Huanglongbing (HLB), referred to as citrus greening disease, is a bacterial disease impacting citrus production worldwide and is fatal to young trees and mature trees of certain varieties. In some areas, the disease is devastating the citrus industry. A successful solution to HLB will be measured in economics: citrus growers need treatments that improve tree health, fruit production, and most importantly, economic yield. The profitability of citrus groves is the ultimate metric that truly matters when searching for solutions to HLB. Scientific approaches used in the laboratory, greenhouse, or field trials are critical to the discovery of those solutions and to estimate the likelihood of success of a treatment aimed at commercialization. Researchers and the citrus industry use a number of proxy evaluations of potential HLB solutions; understanding the strengths and limitations of each assay, as well as how best to compare different assays, is critical for decision-making to advance therapies into field trials and commercialization. This perspective aims to help the reader compare and understand the limitations of different proxy evaluation systems based on the treatment and evaluation under consideration. The researcher must determine the suitability of one or more of these metrics to identify treatments and predict the usefulness of these treatments in having an eventual impact on citrus production and HLB mitigation. As therapies advance to field trials in the next few years, a reevaluation of these metrics will be useful to guide future research efforts on strategies to mitigate HLB and vascular bacterial pathogens in other perennial crops.
AbstractThe Asian citrus psyllid,Diaphorina citri, is a vector of ‘CandidatusLiberibacter asiaticus’ (CLas), a gram-negative, obligate biotroph whose infection inCitrusspecies is associated with citrus greening disease, or Huanglongbing (HLB). Strategies to blockCLas transmission byD. citriremain the best way to prevent the spread of the disease into new citrus growing regions. However, identifying control strategies to block HLB transmission poses significant challenges, such as the discovery and delivery of antimicrobial compounds targeting the bacterium and overcoming consumer hesitancy towards accepting the treatment. Here, we computationally identified and tested a series of 20-mer nodule-specific cysteine-rich peptides (NCRs) derived from the Mediterranean legume,Medicago truncatulaGaertn. (barrelclover) to identify those peptides that could effectively prevent or reduceCLas infection in citrus leaves and/or preventCLas acquisition by the bacterium’s insect vector,D. citri. A set of NCR peptides were tested in a screening pipeline involving three distinct assays: a bacterial culture assay, aCLas-infected excised citrus leaf assay, and aCLas-infected nymph acquisition assay that includedD. citrinymphs, the only stage ofD. citri’s life-cycle that can acquireCLas leading to the development of vector competent adult insects. We demonstrate that a subset ofM. truncatula-derived NCRs inhibit bothCLas growth in citrus leaves andCLas acquisition byD. citrifromCLas-infected leaves. These findings reveal NCR peptides as a new class and source of biopesticide molecules to controlCLas for the prevention and/or treatment of HLB.