Identification of Environmental Factors Related to Claviceps purpurea Ascospore Production in Perennial Ryegrass Seed Fields and Development of Predictive Models

Impact of the United States Department of Agriculture, Agricultural Research Service on Plant Pathology: 2015-2020

There is an increasing need to supply the world with more food as the population continues to grow. Research on mitigating the effects of plant diseases to improve crop yield and quality can help provide more food without increasing the land area devoted to farming. National Program 303 (NP 303) within the U.S. Department of Agriculture, Agricultural Research Service is dedicated to research across multiple fields in plant pathology. This review article highlights the research impact within NP 303 between 2015 and 2020, including case studies on wheat and citrus diseases and the National Plant Disease Recovery System, which provide specific examples of this impact.

Molecular and Alkaloid Characterization of Claviceps purpurea Sensu Lato From Grass Seed Production Areas of the U.S. Pacific Northwest

Ergot, caused by Claviceps purpurea sensu lato, is an economically important seed replacement disease of Kentucky bluegrass (Poa pratensis) and perennial ryegrass (Lolium perenne) seed crops. C. purpurea sensu stricto is considered the primary Claviceps species responsible, but genetic diversity and cryptic species within C. purpurea sensu lato have previously been reported. Fifty-six C. purpurea sensu lato isolates collected from P. pratensis (n = 21) and L. perenne (n = 35) in Oregon and Washington between 2010 and 2014 were characterized via random amplified polymorphic DNA (RAPD), partial internal transcribed spacer (ITS), β-tubulin and elongation factor-1α (EF-1α) sequences, conidial size, and ergot alkaloid chemotype. Based on RAPD analysis, seven isolates from P. pratensis and 33 isolates from L. perenne collected in Oregon corresponded to C. purpurea sensu stricto, and 13 isolates collected from P. pratensis in Washington and Oregon were identified as C. humidiphila. Partial ITS, β-tubulin, and EF-1α sequences identified 10 isolates from P. pratensis as C. humidiphila, and seven isolates from P. pratensis and 33 isolates from L. perenne were identified as C. purpurea sensu stricto. Several isolates generated ambiguous RAPD bands or sequences that prevented identification. Ergot alkaloid chemotype profiling found that ergocornine and its epimer were predominant in sclerotia from P. pratensis, whereas ergotamine and its epimer were most abundant in sclerotia from L. perenne. This study confirms the presence of the C. purpurea sensu lato species complex in the U.S. Pacific Northwest and suggests that more research is needed to characterize and mitigate Claviceps spp. infection of grass seed crops in North America.

Detection and Quantification of Airborne Claviceps purpurea sensu lato Ascospores from Hirst-Type Spore Traps using Real-Time Quantitative PCR

The U.S. Pacific Northwest states of Oregon and Washington are major producers of cool-season grass seed. Ergot, caused by fungi in the Claviceps purpurea sensu lato group, is an important seed replacement disease of grass worldwide. Microscopic methods that are currently used to quantify airborne Claviceps ascospores captured by spore traps are not currently rapid enough to allow for detecting and reporting of spore numbers in a timely manner, hindering growers from using this information to help manage ergot. We developed a SYBR Green real-time quantitative polymerase chain reaction (qPCR)-based assay for fast and efficient detection and quantification of C. purpurea sensu lato ascospores from Hirst-type spore traps. Species-specificity of the qPCR assay was confirmed against 41 C. purpurea sensu lato isolates collected from six hosts and six other Claviceps spp. Significant relationships were observed between cycle threshold (Ct) values and standard curves of serial dilutions of DNA ranging from 1 pg to 10 ng (R² = -0.99; P = 0.0002) and DNA extracted from a conidial suspension representing 8 to 80,000 conidia (R² = -0.99; P = 0.0004). Ct values from qPCR were significantly correlated with results from microscopic examination of spore trap samples from the field (r = -0.68; P < 0.0001) and the procedure was able to detect a single ascospore from spore trap tape samples. The qPCR procedure developed in this study provided a means for quantifying airborne Claviceps ascospores that was highly specific and useful over a wide range of spore densities, and could be performed in a matter of hours instead of days. The qPCR assay developed in this study could be part of an integrated pest management approach to help grass seed growers make risk-based fungicide application decisions for ergot management in grass grown for seed.

Has Selection for Grain Yield Altered Intermediate Wheatgrass?

Perennial grains are demonstrating a greater probability of occupying land currently dedicated to other agricultural production. Arable land that is currently in use for forage or annual crop production becomes utilized. Breeding materials for the introduction of perennial grains directly into the human food chain has required utilizing existing plant materials in the domestication of species or manufacturing diverse crosses to introduce perenniality into existing crops. In the domestication of intermediate wheatgrass (Thinopyrum intermedium (Host), Barkworth and Dewey), existing forage cultivars or plant accessions were used to develop populations selected for grain production. A comparison of Cycle 3 materials from The Land Institute (TLI), Salina, KS, USA to USDA-Germplasm Resources Information Network (GRIN) accessions took place under space-planted field conditions at Carman, MB, Canada from 2011 to 2014. One hundred plants (75 TLI and 25 GRIN identified in May 2012) were followed through three seed harvests cycles with phenological, morphological and agronomic traits measured throughout. Selection for seed productivity (TLI materials) reduced the importance of biomass plant−1 on seed yield plant−1, leading to an increase in harvest index. Principal component analysis demonstrated the separation of the germplasm sources and the differential impact of years on the performance of all accessions. Path coefficient analysis also indicated that plant biomass production was of less importance on seed yield plant−1 in the TLI materials. Analysis removing area plant−1 as a factor increased both the importance of biomass and heads on seed yield cm−2 in the TLI materials, especially in the first two seed production years. Plant differences due to selection appear to have reduced overall plant area and increased harvest index in the TLI materials, indicating progress for grain yield under selection. However, a greater understanding of the dynamics within a seed production field is needed to provide insight into the development of more effective selection criteria for long-term field level production.