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

Abstract

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.

Characterization and report of binucleate Rhizoctonia AG‐Fb affecting soybean in Brazil

“…This scenario is supported by another study that demonstrated a higher prevalence of binucleate Rhizoctonia isolates in natural pasture environments compared to cultivated settings (Kodati et al, 2021). This is the first report of binucleate Rhizoctonia AG-Fb as a pathogen causing root rot in soybean in southern Brazil.…”

“…The isolate of R. solani AG-4 was more aggressive than binucleate Rhizoctonia AG-Fb, which reduced the germination of soybean seeds. Differences in aggressiveness among species and isolates from the same species or AG have been previously reported (Kodati et al, 2021), which demonstrates the wide variability that can be found among organisms belonging to the Rhizoctonia genus. In maize, multinucleate isolates of Rhizoctonia were more aggressive followed by binucleate isolates with moderate aggressiveness and, lastly, uninucleate isolates with low aggressiveness (Zhou et al, 2016).…”

“…After carrying out a survey of the reports of binucleate Rhizoctonia from different AG and host plants (Appendix S4) it was found that, until now, binucleate Rhizoctonia AG‐F was reported as pathogenic to soybean in a study in the United States with strains obtained from prairie grasses (Kodati et al., 2021). Isolates of binucleate Rhizoctonia AG‐Fa have already been reported as causal agents of diseases in common bean ( Phaseolus vulgaris L.) and maize ( Zea mays L.) in Brazil (Blanco & Costa, 2018); common bean (Canpolat et al., 2023), cauliflower ( Brassica oleracea L. var.…”

“…Rhizoctonia solani is the main species of the genus that has been reported in soybean and can cause damping-off, root rot or shoot blight (Basseto et al, 2007) and it can infect soybean plants at any stage of development (Rahman et al, 2020). However, other Rhizoctonia species can also affect soybean, such as binucleate Rhizoctonia, which can cause root rot as already reported in the United States (Kodati et al, 2021). Binucleated Rhizoctonia isolates have been classified into 18 AG, which are identified with a letter ranging from AG-A to AG-W (Aiello et al, 2017).…”

“…Coeficiente de variação (%) 7.55% 9.56% United States (Kodati et al, 2021). In two other studies where the pathogenicity of binucleate Rhizoctonia in soybean was reported, the AG were not identified (Nelson et al, 1996).…”