Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/2/2013
Publication Date: 3/1/2014
Citation: Razi, M.F., Keremane, M.L., Ramadugu, C., Roose, M., Kahn, I., Lee, R.F. 2014. Detection of citrus huanglongbing-associated ‘Candidatus Liberibacter asiaticus’ in citrus and Diaphorina citri in Pakistan, season variability, and implications for disease management. Phytopathology. 104:257-268. Interpretive Summary: We report the results of a study of the Asian citrus psyllid (ACP) populations in the main production area of Pakistan, which has a climate similar in temperatures to the Central Valley citrus production area of California and the citrus growing area of South Texas. Also, we report the characterization of Candidatus Liberibacter asiaticus (Las), the bacterium associated with huanglongbing (HLB) in Pakistan. Two population peaks of ACP were observed, one occurring in the spring (April-May) and the second and highest peak in the fall (October-November). When temperatures reached 50o C in June/July, the ACP tested negative for Las, but the plants tested positive for the bacterium. Analyses of conserved areas of the Las genome suggest that two populations may be present in Pakistan. To our knowledge, this is the first study reported of the populations of ACP from Pakistan and molecular characterization of Las from the main citrus growing area in Pakistan. Because of the similarities of climates in Pakistan, the Central Valley in California, and South Texas, the results from this study may be helpful in California and Texas to manage huanglongbing.
Technical Abstract: We report the detection of the huanglongbing (HLB)-associated bacterium ‘Candidatus Liberibacter asiaticus’ from both plants and insects in Pakistan and the seasonal variability in the numbers of ‘Ca. L. asiaticus’-positive psyllid vector, Diaphorina citri. Our studies showed that ‘Ca. L. asiaticus’ was detectable from trees in areas with maximum temperatures reaching nearly 50°C (average maximum of 42°C). However, the bacterium was present at very low levels in psyllids both in summer (June to August) and autumn (September to November) in contrast to reports from Florida, where the bacterium was detectable at very high levels during October to November. We hypothesize that hot summer temperatures in Pakistan may interfere with acquisition and replication of ‘Ca. L. asiaticus’ in psyllids and may lead to dead or nontransmissible ‘Ca. L. asiaticus’ in plants. Psyllid counts were very low in both summer and winter, showed a population peak (‘Ca. L. asiaticus’-positive vectors) in spring, and showed a larger peak (‘Ca. L. asiaticus’-free psyllids) in autumn. Natural thermotherapy during hot summers and a low vector population during environmental extremes may have played a major role in long-term survival of the citrus industry in Pakistan. These results may be useful in developing management strategies for U.S. citrus industries in Texas and California.