What Causes Weak Wood in Apple?

Some apple scion/rootstock combinations are prone to graft union failure, which is characterized by the scion breaking off of the rootstock.  This can occur at any time in the life of the tree, and no other signs of ill-health may be observed beforehand.  While factors like disease and environmental conditions can lead to graft failure, localized incompatibility is often considered to be the primary cause.  Localized incompatibility occurs when the cambium between the scion and rootstock become discontinuous, causing a proliferation of disorganized parenchyma cells in place of regular xylem differentiation.  Since there are no other external symptoms of localized incompatibility, combinations of scions and rootstocks may be planted without the grower being aware of the future risk.  As new apple cultivars are bred, trees need to be screened for signs of incompatibility. In this study, two methods were used to screen young trees.  The first combined light microscopy and computer image analysis to determine the percentages of the xylem cell types in the wood at the union.  Tissues from the most recent growth ring were sectioned and stained using Toluidine Blue-O and were examined at x200 magnification.  Xylem cells were divided into three tissue types based on their function within the wood: fibrous, conductive, and parenchymatous tissue.  The percentages of the tissue types were calculated between the combinations using ImageJ.  Weak combinations contained more parenchymatous tissue and less fibrous tissues than the strong combinations. Laser Ablation Tomography (LAT) was also evaluated for its use in screening histological traits of incompatibility.  A laser system was used to ablate samples and photographs were taken and stacked to produce a three dimensional view of the union.  Using LAT, we were able to distinguish some of the larger histological differences associated with localized incompatibility, including areas of poorly arranged parenchymatous tissue.