Submitted to: Plant Disease
Publication Type: Peer reviewed journal
Publication Acceptance Date: 3/23/2005
Publication Date: 7/20/2005
Citation: Paulitz, T.C., Schroeder, K.L. 2005. A new method for the quantification of rhizoctonia solani and rhizoctonia oryzae from soil. Plant Dis. 89:767-772. Interpretive Summary: Rhizoctonia solani and R. oryzae are soilborne pathogens that infect the roots of wheat and other small cereals in the Pacific Northwest, causing Rhizoctonia bare patch and root rot. They are difficult to isolate from roots and even more difficult to quantify from soil, because the low inoculum densities preclude dilution plating. We developed a method for quantification of R. solani and R. oryzae from soil samples, based on a toothpick baiting method and plating on a selective medium. This method was tested in three field soils, by adding known levels of inoculum and measuring the number of colonies from the toothpicks. This relationship was logarithmic. This method was also tested in soil from Rhizoctonia bare patches, and showed the hyphal activity of R. solani, but not R. oryzae, was higher in the center of the patches than outside of the patches. This simple and inexpensive technique can be used for detection and diagnosis in grower fields and to study the ecology and epidemiology of Rhizoctonia spp.
Technical Abstract: Rhizoctonia solani anastamosis group (AG) 8 and R. oryzae are important root pathogens on wheat and barley in the dryland production areas of the inland Pacific Northwest. R. solani AG-8 is difficult to isolate from root systems and quantify in soil because of slow growth and low population densities. However, both pathogens form extensive hyphal networks in the soil and can grow a considerable distance from a food base. A quantitative assay of active hyphae was developed, using wooden toothpicks as baits inserted into sample soils. After 2 days in soil, toothpicks were placed on a selective medium, and the numbers of colonies that grew after 24 h were counted under a dissecting microscope. R. solani and R. oryzae could be distinguished from other fungi based on hyphal morphology. This method was tested in natural soils amended with known inoculum densities of R. solani AG-8 and R. oryzae. Regressions were used to compare the inoculum density/toothpick colonization curves to a predicted curve based on the volume of the toothpicks. The slopes and y-intercept of log-log transformed regressions did not differ from the predicted curves in most cases. This technique was used to assess the hyphal activity of R. solani AG-8 and R. oryzae from soil cores taken from various positions in and around Rhizoctonia bare patches at two locations. Activity of R. solani was highest in the center and inside edge of the patch, but there was no effect of patch position on R. oryzae. This simple and inexpensive technique can be used for detection and diagnosis in grower fields and to study the ecology and epidemiology of Rhizoctonia spp.