A New Method for the Quantification of Rhizoctonia solani and R. oryzae from Soil

Area of Science:

• Plant Pathology
• Soil Microbiology
• Agricultural Science

Background:

• Rhizoctonia solani anastomosis group (AG) 8 and R. oryzae are significant root pathogens affecting wheat and barley in the inland Pacific Northwest.
• Current methods for detecting R. solani AG-8 are challenging due to its slow growth and low soil population densities.
• Both pathogens form extensive hyphal networks, enabling them to spread considerable distances from their food source in the soil.

Purpose of the Study:

• To develop a quantitative assay for measuring active hyphae of R. solani AG-8 and R. oryzae in soil.
• To validate the developed assay by comparing its results with known inoculum densities.
• To assess the distribution and activity of these pathogens in relation to Rhizoctonia bare patches in agricultural fields.

Main Methods:

• A novel method using wooden toothpicks as bait inserted into soil samples to capture active fungal hyphae.
• Incubation of toothpicks in soil for 2 days, followed by transfer to a selective medium.
• Quantification of R. solani and R. oryzae colonies based on hyphal morphology observed under a dissecting microscope after 24 hours of growth.

Main Results:

• The toothpick assay demonstrated a strong correlation between inoculum density and hyphal colonization, validating its quantitative accuracy.
• The method successfully distinguished R. solani and R. oryzae from other soil fungi based on distinct hyphal characteristics.
• R. solani activity was highest in the center and inner edge of bare patches, while R. oryzae distribution was not influenced by patch position.

Conclusions:

• The developed toothpick assay is a simple, inexpensive, and effective tool for detecting and quantifying active hyphae of R. solani AG-8 and R. oryzae.
• This technique facilitates disease diagnosis in grower fields and provides valuable insights into the ecology and epidemiology of Rhizoctonia spp.
• The findings contribute to better management strategies for root diseases in wheat and barley production systems.