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Detection of Citrus Huanglongbing-Associated ‘Candidatus Liberibacter asiaticus’ in Citrus and Diaphorina citri in Pakistan, Seasonal Variability, and Implications for Disease Management

First and fifth authors: University of Agriculture, Faisalabad, Pakistan; second and sixth authors: United States Department of Agriculture–Agricultural Research Service, National Clonal Germplasm Repository for Citrus and Dates, Riverside, CA 92507; and third and fourth authors: Department of Botany and Plant Sciences, University of California, Riverside 92507.

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.

Additional keywords: seasonal variation.