VIEW ARTICLE

Cytology and Histology

Condition of the Phloem and the Persistence of Mycoplasmalike Organisms Associated with Apple Proliferation and Pear Decline. Ulrike Schaper, Former graduate student, Biologische Bundesanstalt für Land-und Forstwirtschaft, Research Institute for Plant Protection in Fruit Crops, 6901 Dossenheim, Federal Republic of Germany; E. Seemüller, research plant pathologist, Biologische Bundesanstalt für Land-und Forstwirtschaft, Research Institute for Plant Protection in Fruit Crops, 6901 Dossenheim, Federal Republic of Germany. Phytopathology 72:736-742. Accepted for publication 2 October 1981. Copyright 1982 The American Phytopathological Society. DOI: 10.1094/Phyto-72-736.

The persistence of mycoplasmalike organisms (MLO) associated with apple proliferation and pear decline depends on the condition of the host’s phloem. In October, the secondary phloem of the stem of both species was largely functional, MLO could be readily found in the sieve tubes of diseased trees, and the pathogen could be transmitted easily. In November and December, the sieve tubes of both species were degenerating, but MLO were still detectable in infected trees and could still be transmitted. In January and February, however, in the majority of apple trees and in all pear trees that were investigated, only a few intact sieve tubes were present. In apple trees with a pronounced replacement phloem, there were phloem areas with a large number of functional sieve tubes adjacent to the cambial zone. Accompanying the degeneration of sieve tubes in the fall and winter there was a complete disappearance of MLO from the stem phloem of most trees and a sharp reduction in the levels present in the remainder. Relatively high numbers of MLO still were observed in diseased apple trees with replacement phloem that contained many functional sieve tubes. Stem scions from this latter group of trees gave the only positive results in grafting transmission experiments performed in February. In roots, functional sieve tubes were present in relatively high numbers in all samples taken from apple and pear trees during the winter. On the monthly average, living sieve elements were present in 40–55% of the phloem in the time between October to March. For both diseases, MLO could always be observed in the roots of diseased trees during this period and both diseases could be transmitted readily by root-grafting. It is concluded that the MLO survive mainly in the root systems of affected plants during the winter and that they spread from there to the aerial parts in the spring. Overwintering of the organisms in the stem seems to occur only in exceptional cases.