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

Abstract

Sauer, A.V., Zanutto, C.A., Nocchi, P.T.R., Machado, M.A., Bock, C.H, and Nunes, W.M.C. 201X. Seasonal variation in populations of 'Candidatus Liberibacter asiaticus' in citrus trees in Paraná state, Brazil. Plant Disease XX: xx-xx.Huanglongbing (HLB) is considered one of the most destructive diseases of citrus as the plants rapidly become unproductive, enter a decline and eventually die. HLB is caused by the phloem-limited bacterium 'Candidatus Liberibacter' spp. The objective of this study was to evaluate seasonal variation of the in planta population of 'Ca. Liberibacter asiaticus' (Las) in in the foliage of citrus trees in Brazil using real-time PCR (qPCR). Eleven plants (naturally infected, then screened) in the field with very mild and localized symptoms of HLB were confirmed to be Las-infected by conventional PCR, and the canopies were divided into four quadrants. The bacterial population in the trees was tested on a monthly basis for up to 20 months by quantifying Las DNA using qPCR Las-specific primers (As84F/As180R). The average C t (cycle threshold) values, which relate to Las titer, were analyzed using a mixed model. Significant differences were observed in C t values between seasons (F = 8.77, P = 0.0004), and abrupt changes were observed in C t values in different quadrants of the trees.Autumn had the lowest C t values, indicating the highest Las titer and is thus considered the best period to detect Las infection in citrus trees in southern Brazil. In addition to the seasonal changes in C t values, there was an initial decline in the C t value in the months following initial detection, the rate of decline slowing with time. Concomitant with the increase of the bacterial population in the host, there was an increase in severity of HLB symptoms in the trees over time (Spearman's rank correlation, r = -0.4083, P<0.0001). The

Understanding psyllid transmission of Candidatus Liberibacter as a basis for managing huanglongbing

“…The influence of environmental conditions on the transmission of CLas and CLam is little known, but there is a marked effect on the populations of these bacteria in the host plant. Some authors have reported that CLas is affected by high temperatures (between 24 and 38°C), leading to reduced bacterial titer in plants (Lopes et al 2009b;Gasparoto et al 2012;Sauer et al 2015) and in insect vectors that fed on infected plants (Lopes et al 2013). Water deficit also negatively affected CLas populations in young shoots of citrus plants (Lopes et al 2017).…”

Raman Spectroscopy vs Quantitative Polymerase Chain Reaction In Early Stage Huanglongbing Diagnostics

“…Such qPCR negative results are typical for hot summer months, as the bacteria clears in the shoots due to high temperatures, which were in the range of 100-120 °F during May-August. Although well-documented, this problem substantially complicates qPCR-based diagnostics of HLB [31][32][33][34] . The Raman readings, however, reported stable signal intensity of 1610 cm −1 band above 1.925 ( Fig.…”

Plant hairy roots enable high throughput identification of antimicrobials against Candidatus Liberibacter spp.

“…C Las also is more abundant and more consistently detectable, in roots than in shoots, making root a preferred source tissue in which to perform disease diagnostics tests. Furthermore, roots act as the reservoir for C Las between periods of foliar flushing 86 and to endure the seasonal fluctuations of high temperatures in summer months 84 . Relatively few groups have studied C Lso root infections, but the levels and distribution of C Lso are greater in roots than shoots, much as for C Las 91 .…”

“…A salient feature of the microbial hairy root system is that the efficacy assays are conducted in living (root) tissues that not only mimic the native environment of the pathogen but also encompass features that underpin native plant-microbe interactions, aspects that cannot be replicated in axenic culturing systems. A growing number of studies have indicated the pivotal roles of roots, root-rhizosphere interactions and root-microbiome composition in plant health and disease, including citrus health and HLB progression [84][85][86][87][88][89][90] . For instance, in the majority of presymptomatic citrus trees, roots appear to be the first site of CLas detection, preceding leaves 86 .…”