The Plant Pathology Journal

Huanglongbing (HLB) is a disease caused by the phloem- limited Candidatus Liberibacter asiaticus (CLas) that affects the citrus industry worldwide. To date, only indirect strategies have been implemented to eradicate HLB. Included among these is the population control of the psyllid vector (Diaphorina citri), which usually provides inconsistent results. Even though strategies for direct CLas suppression seem a priori more promising, only a handful of reports have been focused on a confrontation of the pathogen. Recent developments in polymer chemistry have allowed the design of polycationic self-assembled block copolymers with outstanding antibacterial capabilities. Here, we report the use of polymeric nano-sized bactericide particles (PNB) to control CLas directly in the phloem vasculature. The field experiments were performed in Rioverde, San Luis Potosí, and is one of the most important citrusproducing regions in Mexico. An average 52% reduction in the bacterial population was produced when PNB was injected directly into the trunk of 20 infected trees, although, in some cases, reduction levels reached 97%. These results position PNB as a novel and promising nanotechnological tool for citrus crop protection against CLas and other related pathogens.

Huanglongbing (HLB) is one of the most destructive diseases in citrus, which imperils the sustainability of citriculture worldwide. The presumed causal agent of HLB, ‘<i>Candidatus</i> Liberibacter asiaticus’ (CLas) is a non-culturable phloem-limited α-proteobacterium transmitted by Asian citrus psyllids (ACP, <i>Diaphorina citri</i> Kuwayama). A widely adopted method for HLB diagnosis is based on quantitative real-time polymerase chain reaction (qPCR). Although HLB diagnostic qPCR provides high sensitivity and good reproducibility, it is limited by time-consuming DNA preparation from plant tissue or ACP and the requirement of proper lab instruments including a thermal cycler to conduct qPCR. In an attempt to develop a quick assay that can be deployed in the field for CLas detection, we developed a real-time loop-mediated isothermal amplification (rt-LAMP) assay by targeting the CLas five copy <i>nrd</i>B gene. The rt-LAMP assay using various plant sample types and psyllids successfully detected the <i>nrd</i>B target as low as ~2.6 Log<sub>10</sub> copies. Although the rt-LAMP assay was less sensitive than laboratory-based qPCR (detection limit ~10 copies), the data obtained with citrus leaf and bark and ACP showed that the rt-LAMP assay has >96% CLas detection rate, compared to that of laboratory-based qPCR. However, the CLas detection rate in fibrous roots was significantly decreased compared to qPCR due to low CLas titer in some root DNA sample. We also demonstrated that the rt-LAMP assay can be used with a crude leaf DNA extract which is fully deployable in the field for quick and reliable HLB screening.

Changes in physiological and biochemical patterns in lucerne plants caused by the presence of ‘Candidatus Phytoplasma australasia’, which is one of the significant pathogens causing yield losses in lucerne plants, were investigated. Significant differences were evident in total chlorophyll, chlorophyll a, chlorophyll b, and protein amounts between ‘Ca. Phytoplasma australasia’-positive and negative lucerne plants. Stress-related metabolites such as phenol, malondialdehyde, and proline accumulations in ‘Ca. Phytoplasma australasia’-positive plants were remarkably higher than those of phytoplasma-negative plants. As a response to disease attack, phytoplasma-positive plants exhibited higher antioxidant enzymes (peroxidase and catalase) and non-enzymatic metabolite responses such as jasmonic and salicylic acids. We state that partial disease responses were revealed for the first time to breed resistant lucerne lines infected by ‘Ca. Phytoplasma australasia’.