This paper summarizes information on the most relevant phytoplasma diseases of temperate fruit trees, with emphasis on molecular and taxonomic aspects of the associated phytoplasmas. The reviewed diseases are widespread and of considerable economic significance. European stone fruit yellows and apple proliferation are present in the European continent whereas X-disease and peach yellow leaf roll occur in North America. Pear decline is present in both continents. Almond witches’-broom is known almost exclusively from Asian countries. Most of the mentioned diseases appear to be specific because they are induced by specific pathogens, which exhibit a high plant host and insect vector specificity. For each of them, in addition to the specific pathogen, the occurrence of ‘non-specific’ phytoplasmas, which are known to infect a wide range of plant hosts, has also been reported to be associated in some instances. However, the etiological role of these ‘non-specific’ phytoplasmas needs to be validated by pathological studies. Phytoplasma diseases of temperate fruit trees very often escape observation because affected plants show nonspecific symptoms only such as yellowing, stunting and/or decline. Also, the phytoplasma titer in diseased plants is often so low that infections can only be detected through highly sensitive nested PCR assays.
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