Fruit trees and phytoplasmas: a well settled association

The discovery, just fifty year ago, of a new group of plant pathogens related to bacteria led to the finding of polymorphic prokaryotes, located in the phloem of many plant species affected by yellows-type diseases believed to be caused by viruses, considering their infectious nature, and transmission by insects. The first retrievable records of phytoplasma related symptoms in fruit trees were presented during the first ICVF meeting (1954) when these pathogens were not know. In the last years molecular data have provided considerable insights into phytoplasma molecular diversity, and genetic interrelationships; significant taxonomic progress has been achieved by the study of the 16S ribosomal gene and other conserved genes allowing designation of 33 ribosomal groups and 41 ‘Candidatus Phytoplasma’ species. However, there is a gap between taxonomy and diseases since it is not uncommon that the same disease is associated with molecularly differentiable phytoplasmas, and with more than just one phytoplasma especially in woody host plants. Full sequencing of genomes of five phytoplasmas and a number of draft sequences provided the knowledge about putative biochemical pathways, confirming that phytoplasmas are very special microorganism lacking relevant bacterial features such as cell wall, mobility, key enzymes and pathways. They appears to have a small efficient chromosome and tricky metabolisms, allowing them to a trans kingdom life of interaction that often increase activity of their hosts such as enhancing insect fitness, plant shoot production, morphology and plant life cycles. They seems to preparing to become permanent cell hosts; however they are still far from loose independence and freedom as can also act as very dangerous pathogens. Epidemiologic studies of phytoplasma-associated diseases allow confirming the possibility to molecularly identify strains that have the most important roles in fruit tree disease outbreaks. These diseases are mainly widespread presence Europe and include apple proliferation, pear decline, and European stone fruit yellows tentatively classified as ‘Ca. P. mali’, ‘Ca. P. pyri’ and ‘Ca. P. prunorum’, all belonging to ribosomal group 16SrX. Apple proliferation, only reported in Europe is one of the most important disease of this species reducing size, weight and quality of fruit. Although it affects most or all varieties of apple, it is associated with relatively genetically homogeneous phytoplasmas and vectored by the psyllids Cacopsylla picta, C. melanoneura and the leafhopper Fieberiella florii. Pear decline, firstly reported in western USA and Canada, is of relevant importance in European pear orchards and was recently identified also in South Amrica. Main symptoms enclose poor shoot and spur growth, dieback, premature reddening and upper rolling of leaves, reduced leaf and fruit size and number. Insect vectors are C. pyricola and C. pyri. European stone fruit yellows is a disease seriously affecting apricot, plum, and peach; apricot and Japanese plum. Although symptom severity is fairly variable, infected trees show typical yellows accompanied by leaf roll followed by leaf reddening, and often winter sprouting. ‘Ca. P. prunorum’ vectored by C. pruni is the associated organism, together with some other phytoplasma among which the “stolbur” (‘Ca. P. solani’, 16SrXII-A) is very often present. Severe epidemic of ‘Ca. P. phoenicium (16SrIX) are also recently reported in some areas of the Arabian peninsula.
\nThe phytoplasma growth in artificial media achieved from periwinkle shoots and from field infected plants should allow the confirmation of the molecular information on phytoplasma biology and possibly pathogenicity gained in the last years. This knowledge will help in defining feasible solutions to reduce the phytoplasma disease impact and in devising the best management strategies, especially in fruit tree crops considering their several years’ permanence in orchards.

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