Histopathology of Apple Proliferation in Malus Taxa and Hybrids of Different Susceptibility

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Abstract
Malus taxa and hybrids (“taxa”) grafted with M. pumila cv.‘Golden Delicious’differ significantly in their susceptibility to apple proliferation which is caused by a mycoplasma‐like organism (MLO). These differences are correlated with the severity of anatomical aberrations and the numbers of MLOs in the phloem. The roots of declining trees of highly susceptible taxa with a mortality of more than 50 % are characterized by extensive phloem necrosis and the depletion of starch. MLOs are either not detectable or are present in low numbers, or the population appears degenerate when viewed by fluorescence microscopy. In comparable trees of a hybrid of M. sieboldii×M. pumila which shows a high recovery rate, both phloem necrosis and starch depletion are less pronounced, and the MLO numbers are low or the organisms are not detectable. Decline‐tolerant taxa such as M. silvestris or M. pumila×M. baccata are little affected. Their phloem conditions and starch content do not differ significantly from that of healthy trees. However, the MLO titer is high. The histopathology of the scion cultivar of all groups examined is rather similar to that of the roots of the decline‐tolerant taxa. Only in a late stage of decline, phloem necrosis increases while starch content and MLO numbers decrease in the scions grafted onto highly susceptible stockls.