Incidence of psyllids and ‘ Candidatus Liberibacter asiaticus’ in mandarin orchards at different altitudes in Tsirang, Bhutan

Citrus in Bhutan is cultivated on steep hillsides up to ~ 1550 m above sea level (ASL). Since 2000, the Asiatic form of huanglongbing caused by ‘Candidatus Liberibacter asiaticus’ (CLas) has led to dramatic losses of trees below 1200 m ASL. Surveys prior to this study indicated a low incidence of the disease and the psyllid Diaphorina citri (the principal vector of the pathogen), above 1200 m ASL, the presence of Diaphorina communis below 1200 m ASL and Cacopsylla heterogena above 1200 m ASL. This study was conducted in the Tsirang Dzongkhag (a southern administrative district) to determine relationships between ambient air and leaf temperatures, relative humidity, rainfall, plant growth and altitude on the incidence of psyllids and huanglongbing. In May 2013, CLas free seedling mandarin trees were planted at eight altitudes from 783 to 1473 m ASL to determine presence of the psyllids and development of CLas. Root and trunk tissues from these seedlings and mature trees in the dzongkhag were tested for CLas. Leaf temperatures of the seedling trees were lower than ambient air temperatures and were not related to the incidence of CLas and D. citri. Ambient air temperatures and relative humidities up to 1473 m ASL were suitable for development and survival of D. citri, but the psyllid was not recorded on the seedlings above 1053 m ASL and rarely on mature trees above this altitude. CLas was recorded in seedling trees below 1053 m ASL. In mature trees, it was generally restricted to altitudes ≤ 1183 m ASL. Diaphorina communis was not recorded in the experimental plots. Cacopsylla heterogena nymphs were observed on the seedlings at 1378 m ASL in April 2015. Results and observations indicate that natural incidence of CLas in Bhutan is predominantly related to D. citri. Factors other than ambient air temperature and relative humidity directly or indirectly limit the occurrence of the psyllid and CLas at altitudes ≥ 1200 m ASL; UV radiation may be a factor.