Horticulture

Huanglongbing (HLB) or citrus greening disease is the most devastating disease of citrus worldwide. This disease, caused by the bacterium ‘Candidatus Liberibacter asiaticus’ (CLas), leads to low fruit quality and unproductive trees. In 2008, HLB was found in a residential citrus tree in Savannah, Georgia, and, as a result, the state has been quarantined for this disease since 2009. Nonetheless, little is known about the distribution of CLas within Georgia, even though the commercial planting of citrus in Georgia has increased exponentially in recent years. In 2019, 94 samples from commercial and residential citrus trees within 11 counties in coastal and southern Georgia were collected and tested for the presence of CLas. Molecular testing results revealed the presence of CLas in three counties where HLB had not been previously reported and in 9% of samples overall. This is the first definitive report confirming HLB in southern Georgia counties besides those along the coast.

AbstractA previously unknown haplotype of the plant pathogen ‘Candidatus Liberibacter solanacearum’ (Lso) was found in cultivated carrots and parsnips in eastern Finland. That same haplotype was found in western Finland, over 300 km away, in the family Polygonaceae, the species Fallopia convolvulus (wild buckwheat) and Persicaria lapathifolia (pale persicaria) growing as weeds within carrot and parsnip fields. The infected plants, both apiaceous and polygonaceous, showed symptoms of foliar discolouration. This is the first report of Lso bacteria in plants of the family Polygonaceae. The finding that the polygonaceous plants infected with a previously unknown haplotype of Lso were growing among the apiaceous plants infected with Lso haplotype C suggests that these two haplotypes might be transmitted by different vectors. Phylogenetic analyses showed that the new haplotype, called haplotype H, is distinct from the previously characterized haplotypes and appears to have diverged early from their common ancestor. Multi-locus sequence analysis revealed four different sequence types (strains) within the haplotype H. These findings suggest that the haplotype H is likely to be endemic in northern Europe and that the genetic diversity within the Lso species is higher than previously assumed.

During 2015, samples from 22 apple trees showing proliferation symptoms were collected in southwest Bulgaria and Central and South Poland and tested for phytoplasma presence. ‘Candidatus Phytoplasma mali’ was identified in 18 samples based on results of restriction fragment length polymorphism (RFLP) analysis of the 16S rRNA gene amplified in nested PCR using primer pair P1/P7 followed by R16F2n/R16R2 and F1/B6 primer pairs. The nitroreductase and rhodonase like genes and ribosomal protein genes rpl22 and rps3 were then analyzed using PCR-RFLP technique to study the genetic variability of the  phytoplasma strains. Two restriction profiles, P-I or P-II, were obtained for fragments of 16S rDNA plus 16S-23S spacer region digested with HpaII enzyme. Restriction fragment length polymorphism analysis of nitroreductase and rhodonase like genes using digestion with HincII endonuclease revealed that the all ‘Ca. P. mali’ strains belonged to subtype AP-15. Analysis of rpl22 and rps3 ribosomal protein genes digested with AluI enzyme resulted in classification of detected phytoplasma strains to rpX-A subgroup.

Monitoring the health of Huanglongbing-affected citrus trees by following changes in leaf Candidatus Liberibacter asiaticus (CLas) titer has an inherent element of imprecision because CLas titer varies considerably within the tree canopy and with calendar seasons. In addition, the destructive sampling method used to determine CLas titer entails a different set of leaves per sampling period adding to the inconsistency and inexactitude of the results. To overcome these ambiguities and to reduce the numerical variability between samples, we developed an experimental method that analyzes portions of the same treated leaves for up to four sampling periods. By assaying subsamples of adjacent locations of the same leaf, random variability was significantly reduced, and comparative analysis can be carried out with greater precision.