Genetic studies on peach yellow leaf roll phytoplasma
By using molecular techniques, significant advances have been made in the characterization of phytoplasmas affecting stone fruits. In Europe, most or all phytoplasma diseases of stone fruits are caused by a relatively homogeneous pathogen, the European stone fruit yellows phytoplasma (EPPO A2 quarantine pest). This pathogen is closely related to phytoplasmas inducing apple proliferation and pear decline, and it is considered that it belongs to the apple proliferation group.
In North America, the X-disease (peach X-disease phytoplasma is listed as an EPPO A1 quarantine pest) is thought to be induced by a highly variable pathogen whose numerous strains vary considerably in virulence, symptomatology, geographical distribution. Therefore, the various disease manifestations have been described under several names: X-disease, western X-disease, eastern X-disease, cherry buckskin, peach yellow leaf roll, peach leaf casting yellows, etc. It has been shown that the phytoplasmas inducing typical X-disease in western and eastern USA and Canada are genetically similar. Most of these strains belong to a different phylogenetic cluster, the western X-disease phytoplasma group (which is distinct from the apple proliferation group).
Concerning peach yellow leaf roll disease, it appeared in previous studies that different phytoplasmas could be involved. In California, one form of the disease is caused by a strain of the X-disease phytoplasma (called WX1 strain), but it was observed that two strains from peach trees also showing symptoms of peach yellow leaf roll were closely related to the European fruit tree phytoplasmas of the apple proliferation group. Studies were therefore carried out to compare the genetic relationships between isolates of peach yellow leaf roll phytoplasma from California and phytoplasmas causing fruit tree diseases in Europe, by using molecular techniques (PCR, RFLP, Southern blot). Twelve diseased peach trees showing typical peach yellow leaf roll symptoms were sampled in October 1993 in four orchards located in the Sacramento valley, California (US). Results showed that peach yellow leaf roll phytoplasma is closely related to apple proliferation, pear decline and European stone fruit yellows phytoplasmas, and that it is a member of the apple proliferation group. The peach yellow leaf roll phytoplasma could be clearly distinguished from the apple proliferation and the European stone fruit yellows phytoplasmas (by RFLP of ribosomal DNA and by Southern blot) but was indistinguishable from the pear decline phytoplasma (by RFLP of ribosomal DNA).
The authors noted that this is not the first time that two genetically different pathogens (the peach yellow leaf roll phytoplasma and the WX1 strain from peach showing symptoms of peach yellow leaf roll) are able to cause similar symptoms. This is also observed in grapevine where grapevine flavescence dorée phytoplasma (member of the elm yellows group) and the phytoplasmas causing bois noir or Vergilbungskrankheit, which are closely related to or identical with the stolbur phytoplasma (members of the aster yellows group), induce very similar symptoms.
Sources
Kison, H.; Kirkpatrick, B.C.; Seemüller, E. (1997) Genetic comparison of the peach yellow leaf roll agent with European fruit tree phytoplasmas of the apple proliferation group.
Plant Pathology, 46(4), 538-544.