at http://hortsci.ashspublications.org/cgi/content/abstract/31/7/1160.
Dr. Wesley Autio, University of Massachusetts Amherst
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