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.

Factors Affecting Sugar Accumulation and Fluxes in Warm- and Cool-Season Forages Grown in a Silvopastoral System

Forage management and environmental conditions affect water soluble carbohydrate (WSC) storage, and, in turn, influence ruminant forage utilization in silvopastoral systems. The objective was to determine effects of four dependent variables: forage species [(non-native, C3 (orchardgrass (Dactylis glomerata L.)) and native C4 mix (8:1:1 big bluestem (Andropogon gerardii Vitman), little bluestem (Schizachyrium scoparium Michx. Nash) and indiangrass (Sorghastrum nutans L.))]; fertility (poultry litter and an unfertilized control); forage sampling date (mid-May, late-May, early-June, mid-June, and late-June); and hour of day (0800, 1100, 1400, and 1700 h) on WSC accumulation in a silvopasture. Concentrations of WSC (g kg DM−1) were greater (p ≤ 0.05) for C3 forages, with poultry litter not impacting WSC accumulation. Overall, WSC was greatest in mid-June, with the lowest WSC concentration observed at 0800 compared to 1100, 1400, and 1700 h (p ≤ 0.05). Therefore, harvesting forages later in the day resulted in greater WSC. A stepwise regression model indicated acid detergent fiber, ash, and forage P concentration were the best predictors (R2 = 0.85, p ≤ 0.05) of forage WSC. These results may be useful in future studies aimed at explaining diurnal cattle grazing preference and optimum forage harvest timing in silvopastoral systems.

87 Evaluation of two stocking rates for grazing heifers on mixed sward native warm-season grass pastures in the Black Belt Prairie region

Optimal use of native warm-season grasses in pasture systems involves stocking grazing livestock at suitable rates. The study objective was to evaluate forage nutritive value and heifer ADG at two stocking rates on mixed-sward pastures of big bluestem (Andropogon gerardi Vitman), little bluestem (Andropogon scoparius), and indiangrass (Sorghastrum nutans L.). Pastures (3 replications) were stocked for 56 d during June and July in 2 yr with crossbred (Bos taurus) heifers (n = 24 heifers/year) stratified by initial BW (288.3 ± 1.7 kg) to one of two continuous stocking rates: 1.9 heifers/ha (HIGH) and 1.2 heifers/ha (LOW). Mean forage nutritive values on a DM basis were not different between HIGH and LOW stocking rates, respectively, for CP (7.0 ± 0.2% vs 6.7 ± 0.2%; P = 0.27), ADF (41.0 ± 0.6 vs. 41.4 ± 0.6; P = 0.64), NDF (69.9 ± 0.5 vs. 68.7 ± 0.5; P = 0.09), or relative feed value (RFV) (76.0 ± 1.0 vs. 76.9 ± 1.0; P = 0.53). There was a year effect (P < 0.01) and stocking rate x day effect (P < 0.01) for TDN. At LOW, TDN decreased from day 0 to day 28 (P = 0.02) and day 28 to day 56 (P = 0.02). At HIGH, TDN decreased (P < 0.01) from day 0 to day 28 but remained steady until day 56 (P = 0.21). There was a stocking rate x day interaction (P < 0.01) with ADG: LOW day 28 to 56 (1.20 ± 0.08 kg/day), HIGH day 0 to 28 (0.89 ± 0.08 kg/day), HIGH day 28 to 56 (0.44 ± 0.08 kg/day), and LOW day 0 to 28 (0.30 ± 0.08 kg/day). Further assessment of cattle ADG using more divergent stocking rates and plant persistence measures is warranted to inform ideal native grass stocking rate recommendations.

20 Comparison of native warm-season grasses and bermudagrass for stocker calf grazing in the Black Belt Prairie region

Native grasses are touted for use in drought mitigation strategies for grazing cattle. It is important to determine how these forages compare in specific production environments to more widely used improved grasses such as bermudagrass [Cynodon dactylon (L.) Pers.] that they may replace as pasture. Native warm-season grasses including big bluestem (Andropogon gerardi Vitman), little bluestem (Andropogon scoparius), and indiangrass (Sorghastrum nutans L.) were evaluated against bermudagrass pastures in the Black Belt Prairie region of Mississippi for stocker cattle grazing. Objectives were to compare bermudagrass (BG), indiangrass (IG) and mixed-sward native warm-season grass (mix of big bluestem, little bluestem, and indiangrass) (NGMIX) pasture for forage nutritive value and steer ADG. Crossbred (Bos taurus) steers (n = 36 steers/year) were stratified by initial BW (339.7 ± 4.3 kg) to 2.02-ha pastures (3 replications) during 56-d grazing periods in June and July for 2 yr. Mean forage nutritive values on a DM basis were: BG [8.8% CP, 43.4% ADF, 67.3% NDF, 58.7% TDN, and 77 relative feed value (RFV)], IG (6.7% CP, 39.6% ADF, 68.3% NDF, 58.2% TDN, and 79 RFV) and MIXNG (7.3% CP, 40.5% ADF, 69.5% NDF, 58.1% TDN, and 77 RFV). A forage treatment x day x year effect existed (P < 0.01) for each these nutritive values. There was no effect (P = 0.47) of forage treatment on steer ADG (BG: 0.4 ± 0.1 kg/d; IG: 0.5 ± 0.1 kg/d; NGMIX: 0.5 ± 0.1 kg/d). Thus, no net advantage or disadvantage in steer growth rates was observed due to forage species. Other factors such as forage establishment cost and animal stocking rates supported may be relevant to consider in decisions to replace bermudagrass with warm-season native grasses.

Infection, Survival, and Growth of Clavibacter nebraskensis on Crop, Weed, and Prairie Plant Species

Clavibacter nebraskensis is the causal agent of Goss’s leaf blight and wilt, an important disease of maize in the United States and Canada. The epidemiology and ecology of this bacterial pathogen are poorly understood. Infested maize residue is often considered to be the primary source of inoculum for maize; however, the potential for many other plant species to be infected and serve as inoculum sources is unknown. The goal of this study was to determine if C. nebraskensis could infect, survive, and grow on common weed, crop, and grass species. Seedling leaves of 18 plant species that grow in maize production areas in the United States were inoculated with this pathogen in a controlled environment and in the field. Lesion development, bacterial streaming, and pathogen population size on leaves were then determined and used as criteria to evaluate host–pathogen interactions. Woolly cupgrass (Eriochloa villosa) and the native prairie grasses big bluestem (Andropogon gerardii) and little bluestem (Schizachyrium scoparium) developed lesions and bacterial streaming, identifying them as hosts and susceptible to infection. To our knowledge, this is the first report of these grass species being hosts of C. nebraskensis. Ten other grass species, including wheat and oats, were identified as potential sustaining hosts that maintained epiphytic or endophytic pathogen populations >106 colony forming units per leaf sample but displayed no evidence of infection. Five broadleaf species tested were nonhosts based on the three criteria. This study suggests that multiple plant species support infection and growth of C. nebraskensis and further elucidates the ecology of this pathogen and the epidemiology of Goss’s wilt.