Research and IPM

Grants Programs: Pierce's Disease Research

CDFA PD/GWSS RESEARCH PRIORITIES

(Based on: PD/GWSS Research Scientific Review, Final Report, August 2007, Research Scientific Advisory Panel; updated for this RFP)

• Research proposals that address the following key research areas will be given funding priority by the CDFA program. Proposals in other areas will not be rejected a priori. However, all proposals must include an explanation of how the proposed research can lead to reductions in the PD problem and development of a sustainable PD management strategy. Both the CDFA and UC programs will take into account the perceived applicability of the anticipated results when making awards.
• Information on past and current research is available at http://piercesdisease.org/. Researchers are encouraged to review this information to ensure proposed research represents new ideas or approaches.

Biological Research Priorities

Exploiting Xylella fastidiosa (Xf) virulence factors to control Pierce’s disease.

In the last four years, several labs have participated in the effort to knock out Xf virulence genes and/or overexpress them, followed by testing the mutant strains for virulence on grape. This work has led to several important insights that can potentially be applied to new PD control strategies. Various transgenic and non-transgenic strategies can be envisioned for interfering with the function of protein-based factors, and thus conferring resistance to Xf infection; however, most research projects have not yet advanced to the point of demonstrating such a control method.

Priority areas include:

• Use of Diffusible signal factor (Dsf) for disrupting Xf colonization, including delivery by plant associated microbes, transgenic rootstocks, and application of chemical analogs.
• Inhibition of Xf polygalacturonase (PG). This research area includes identification of PGIPs with high activity against Xf PG, delivery of PGIP to grape plant scions from transgenic rootstocks, and development of small molecule inhibitors of Xf PG.
• Targeting other Xf proteins required for virulence. This research area includes development of protein/peptide-based inhibitors of cell surface proteins such as pilins and adhesins, along with identification of chemical inhibitors of these proteins.

Biological control of GWSS using parasitoids.

The use of parasitoids to reduce population densities of GWSS continues to show promise, especially in settings where synthetic insecticidal sprays cannot be used (e.g. organic farms, urban areas, or other non crop habitat). The laborintensive methods required to produce parasitoids are currently a major limitation of this approach.

Priority areas include:

• Elucidation of biochemical cues that parasitoids utilize to identify and parasitize their hosts.
• Production of parasitoids, with a particular emphasis on developing efficient means of mass producing GWSS eggs or an alternative suitable host for large-scale production of parasitoids.
• The utility of natural enemies (with an emphasis on native and introduced parasitoids) to suppress PD should be measured, particularly with respect to impact on GWSS populations in the field and under diverse environmental conditions (cultural practices and climatic differences). Further work should be conducted to quantify the value of natural enemies as an integral component of PD disease control programs in urban and rural communities. Further, limited research on conservation of existing parasitoids is warranted (e.g. by understory plantings that provide key resources, nectaries, over-wintering sites, etc.). The evaluation of new, imported species of parasitoids should focus on realistic assessments of their potential for greater impacts on PD than from currently established natural enemies (such as with the aid of models). Potential agents hypothesized to be more effective early in the season and suited to the California climate should be a priority. The potential impact of imported parasitoids on native non-pest leafhoppers should be assessed before release is allowed, using realistic host specificity trials.
• Studies that involve altering the environment to provide other food sources for parasitoids and/or artificial feeding of parasitoids. There is a significant body of work relating to conservation of natural enemies, and increasing their numbers and activity in agricultural crops with carefully selected cover crops. Such studies as they pertain to PD/GWSS should be pursued.

The relevance of Xf genotype to disease control. Targeted  comparative analysis with a focus on genes that are important in current promising strategies for control, such as the rpf and polygalacturonase-encoding genes. The goal is to assess the potential impact of genotypic diversity on proposed control strategies.

Plant-GWSS interactions: Determinants of host specificity. An understudied area is what makes host plants good and/or attractive hosts versus poor and/or unattractive hosts for GWSS.  Do poor hosts chemically repel GWSS?  Are there aspects of plant physiology that can be manipulated or compounds that can be generated in planta to prevent GWSS feeding and/or reproduction? Can host plant physiology be manipulated to reduce GWSS attraction and/or growth?

Priority areas include:

• Determine the mechanisms by which GWSS locates its plant hosts. The chemical ecology of GWSS-grapevine interactions/relationships should be examined (e.g., plant host factors that influence their attractiveness to females for oviposition and egg maturation).
• Identification of GWSS repellents.
• Identification of factors that reduce GWSS attraction and/or fitness that can be delivered through host xylem: for example, novel compounds, sublethal effects of pesticides (particularly the new generation systemic pesticides),

Plant-mediated disruption of GWSS life cycle or Xf transmission.

There is precedence in studies of plant pathogenic nematodes and fungi for the notion that RNAs produced by the host and ingested by the parasite can silence specific corresponding parasite genes and prevent disease. Recent scientific articles [Huang (2006), Nowara (2010) and Tinoco (2010)] demonstrate potential utility RNAi-mediated silencing of pathogen genes.

Priority areas include:

• Assessment of the ability of plant-produced double-stranded RNAs to silence GWSS genes.
• Identification of candidate targets for silencing, including GWSS genes important in Xf transmission, and GWSS genes required for maturation and reproduction

Host resistance to Pierce’s disease.

In annual crop species, the most cost effective and environmentally safe method for preventing disease is breeding for resistance. Traditional breeding strategies can be dramatically accelerated if the genes controlling resistance have been linked with DNA-based molecular markers that can be scored in a high throughput fashion. A second area that merits more attention in the short-term is collection and dissemination of information on PD resistance in existing commercial varieties of grapes. There appears to be significant anecdotal information about which commercial grape varieties are most susceptible to PD, but it does not appear that any one has performed a carefully controlled study of commercial grape varieties and disseminated the results. Several chemical and biotic inducers of systemic plant resistance have been applied with limited success in a few crops. Existing commercial varieties of grapes should also be screened for effective responses to such inducers.

Priority areas include:

• Marker Assisted Selection-based breeding for resistance. The RSAP recommends recruitment of additional breeders so that genes in addition to PdR1 can be mapped, tagged with molecular markers, and the process of introgression into multiple commercial backgrounds initiated.
• Assessment of PD resistance in existing commercial grape varieties. The RSAP envisions greenhouse studies employing both GWSS-mediated inoculations in one set of experiments and mechanical inoculations in another set, to distinguish between resistance derived from reduced attractiveness to the vector versus reduced susceptibility to colonization by the bacterium. Data on both PD symptoms and Xf growth should be obtained to distinguish also between tolerance and resistance, as tolerant varieties could become problematic reservoirs of the pathogen.
• Assessment of PD resistance in existing commercial grape varieties following treatment with chemical or biotic inducers of resistance. Given the limited success of inducers in other crops, a proposal in this area should include substantial preliminary data on the effectiveness of one or more agents.

Economic Research Priorities

Economic analysis of the impact of PD/GWSS on agriculture, both in terms of real and potential economic effects and economic losses due to PD and the effects of current and prospective control measures, including losses to growers and other market participants.

These specific topics are illustrative and are not listed in priority order:

• Modeling and measuring the economic effects of the current PD/GWSS disease situation. How has PD/GWSS affected costs, acreages, prices and quantities? How much cost has been incurred so far and who has incurred those costs among consumers, producers, taxpayers, and other stakeholders by crop?
• What are the impacts on disease control and economic effects of alternative government and industry-wide policies for dealing with PD/GWSS? What policies complement alternative research and development strategies?
• Simulating alternative ex ante scenarios of the economic effects if PD/GWSS were to continue unabated. What are the likely impacts on costs, acreages, prices and quantities? How much cost is likely to be incurred, who is likely to incur those costs among consumers, producers, taxpayers, and other stakeholders by crop?
• Evaluating, in an ex ante sense using simulation models, the likely contributions of alternative investments in PD/GWSS research and development. Such a project could evaluate the potential contributions of several alternative R&D efforts that have different impacts on control of PD/GWSS and different time horizons. Such a project would not attempt to evaluate the likely scientific merit of alternative research efforts, but rather assess the payoff for the industry, including consumers, if reasonable success is obtained.

Huang, G., Allen, R., Davis, E.L., Baum, T.J., and Hussey, R.S. (2006). Engineering broad root-knot resistance in transgenic plants by RNAi silencing of a conserved and essential root-knot nematode parasitism gene. Proceedings of the National Academy of Sciences 103, 14302-14306.

Nowara, D., Gay, A., Lacomme, C., Shaw, J., Ridout, C., Douchkov, D., Hensel, G., Kumlehn, J., and Schweizer, P., (2010). HIGS: Host-Induced Gene Silencing in the obligate Biotropphic Fungal Pathogen Blumeria graminis. The Plant Cell, (in press—advanced publichation on lin at dx.doi.org/10.1105/tpc.100.077040).

Tinoco, M., Dias, B., Dall’Astta, R., Pamphile, J., and Aragao, F. (2010). In vivo trans-speicfic gene silencing in fungal cells by in planta expression of a double-stranded RNA. BMC Biol. 8, 27.

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