PHYTOPLASMAL DISEASES OF PEACH AND ASSOCIATED PHYTOPLASMA TAXA
C. Marcone, L.J. Guerra, J.K. Uyemoto
Phytoplasmal diseases occur wherever peach (Prunus persica) trees are grown. However, the causal agents differ considerably in taxonomy, insect vector relationships and geographic locations. X-disease of peach is widespread in North America, but does not occur elsewhere in the world. X-disease is induced by ‘Candidatus Phytoplasma pruni’, a member of the X-disease phytoplasma group (16SrIII group, subgroup 16SrIII-A). Peach rosette, peach red suture, and peach yellows, which occur in eastern United States and Canada are all caused by the X-disease phytoplasma. Another North America disease, peach yellow leaf roll (PYLR) is present in a limited area of northern California. Its causal agent is classified in the apple proliferation (AP) group, 16SrX group, as a subtype of the pear decline phytoplasma. In Europe, phytoplasmal diseases of peach are reported under the name European stone fruit yellows and incited by ‘Ca. P. prunorum’, a member of the AP group, subgroup 16SrX-B. ‘Ca. P. prunorum’ is closely related to the PYLR agent. In Lebanon and Iran, peach trees are affected by almond witches’ broom, a lethal disease incited by ‘Ca. P. phoenicium’, a member of the 16SrIX group, subgroup 16SrIX-B. Phytoplasmas of other phylogenetic groups, known to infect a wide range of plant hosts, have been identified in declining peach trees in several fruit-growing areas of the world. The pathological relevance of several ‘nonpeach’ phytoplasmas requires further investigations as their presence was ascertained by nested PCR assays only.