Identification Of Rhizoctonia Species PDF Download

Rhizoctonia-like organisms include soilborne fungi that live as saprophytes, necrotrophic pathogens with varying host ranges and aggressiveness, and symbiotic mycorrhizae. Accurate identification of species of Rhizoctonia causing disease is important for management recommendations. Research in Chapter 2 was conducted to identify the diversity of Rhizoctonia spp. from soybean growing regions, evaluate their aggressiveness on soybean and cross-pathogenicity to other hosts. A total of 139 isolates were collected and identified using morphology and gene sequencing. Results showed R. solani AG-4 and R. zeae were most abundant. Interestingly, R. zeae isolates were most aggressive to soybean when using higher temperatures for in planta evaluations compared to those used for R. solani. Reported mostly as a pathogen of turf grasses, R. zeae was not previously considered an aggressive pathogen on row crops. In addition to row crops, a significant amount of land in Nebraska is comprised of Sandhills and yet information on pathogen diversity is lacking. Thus, Chapter 3 of this dissertation focused the diversity and pathogenicity of Rhizoctonia spp. from the Sandhills. Most Rhizoctonia spp. isolated from the Sandhills (12 of 17 the isolates) were binucleate Rhizoctonia. Isolates were pathogenic on native grasses (sand bluestem and needle-and-thread) and soybean. The Sandhills grasslands was shown to harbor a unique composition of Rhizoctonia spp. that have potential to cause disease on soybean when this area is converted to cultivated land. In Chapter 4, GC-MS analysis was performed on extracts of R. zeae and their phytotoxic activity on soybean leaves was evaluated. Diverse functional groups were identified and some compounds in functional groups like pyridine, furan, fatty acid, sugar acids were identified only in the extracts of aggressive isolates. Phytotoxic activity of these extracts in detached leaf assay was positively correlated with aggressiveness of the isolates evaluated on seedlings, suggesting these exudates contain phytotoxins and can be used in the future for in vitro screening of soybean breeding lines. Collectively, information generated by the research in this dissertation is important for advancing our knowledge of Rhizoctonia diseases.