Agronomy and Crop Science

Huanglongbing (HLB) is considered one of the most destructive diseases of citrus because 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’ in the foliage of citrus trees in Brazil using real-time polymerase chain reaction (qPCR). Eleven plants (naturally infected, then screened) in the field with very mild and localized symptoms of HLB were confirmed to be ‘Ca. L. asiaticus’ 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 ‘Ca. L. asiaticus’ DNA using qPCR ‘Ca. L. asiaticus’-specific primers (As84F/As180R). The average cycle threshold (Ct) values, which relate to ‘Ca. L. asiaticus’ titer, were analyzed using a mixed model. Significant differences were observed in Ct values between seasons (F = 8.77, P = 0.0004), and abrupt changes were observed in Ct values in different quadrants of the trees. Autumn had the lowest Ct values, indicating the highest ‘Ca. L. asiaticus’ titer, and, thus, is considered the best period to detect ‘Ca. L. asiaticus’ infection in foliage of citrus trees in southern Brazil. In addition to the seasonal changes in Ct values, there was an initial decline in the Ct 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 results identify the optimal season to sample foliage for ‘Ca. L. asiaticus’ in southern Brazil (autumn) and confirm the importance of sample timing to maximize detection of ‘Ca. L. asiaticus’ and, thus, contribute to the search for effective measures to manage HLB.

North American grapevine yellows (NAGY) disease has sometimes been attributed to infection of Vitis vinifera L. by Prunus X-disease phytoplasma (‘Candidatus Phytoplasma pruni’) but this attribution may not be fully adequate. In this study, phytoplasma strains related to ‘Ca. Phytoplasma pruni’ were found in NAGY-diseased grapevines in Maryland, Pennsylvania, Virginia, Ohio, Missouri, and New York State. Based on restriction fragment length polymorphism analysis of 16S ribosomal RNA gene (16S rDNA) sequences, the strains (termed NAGYIII strains) were classified in group 16SrIII (X-disease group) but they contained a recognition site for the restriction endonuclease MseI that is not present in the 16S rDNA of ‘Ca. Phytoplasma pruni’. The 16S rDNA of the strains differed by three or four nucleotides from that of ‘Ca. Phytoplasma pruni’, indicating that they belonged to two novel 16S rDNA sequevars, designated NAGYIIIα and NAGYIIIβ. Both sequevars differed from ‘Ca. Phytoplasma pruni’ by a single base in each of three regions corresponding to species-unique (signature) sequences described for ‘Ca. Phytoplasma pruni’. Phylogenetic analyses of 16S rRNA genes and SecY proteins, and single-nucleotide polymorphism analyses of secY and ribosomal protein genes, further distinguished the two grapevine sequevar lineages from one another and from ‘Ca. Phytoplasma pruni’. The NAGYIIIα and NAGYIIIβ sequevars also differed from ‘Ca. Phytoplasma pruni’ in regions of the folded SecY protein that are predicted to be near or exposed at the outer surface of the phytoplasma membrane. No evidence indicated that diseased grapevines contained any phytoplasma strain conforming to ‘Ca. Phytoplasma pruni’ sensu stricto. Because the NAGYIII sequevars have not been reported in X-disease, a question is raised as to whether NAGYIII and Prunus X-disease are caused by different phytoplasma genotypes.

Zebra chip disease of potato is caused by the bacterial pathogen ‘Candidatus Liberibacter solanacearum’ and is a growing concern for commercial potato production in several countries in North and Central America and New Zealand. ‘Ca. L. solanacearum’ is vectored by the potato psyllid Bactericera cockerelli, which transmits the pathogen to several cultivated and wild solanaceaous host plants. Silverleaf nightshade (SLN), Solanum elaeagnifolium, is a common weed in the Lower Rio Grande Valley of Texas and a host for both the potato psyllid and ‘Ca. L. solanacearum’. SLN plants were successfully inoculated with ‘Ca. L. solanacearum’ under laboratory conditions. Retention studies demonstrated that ‘Ca. L. solanacearum’-infected SLN planted in the field in January 2013, concurrent with commercial potato planting, retained the pathogen under field conditions throughout the year despite extensive dieback during summer. The presence of ‘Ca. L. solanacearum’ was confirmed in leaves, roots, and stolons of SLN plants collected the following year using polymerase chain reaction. Acquisition assays using B. cockerelli adults also revealed that SLN retained the pathogen. Transmission studies determined that B. cockerelli can acquire ‘Ca. L. solanacearum’ within a 2-week acquisition access period on ‘Ca. L. solanacearum’-infected SLN and subsequently transmit the pathogen to potato. These results demonstrate that SLN plants can serve as a reservoir for ‘Ca. L. solanacearum’, providing a source of inoculum for B. cockerelli adults colonizing potato the next season. The presence of SLN plants all year round in the LRGV makes the weed an epidemiologically important host. These findings underscore the importance of eradicating or managing SLN plants growing in the vicinity of potato fields to prevent spread of ‘Ca. L. solanacearum’ and damage caused by zebra chip.