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

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 non transmissible '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.

Molecular Detection of Candidatus Liberibacter Asiaticus, the Causal Organism of Huanglongbing (Citrus Greening) in Faisalabad, Pakistan for Huanglongbing Management

“…For population dynamic studies of psyllid vector, psyllid samples were collected from different areas of Punjab, Pakistan and molecular study for detection of Candidatus Liberibacter asiaticus in plants and Diaphorina citri Kuwayama was conducted at USDA-ARS, Riverside, CA. This study showed that Las was detectable from trees at average maximum temperatures of 42°C (Razi et al, 2014). Diagnosis of the disease is not easy due to the uneven distribution of pathogen in the affected citrus plants (McClean, 1970;Hartung et al, 1993).…”

Seasonal Variation in Populations of ‘Candidatus Liberibacter asiaticus’ in Citrus Trees in Paraná State, Brazil

“…Other studies, two from Florida (22,44) and one from Pakistan (38) Temperature can thus be related to changes in bacterial titers, affecting the population of the pathogen and its reproduction (32), which may also in part be due to the influence of seasonal conditions on transport of the pathogen with carbohydrate to and from the roots (24). The low incidence and slow spread of Las by D. citri in production fields in warmer regions of São Paulo State might also be due to a direct influence of the ambient temperature on the titers of Las in leaves and possibly in the vector (22,44,34,38). The mean C t values measured in both sweet orange and Tahiti lime trees decreased with sample date (i.e.…”

“…Recently temperature has been shown to impact populations of Las in both the vector and the host (33,38), and in many parts of the world where citrus is grown there is a large seasonal fluctuation in temperature. Irey et al (22) reported that in Florida the most symptomatic samples were received in their diagnostic lab from August to January (mid-summer, autumn and early winter) and these had greatest percentage of HLB positives using qPCR.…”

“…Similarly, but based on incidence of detection in trees, in Pakistan it was found that during spring, only 18% of tree samples were Las positive, but in the summer 32% of tree samples were positive (38). Recent work in Florida has provided compelling evidence that the population of Las is not only greater in the roots (24), but also more consistent, perhaps making the roots a more reliable sample source to test for HLB.…”

“…The highest temperatures were recorded in the summer months (approximately 25 °C). The greater efficiency for infection of citrus with Las is 22 to 27 °C(13), which is thought to play an important role in the epidemiology of the disease, supporting survival and availability of inoculum in the field(3), and in the vector(38).…”