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Research Project: Evaluation of NPGS Parthenium argentatum Accessions for Resistance to Damping-off and Seedling Root Rot

Location: Office of The Director

Project Number: 2034-21000-012-001-S
Project Type: Non-Assistance Cooperative Agreement

Start Date: Sep 1, 2019
End Date: Sep 30, 2023

Guayule (Parthenium argentatum) is closer to commercial development in the U.S. than any other time in its long history as a source of natural rubber, resins, and biomass for fuel and energy. A commercial partner has invested millions of dollars each year since 2012 to commercialize guayule and develop a North American natural rubber industry. It is very conceivable within the next 5 years that the first commercial extraction plant will be built in Arizona requiring up to 50,000 acres of shrubs for the first year of operation and another 50,000 acres for year 2. The USDA, ARS guayule collection is the source of germplasm for guayule breeding worldwide. The collection has little information on resistance or susceptibility to diseases. In order to reduce production costs, direct-seeded guayule is preferred to seedling transplant. However, successful seed germination requires shallow sowing of seed on fine-textured, firm beds with the maintenance of high soil-surface moisture until emergence. One unintended consequence of high moisture levels is damping-off diseases, which were found to be responsible for poor survival of these young stands and high seedling mortality. For example, seedling mortality was observed in field planting on both sandy and clay soils. In a fall 2018 planting at Maricopa Ag Center, it was observed that seedling mortality due to possible seedling disease could be as high as 28%. However, these seedlings were not analyzed for disease identification. Several fungal pathogens including Rhizoctonia sp., Fusarium sp., Pythium sp., Phytophthora sp. have been added to the list of guayule seedling pathogens in the United States (Norton and Frank 1953, Alcorn 1982, Mihail et al. 1985 and 1989, Beaupre and Cheo 1983). The objective of this proposal is to work with the Cooperator and the commercial partner to evaluate the ARS guayule accessions for seedling fungal diseases (Rhizoctonia and Fusarium) and add this information to GRIN-Global to improve utilization of this valuable collection.

Cooperator will evaluate and characterize all available accessions of guayule in ARS' collection for variation in response to seedling fungal pathogens. The data will be added to GRIN-Global, improving utilization and transfer of the existing germplasm for breeding programs. An estimated 55 available accessions will be evaluated in a greenhouse. Isolates of damping-off and seedling root rot pathogens will be isolated from guayule seedlings showing symptoms of damping-off from several guayule field locations in Eloy. Experiment 1: pathogenicity test (Oct 1 – Oct 30, 2019) Pathogenicity of all Rhizoctonia solani and Fusarium solani isolates will be tested on two guayule accessions in a growth chamber. The inoculum will be prepared by growing each isolate on autoclaved rye grain for 15 days at room temperature. Pots of both accessions grown similarly but without inoculum will be used as check. A randomized complete block design will be used with 4 replicates. Emergence counts will be taken 2 weeks after seeding. Experiment 2: preparation of inoculum (Nov 1 – Dec 31, 2019) This experiment will determine the amount of inoculum required to produce a level of disease sufficient to differentiate accessions. For large-scale screening of guayule accessions, the inoculum will be prepared as follows: Single hyphal tip cultures of 10 pathogenic R. solani isolates and 10 F. solani isolates will be grown separately on autoclaved rye grain for 15 days at room temperature, then air-dried, ground, and sieved for particles with size ranging from 0.5mm to 1.2mm. Particles from all 20 isolates will be mixed in equal amounts, and the mixed inoculum will be stored at 5oC until use. Dilution plating will be used to determine the number of vial propagules per 1 g of mixed inoculum. We will start the initial infection studies with 5.103 viable propagules per 1 L of potting mix. Growth chamber studies will be conducted repeatedly to find a satisfactory inoculum level. Pathogen-infested potting mix will not be used as a planting medium, but only as a thin cover over the seeds. We will adjust the inoculum concentration so that 100 mL of infested-potting mix giving a 1-cm thick cover over the guayule seeds produces a level of disease sufficient to differentiate various genotypes. Experiment 3: greenhouse screening (Jan 1, 2020 – Sep 30, 2020) Twenty-five seeds per pot of each genotype will be placed on non-infested potting mix, then covered with 100mL of inoculum. Pots of each genotype seeded similarly but without inoculum will be used as controls. The pots for each replication will be randomized in a tray. A randomized complete block design will be used with 4 replicates. The experiment will be conducted in early spring to simulate the field conditions and will follow standard practices for growing guayule under greenhouse conditions. The criteria used to evaluate disease response will be the percent emergence after 1 week, leaf and plant senescence after 2 weeks, and the percentage of healthy plants and the percent survival at 3 weeks after seeding. ANOVA will be performed and significant differences between means will be determined using Duncan’s Multiple Range Test