Global Scenario on Phytoplasmal Diseases in Palms

Abstract

Worldwide phytoplasma diseases pose great threat to the sustainability of the palm family members viz., coconut, arecanut, date palm, oil palm and ornamental palms causing setbacks in economic and livelihood security. So far, at least eight Candidatus Phytoplasma species belonging to different 16Sr groups have been reported to be associated with coconut. This includes Ca. P. asteris (lethal wilt disease in India; 16Sr IB), Ca. P. oryzae (Coconut root (wilt) disease in India, Weligama leaf wilt disease in Sri Lanka; 16Sr XIB), Ca. P. cynodontis (Coconut yellow decline in Malaysia; 16Sr XIV), Ca. P. malaysianum (Malayan yellow dwarf phytoplasma in Malaysia; 16Sr XXXIIB), Ca. P. palmicola (Cape St. Paul wilt in Ghana, Awka wilt in Nigeria; 16Sr XXIIB), Ca. P. cocostanzaniae (lethal yellowing in East Africa; 16Sr IVC), Ca. P. noviguineense (Coconut bogia syndrome in Papua new Guinea) and Ca. P. palmae (Coconut lethal yellowing in Americas and Caribbean region; 16Sr IVA,B,E). Though there are several reports on insects transmitting coconut phytoplasmal diseases, except for LY in Florida and RWD in India, the vectors remain elusive. Susceptibility of date palms to phytoplasmal diseases viz., lethal yellowing, Alwijam, White Tip Dieback (WTD) and Slow Decline (SD) or El Arkish’ are emerging concerns in Arabian countries and North Africa. Oil palm is a potential host of diverse groups (16SrI, 16SrXI, 16Sr XIV and 16SrXXXII) of phytoplasma in many parts of the world. Yellow leaf disease caused by 16SrI, 16SrXI and16SrXIV group phytoplasmas is one of the major diseases limiting the productivity of arecanut in south India. Many ornamental palm species harbour a wide array of phytoplasma groups that could serve as an intermediary mode of transmission to cultivated palms. Large scale movement of ornamental palms through international trade warrants the need for strengthening the international quarantine networks and bio-security measures to contain the spread of these phytopathogenic mollicutes. Despite rapid progress accomplished in taxonomy using molecular approaches many palm phytoplasma isolates are yet to be characterized for the assignment of specific taxonomic status. Due to its confinement to phloem, non-uniform distribution and sub-minimal titres in palms, molecular detection is many atimes intriguing. Palm-phytoplasmal interaction and transmission mechanism by auchenorrhynchan fauna are poorly understood. As a victim of climate change, phytoplasma diseases in synergy with survival superiority of insect vectors are extremely challenging to comprehend and evolve effective suppression mechanism. Management approaches involving surveillance, eradication and containment of the disease in newly emerging areas along with resistance breeding to ensure sustainable income to the farmers are discussed in this chapter.