Phytoplasmas and their insect vectors: implications for date palm.

Phytoplasmas are transmitted chiefl y by insects, most commonly planthoppers and leafhoppers. Molecular genetic analyses have improved the understanding of phytoplasma taxonomy, and also enhanced the ability to identify phytoplasmas detected in hosts and insect vectors. Date palm is affected by Al-Wijam disease in Saudi Arabia and molecularly indistinguishable phytoplasmas (16SrI group) were recovered from affected palms and from Cicadulina bipunctata , an insect commonly found on the palms. The phytoplasma that is associated with the lethal yellowing disease in coconut palm (16SrIV-A) can also infect date palm. In the Americas lethal yellowing is likely to be transmitted by Haplaxius crudus (formerly Myndus crudus ). Texas phoenix decline is reported from warm regions of South-East USA and may be transmitted by two species of Derbidae. Phytoplasmas belonging to the 16SrIV-F and 16SrXIV groups have also been identifi ed from date palm growing in the USA and Africa respectively, though vectors have not been identifi ed. Preventing spread in infected vegetative planting material and of vectors is key to limiting the impact of phytoplasma diseases. Management in affected areas can use antibiotics on high value trees, but this is not economical for extensive crops. In these situations, vector control by insecticide use or habitat management can be useful, but the long lifespan of individual palms means that even low vector pressure can lead to infection over successive years. The development of resistant varieties and replanting is the most effective long-term approach developed so far for phytoplasma disease management in this plant species.