Location: Crop Diseases, Pests and Genetics Research
2022 Annual Report
Objectives
Grapes and citrus are major crops in California with an estimated value of $8.7 billion per year. Inputs for protection against pathogens and insect pests can be significant, and often environmentally friendly control strategies are lacking. To address the plant disease management challenges confronted by grape and citrus producers in California, research will focus on the following four objectives and associated sub-objectives.
Objective 1: Create novel grapevine and citrus pathogen identification and disease diagnosis methods.
Subobjective 1A: Develop novel species identification techniques for Xylella fastidiosa (Xf) and fungal pathogens of grapevines including utilizing FAME profiling and phenotype microarrays.
Subobjective 1B: Improve diagnostics of citrus pathogens through field deployable technologies such as isothermal amplification, robotics, and artificial intelligence.
Subobjective 1C: Develop sensitive and accurate detection systems for grape and citrus pathogens using genomic information.
Objective 2: Elucidate vector feeding and movement behaviors associated with transmission and spread of Xylella fastidiosa.
Subobjective 2A: Compare vector probing behaviors of blue-green sharpshooter (BGSS), with or without Xf, on Pierce’s disease (PD)-resistant or -susceptible grapevines.
Subobjective 2B: Develop an electropenetrography (EPG)-based Resistance Index (EPG-RI) to rapidly detect grapevine resistance to Xf inoculation behaviors of vectors.
Subobjective 2C: Determine the role of glassy-winged sharpshooter (GWSS) nymphs in spreading Xf within vineyards.
Subobjective 2D: Develop a flexible individual-based model to evaluate implications of results from studies conducted during completion of this project on pathogen spread.
Objective 3: Develop vibrational control methods for grapevine pests for integration into vineyard management practices.
Subobjective 3A: Evaluate female BGSS re-mating receptivity and communication.
Subobjective 3B: Determine efficacy of natural tremulatory signals in disrupting mating of the BGSS.
Subobjective 3C: Assess BGSS male attractiveness to playback of female vibrational signals.
Subobjective 3D: Develop methods for transmission of GWSS and BGSS disruptive signals to crops and ground vegetation.
Objective 4: Develop sustainable management tactics for pests and diseases of grapevine and citrus.
Subobjective 4A: Develop formulations to improve the use of region-specific biological control strains of Trichoderma spp. as a disease management tool for combating bacterial and fungal diseases of grapevines.
Subobjective 4B: Determine susceptibility of vine mealybug (VMB) to soil-applied imidacloprid.
Subobjective 4C: Target bacterial endosymbionts for control of VMB.
Subobjective 4D: Evaluate sub-lethal effects of soil-applied imidacloprid on GWSS fecundity, survival, and movement behavior.
Subobjective 4E: Determine distribution and genetic diversity of citrus tristeza virus (CTV) in citrus in California and assess the environmental/economic impact for future deployment of genetically-engineered (GE)-CTV.
Approach
The approach is to synergistically exploit weak links between main components of grapevine and citrus pathosystem (pathogen, vector, plant) and insect pests to induce an unstable or neutral interaction that can lead to disruption of destructive processes affecting grape and citrus production. First, disease management requires accurate, sensitive, and cost-effective diagnostic tests to identify causal agents. The most recent genomic information will be used to improve pathogen detection methods that are based on DNA sequences, whereas studies of pathogen phenotypes will lead to development of novel complimentary diagnostic methods. Second, developing plant disease management strategies requires a fundamental understanding of pathogen spread. Mathematical models will be used to evaluate the role of environmental factors on pathogen spread and to simulate management approaches. Experiments will compare insect vector feeding behaviors on resistant and susceptible plants and evaluate the role of juvenile stages in pathogen spread. Third, management of insect-transmitted pathogens requires novel sustainable methods for suppressing vector population growth. Methods to disrupt mating by interfering with insect vector behaviors will be developed. A final objective will assess current control methods to minimize risk of insecticide resistance, develop novel molecular technology targeting vector endosymbionts, identify biological control agents of fungal cankers, and evaluate safety and efficacy of genetically-engineered viruses to manage citrus diseases. The research will benefit grape and citrus growers by addressing current needs and developing novel technology to meet the demand for sustainable farming practices.
Progress Report
This project started in February 2022 and continues research from expired project 2034-22000-012-000D, Identification of Novel Management Strategies for Key Pests and Pathogens of Grapevine with Emphasis on the Xylella Fastidiosa Pathosystem.
Under Objective 1, ARS researchers in Parlier, California, collected samples of citrus plants infected by Citrus yellow vein clearing virus (CYVCV), a new virus reported from the United States. Virus-infected citrus plants were established in a containment facility for further insect transmission experiments and studies aimed at improving diagnostics. This research focuses on the spread dynamics of this new vector-pathogen complex that has local, state, and national importance. For grapevines, fatty acid methyl ester (FAME) profiles from isolates of 20 different fungal pathogens including Diplodia seriata, Eutypa lata, Neofusicoccum parvum, and Phaeomoniella chlamydospore were collected. In addition, five isolates of the bacterium Xylella fastidiosa (Xf) had FAME profiles collected. A minimum of five replicates of FAMEs were conducted for each. Software-based analysis demonstrated that consistent identifications could be made for each isolate. Eventually, with further development, FAMEs could be used to identify fungal and bacterial species and isolates. Also under Objective 1, progress was made in the development of polymerase chain reaction (PCR) systems for detection of Xylella taiwanensis (Xt), the pathogen causing pear leaf scorch disease (PLSD). The genome sequence of Xt strain PLS229 was downloaded from GenBank genome sequence database. Three conserved genomic loci, 16S rRNA gene (rrs), and 16S-23S RNA intergenic transcribed sequence (16S-23S rRNA ITS) were identified. Gene sequences were used to develop five Xt-specific TaqMan quantitative PCR systems (primers/probe sets). Sensitivity, specificity, and effectiveness of the PCR systems were validated using pure cultures from 17 Xt strains, one Xf strain that causes Pierce’s disease of grapevine, one strain of Xanthomonas campestris, and 136 PLSD plant samples collected from 23 pear orchards in four counties in central Taiwan.
Under Objective 2, progress was made in determining the role of glassy-winged sharpshooter (GWSS) nymphs in spreading Xf within vineyards. Studies were initiated to quantify movement of GWSS nymphs between (horizontal movement) and within (vertical movement) host plants. A computer code was written for an individual-based simulation model on the spread of a plant pathogen. Preliminary evaluations of the model suggest that performance can be greatly improved by using a super-computer.
Under Objective 3, progress was made in the development of vibrational control methods for grapevine pests. Studies of the reproductive biology of blue-green sharpshooter, an important vector of the bacterium Xf, showed that previously mated males and females are receptive to further copulation events at least one week after the first copulation. Information on insect re-mating receptivity is important for developing mating disruption or trap-and-kill approaches to integrate into vineyard management practices.
Under Objective 4, ARS researchers in Parlier, California, collected isolates of 24 Trichoderma species with potential for use as biological control agents of fungal diseases of grapevines. In plate assays, 10 of 24 isolates demonstrated capacity to consume or inhibit fungal pathogen growth. Two isolates were later shown to potentially inhibit pathogen infections in plants. These results demonstrate the potential for isolates to form the basis of a biological control program for integration to grapevine disease management strategies. Regarding citrus, a field survey of citrus tristeza virus (CTV) in commercial citrus orchards in California was completed. The predominant CTV strain was T30, which was detected with other strains in over 95% of the collected samples. Sweet orange had the highest level of infections (approximately 80%), followed by grapefruit (approximately 21%), and CTV was rare in lemons and was not detected in mandarins sampled. This information will be used to support an environmental assessment of CTV in California and the potential use of CTV viral vectors to express antimicrobial peptides to help control Huanglongbing in commercial citrus in California. Lastly, studies were conducted to evaluate the interaction of glassy-winged sharpshooter and mealybugs with imidacloprid-treated plants, which advanced efforts to determine the susceptibility of grapevine pests to a soil-applied systemic insecticide such as imidacloprid.
Accomplishments
