Plant Science

Huanglongbing (HLB) or greening currently is the most devastating citrus disease worldwide. The fastidious phloem-colonizing bacterium ‘Candidatus Liberibacter asiaticus’ (CLas) is the causal agent of citrus HLB in Florida. Bactericides containing the active ingredient oxytetracycline (OTC) have been used in foliar spray to control citrus HLB in Florida since 2016. However, the minimum concentration of OTC required to suppress CLas in planta remains unknown. We developed a new method for evaluating the effects of OTC treatment on CLas titers in infected plants and determined the relationship between OTC residue levels and control levels achieved for CLas using mathematical modeling in greenhouse and field experiments. In both greenhouse and field, OTC spray did not reduce the titers of CLas, and it produced undetectable or mild levels of OTC residue in leaves within 7 days post-application (DPA). In greenhouse, OTC injection at 0.05 g per tree decreased CLas titers to an undetectable level (cycle threshold value ≥ 36.0) from 7 to 30 DPA and produced a residue level of OTC at 0.68 to 0.73 µg/g of fresh tissue over this period. In the field, OTC injection at 0.50 g per tree resulted in the decline of CLas titers by 1.52 log reduction from 14 to 60 DPA, with residue levels of OTC at 0.27 to 0.33 µg/g of fresh tissue. In both trials, a first-order compart model of OTC residue dynamics in leaves of trunk-injected trees was specified for estimating the retention of effective concentrations. Furthermore, nonlinear modeling revealed significant positive correlations between OTC residue levels in leaves and the control levels for CLas achieved. The results suggested that the minimum concentrations of OTC required to suppress CLas populations in planta to below the detection limit are 0.68 and 0.86 µg/g and that the minimum concentrations of OTC required for initial inhibition of CLas growth in planta are ∼0.17 and ∼0.215 µg/g in leaf tissues under greenhouse and field conditions, respectively. This finding highlights that a minimum concentration of OTC should be guaranteed to be delivered to target CLas in infected plants for effective control of citrus HLB.

Disease caused by the bacterial pathogen “Candidatus Liberibacter solanacearum” (Lso) represents a serious threat to solanaceous crop production. Insecticide applications to control the psyllid vector, Bactericera cockerelli Šulc (Hemiptera: Triozidae) has led to the emergence of resistance in psyllids populations. Efforts to select natural resistant cultivars have been marginally successful and have been complicated by the presence of distinct Lso haplotypes (LsoA, LsoB) differing in symptoms severity on potato and tomato. A potentially promising management tool is to boost host resistance to the pathogen and/or the insect vector by promoting mycorrhization. Here we tested the hypothesis that mycorrhizal fungi can mitigate the effect of Lso infection on tomato plants. The presence of mycorrhizal fungi substantially delayed and reduced the incidence of Lso-induced symptoms on tomato as compared to non-mycorrhized plants. However, PCR with specific Lso primers revealed that mycorrhization did not prevent Lso transmission or translocation to newly formed leaves. Mycorrhization significantly reduced oviposition by psyllids harboring LsoA and survival of nymphs from these eggs. However, mycorrhization had no effect on oviposition by psyllids harboring LsoB or the survival of nymphs from parents harboring LsoB. These findings indicate the use of mycorrhizal fungi is a promising strategy for the mitigation of disease caused by both LsoA and LsoB and warrants additional field testing.

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.

‘Candidatus Liberibacter solanacearum’ (Lso) is an uncultured, phloem-associated bacterium causing a severe tuber disease in potato called zebra chip (ZC). Seven haplotypes of Lso have been described in different hosts, with haplotypes A and B found associated with infections in potato and tomato. In the field, Lso is transmitted by the potato psyllid (Bactericera cockerelli), and between 2011 and 2015, a significant change in Lso haplotype prevalence was previously reported in Idaho: from exclusively A haplotype found in tested psyllids in 2012 to mainly B haplotype found in collected psyllids in 2015. However, prevalence of Lso haplotypes in Idaho was not analyzed in potato tubers exhibiting symptoms of ZC. To fill in this knowledge gap, prevalence of Lso haplotypes was investigated in potato tubers harvested in southern Idaho between 2012 and 2018, and it was found to change from exclusively A haplotype in the 2012 season to an almost equal A and B haplotype distribution during the 2016 season. During the same period, haplotype distribution of Lso in psyllid vectors collected using yellow sticky traps also changed, but in psyllids, the shift from A haplotype of Lso to B haplotype was complete, with no A haplotype detected in 2016 to 2018. The changes in the haplotype prevalence of the Lso circulating in potato fields in southern Idaho may be, among other factors, responsible for a decrease in the ZC incidence in Idaho potato fields between an outbreak of the disease in 2012 and a very low level of ZC afterward.