Origin of agricultural plant pathogens: Diversity and pathogenicity of Rhizoctonia fungi associated with native prairie grasses in the Sandhills of Nebraska

The Sandhills of Nebraska is a complex ecosystem, covering 50,000 km2 in central and western Nebraska and predominantly of virgin grassland. Grasslands are the most widespread vegetation in the U.S. and once dominated regions are currently cultivated croplands, so it stands to reason that some of the current plant pathogens of cultivated crops originated from grasslands, particularly soilborne plant pathogens. The anamorphic genus Rhizoctonia includes genetically diverse organisms that are known to be necrotrophic fungal pathogens, saprophytes, mycorrhiza of orchids, and biocontrol agents. This study aimed to evaluate the diversity of Rhizoctonia spp. on four native grasses in the Sandhills of Nebraska and determine pathogenicity to native grasses and soybean. In 2016 and 2017, a total of 84 samples were collected from 11 sites in the Sandhills, located in eight counties of Nebraska. The samples included soil and symptomatic roots from the four dominant native grasses: sand bluestem, little bluestem, prairie sandreed, and needle-and-thread. Obtained were 17 Rhizoctonia-like isolates identified, including five isolates of binucleate Rhizoctonia AG-F; two isolates each from binucleate Rhizoctonia AG-B, AG-C, and AG-K, Rhizoctonia solani AGs: AG-3, and AG-4; one isolate of binucleate Rhizoctonia AG-L, and one isolate of R. zeae. Disease severity was assessed for representative isolates of each AG in a greenhouse assay using sand bluestem, needle-and-thread, and soybean; prairie sandreed and little bluestem were unable to germinate under artificial conditions. On native grasses, all but two isolates were either mildly aggressive (causing 5–21% disease severity) or aggressive (21–35% disease severity). Among those, three isolates were cross-pathogenic on soybean, with R. solani AG-4 shown to be highly aggressive (86% disease severity). Thus, it is presumed that Rhizoctonia spp. are native to the sandhills grasslands and an emerging pathogen of crops cultivated may have survived in the soil and originate from grasslands.

Stomatal variability of native warm-season grasses from the Nebraska Sandhills

Soil moisture deficit is usually the major limiting factor for herbage production in the Sandhills of Nebraska. We examined inter-population and interspecific variability in stomatal characteristics and drought tolerance in sand bluestem (Andropogon hallii Vitman), little bluestem [Schizachyrium scoparium (Michx.) Nash], prairie sandreed [Calamovilfa longifolia (Hook) Scribn.], and switchgrass (Panicum virgatum L.). Ramets were collected during the dormant season across an aridity gradient from east to west (ranging from 560 mm to 340 mm average annual precipitation) in the Sandhills of Nebraska. Plants were grown in individual pots under greenhouse conditions. Once plants were well established, stomatal characteristics were determined and stomatal conductance (gs) was measured through a dry-down period of no watering. Populations did not differ in stomatal characteristics across the gradient, except for stomatal density on the adaxial leaf surface of prairie sandreed and the abaxial leaf surface of sand bluestem. Leaves of switchgrass and prairie sandreed were amphistomatic (stomata on both leaf surfaces), whereas leaves of little bluestem and sand bluestem were hypostomatic (stomata on the lower leaf surface). In the absence of drought, gs was 17 to 31% higher in little bluestem than in other species. Differences among species in gs were found mainly when watered and disappeared as the dry-down progressed. There was a positive relationship between stomatal density and gs in all species except prairie sandreed. Prairie sandreed maintained 35% of the initial relative water content of its leaves after 17 d of dry-down, compared to 23% for sand bluestem, 14% for switchgrass, and 9% for little bluestem. Variation in stomatal traits within species did not explain ecotypic adaptation to sites with a range of precipitation in the Nebraska Sandhills. Key words: Stomatal characteristics, water stress, warm season grasses, Nebraska Sandhills