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’ ( C Las) 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, C Las free seedling mandarin trees were planted at eight altitudes from 783 to 1473 m ASL to determine presence of the psyllids and development of C Las. Root and trunk tissues from these seedlings and mature trees in the dzongkhag were tested for C Las. Leaf temperatures of the seedling trees were lower than ambient air temperatures and were not related to the incidence of C Las 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. C Las 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 C Las 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 C Las at altitudes ≥ 1200 m ASL; UV radiation may be a factor.

中文翻译：

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不丹Tsirang不同海拔柑橘园木虱和'Candidatus Liberibacter asiaticus'的发生率

不丹的柑橘种植在海拔约 1550 米 (ASL) 的陡峭山坡上。自 2000 年以来，由 'Candidatus Liberibacter asiaticus' (C Las) 引起的亚洲型黄龙病已导致 1200 米 ASL 以下的树木急剧减少。本研究之前的调查表明，该病的发病率较低，木虱 Diaphorina citri（病原体的主要载体）在 1200 m ASL 以上，在 1200 m ASL 以下存在 Diaphorina community，在 1200 m ASL 以上存在 Cacopsylla heterogena。本研究在 Tsirang Dzongkhag（南部行政区）进行，以确定环境空气与叶片温度、相对湿度、降雨量、植物生长和海拔之间的关系，与木虱和黄龙病的发病率之间的关系。2013 年 5 月，在 783 至 1473 m ASL 的八个海拔处种植 C Las 无幼苗柑橘树以确定木虱的存在和 C Las 的发育。对 dzongkhag 中这些幼苗和成熟树木的根和树干组织进行了 C Las 测试。苗木叶温低于环境气温，与C Las和D. citri的发病无关。环境气温和相对湿度高达 1473 m ASL 适合 D. citri 的发育和存活，但木虱在 1053 m ASL 以上的幼苗上没有记录，在这个高度以上的成熟树上很少有木虱记录。C Las 记录在 1053 m ASL 以下的幼苗中。在成熟的树木中，它通常限于海拔≤ 1183 m ASL。试验地块中没有记录 Diaphorinacommunis。2015 年 4 月，在 1378 m ASL 的幼苗上观察到 Cacopsylla heterogena 若虫。结果和观察表明，不丹 C Las 的自然发病主要与 D. citri 相关。环境空气温度和相对湿度以外的因素直接或间接限制了木虱和 C Las 在海拔 ≥ 1200 m ASL 的发生；紫外线辐射可能是一个因素。