Abstracts Category : Other

The roles of psyllids, host plants and environment in the aetiology of huanglongbing in Bhutan

by Namgay Om

Candidatus Liberibacter asiaticus (CLas) (-Proteobacteria) is the pathogen associated with the Asiatic form of huanglongbing (HLB), the most serious disease of citrus in Asia and the Americas, in Bhutan. The principal vector of the disease, Diaphorina citri Kuwayama (Hemiptera: Sternorrhyncha: Liviidae), is also present in Bhutan. Recent detection of CLas in the black psyllid, Diaphorina communis Mathur (Hemiptera: Liviidae), prompted my PhD study. I examined the molecular characteristics of Diaphorina communis and Diaphorina citri and found that the mitochondrial COI and 16S regions can distinguish the two species, and is useful for the identification of nymphs in the absence of adults and fifth instar nymphs. Investigations into whether Diaphorina communis can acquire CLas and transmit it from CLas-infected mandarin (Citrus reticulata Blanco) to healthy seedlings of mandarin and curry leaf (Bergera koenigii L.) and vice versa were conducted under field conditions at Baychhu (27.2975N, 89.9669E, 784 m ASL), Wangdue Phodrang Dzongkhag. Several experiments were conducted starting with preliminary experiments in 2013, and transmission experiments in 2014 and 2015. Results showed that Diaphorina communis did not reproduce on mandarin and preferred curry leaf to mandarin given the choice. Further, a low (~6%) proportion of Diaphorina communis acquired CLas after 1647 d and 39 d of acquisition and transmission intervals in 2014 and 2015, respectively. No transmission of CLas from CLas-infected mandarin to healthy mandarin and curry leaf by Diaphorina communis could be established based on periodic testing of mandarin and curry leaf seedlings. No transmission of CLas from curry leaf to mandarin or curry leaf could be achieved. Phylogenetic analyses based on ITS 1 and 2, and the COI region as well as the morphological descriptions revealed that the individuals of the parasitoid reared from parasitised nymphs of Diaphorina communis belonged to a new species of Tamarixia. The name of the new species has been proposed as Tamarixia drukyulensis sp. n. (Hymenoptera: Eulophidae). Assessment of the effect of altitude, ambient temperature, relative humidity and rainfall on leaf temperatures of mandarin plants, HLB, and distribution of psyllids, was made using eight sites ranging from 800 m to 1500 m ASL along Droopchhugang-Phuensoomgang in Tsirang. My investigations revealed that, in contrast to my hypotheses, leaf temperatures were lower than ambient temperature at all elevations and did not influence the low incidences of CLas and Diaphorina citri at higher elevations; mandarin seedlings grew better at 8001000 m ASL than seedlings planted at 1100 and 1500 m ASL in spite of higher incidences of CLas and Diaphorina citri at 8001100 m ASL; ambient temperatures and relative humidity at all elevations were within the range of conditions favourable for development and survival of Diaphorina citri. UV radiation could have a role in the distribution of psyllids in addition to the interaction of rainfall andAdvisors/Committee Members: Western Sydney University. School of Science and Health (Host institution).