Abstracts of Selected Papers Relating to Rhizoctonia Root Rot

Smiley, R.W., and D.E. Wilkins. 1992. Impact of sulfonylurea herbicides on Rhizoctonia root rot, growth, and yield of winter wheat. Plant Disease 76:399-404.

Chlorsulfuron and metsulfuron-methyl were examined for their potential to predispose winter wheat to Rhizoctonia root rot in eastern Oregon. Soil treated or not treated with chlorsulfuron in the field was collected as intact cores, infested with Rhizoctonia solani AG-8, R. oryzae, or neither, and evaluated in the greenhouse for effects on disease severity, growth, and development of winter wheat. Seedlings in chlorsulfuron-treated soil had more severe root rot and reduced growth, as compared to those in non-treated soil. Eight field experiments were performed at two root-rot affected sites to examine the effect of pre-plant and post-emergence applications of chlorsulfuron on tilled and non-tilled soil. These studies were performed over two crop seasons, and chlorsulfuron + metsulfuron methyl was included during 1 yr. Seedlings always had more severe Rhizoctonia root rot and grew less vigorously in plots treated with sulfonylurea herbicides. With time, however, the negative impacts dissipated and herbicide-treated plots had grain yields that differed from non-treated plots in only 1 of 8 experiments. Yield was increased by sulfonylurea herbicides in one experiment and this was apparently not related entirely to differences in weed populations. Although sulfonylurea herbicides interacted with Rhizoctonia species to damage roots of winter wheat seedlings, this did not have a direct relationship with the yield of grain under the conditions studied.

Smiley, R. W., A.G. Ogg, Jr., and R.J. Cook. 1992. Influence of glyphosate on Rhizoctonia root rot, growth and yield of barley. Plant Disease 76:937-942.

Time intervals between application of glyphosate to kill volunteer cereals and weeds and then planting spring barley by direct drilling (no-till) into Rhizoctonia-infested soil were evaluated in field plots at Pendleton, OR and Lacrosse and Lind, WA. As the interval was shortened from autumn to spring application, or from 3 wk to 3 days before planting in the spring, Rhizoctonia root rot severity increased and grain yield decreased. When glyphosate applications were delayed until 2 to 3 days before planting (commonly used in production of spring barley in the Pacific Northwest) spring barley yields were reduced as much as 50% compared to when glyphosate was applied in the autumn or early spring. Disease was not as prevalent when glyphosate was applied 1 or 2 days after direct drilling compared to applications made 3 days before planting. Rhizoctonia root rot was least on spring barley when tillage or application of glyphosate was performed in the autumn or in spring 3 wk before planting. Tilling soil 2 days before planting at one site nullified the yield-depressing affect of a preplant glyphosate application. These results suggest that the inoculum potential for R. solani AG-8 as a pathogen of spring barley is strongly influenced by the timing of volunteer cereal and weed elimination, and that adjustments in such practices can minimize crop damage and maximize yield.

Smiley, R.W., and W. Uddin. 1993. Influence of soil temperature on Rhizoctonia root rot (R. solani AG-8 and R. oryzae) of winter wheat. Phytopathology 83:777-785.

Rhizoctonia solani AG-8 and R. oryzae are associated with Rhizoctonia root rot of cereals in the northwestern U.S.A. These pathogens were evaluated in the greenhouse and field for effects on disease severity and growth, development, and yield of winter wheat. Experiments in the greenhouse were performed either with pasteurized soil mixture or intact cores of natural soil collected from the field. Soil temperatures were controlled at diurnal cycles of 27/16, 23/11, or 19/6 C, or experiments were performed at ambient temperatures of 28/15 C. Inocula of the Rhizoctonia species were also placed in a band below winter wheat seed in the field. R. solani caused moderate to severe disease and reduced plant growth and development in all six greenhouse and field tests. R. solani in natural soil caused more severe root rot at low than at high temperature. R. oryzae caused slight to moderate root rot in natural soil and severe root rot in pasteurized soil at high temperature, but never significantly (P<0.05) suppressed plant growth or development. Grain yields for winter wheat growing in soil infested with R. solani, R. oryzae, or neither pathogen were 624, 8724, and 9444 kg/ha, respectively, during one field test. The yield component most closely associated with root rot during both field seasons was tiller development. We conclude that R. solani AG-8 is the principal incitant of Rhizoctonia root rot on winter wheat in the Pacific Northwest.

Lucas, P., R.W. Smiley, and H.P. Collins. 1993. Decline of Rhizoctonia root rot on wheat in soils infested with Rhizoctonia solani AG-8. Phytopathology 83:260-265.

Soils collected from two sites were used for up to six successive plantings of wheat in the greenhouse. Soils were infested before each of the first four plantings with Rhizoctonia solani AG-8 or R. oryzae, or left noninfested. R. solani stunted seedlings during the first planting. After a second or third crop, depending upon the soil origin, shoot weights were significantly higher in infested soils than in controls. Shoot growth was never suppressed when soils were infested with R. oryzae. Disease suppressiveness tests performed during a fifth crop demonstrated that successive plantings of wheat into soils infested with R. solani AG-8 caused a decline of the disease. Both a susceptible host (wheat) and virulent pathogen (R. solani AG-8) were necessary to achieve disease suppressiveness. In contrast, R. oryzae did not induce soil suppressive to R. solani. Induction of Rhizoctonia root rot decline occurred earlier in tilled than non-tilled soil, and rates of applied nitrogen had little effect on the decline process.

Smith, J.D., K.K. Kidwell, M.A. Evans, R.J. Cook, and R.W. Smiley. 2003. Evaluation of spring cereal grains and wild Triticum germplasm for resistance to Rhizoctonia solani AG-8. Crop Science 43:701-709.

Rhizoctonia root rot, caused by Rhizoctonia solani Kühn AG-8 (Anastomosis Group 8), is a yield-limiting disease of direct-seeded cereals in the Pacific Northwest (PNW) region of the USA, and to date, no resistant Triticum germplasm has been identified. The objective of this research was to identify potential sources of genetic resistance among selected members of the primary, secondary and tertiary gene pools of wheat (Triticum aestivum L.) for use in cultivar improvement. Members of the primary gene pool (spring wheat germplasm, synthetic hexaploids, triticale (X Triticosecale spp.), Triticum turgidum var. durum (Desf.) Bowden [= T. turgidum subsp. durum (Desf.) Bowden], secondary gene pool [Aegilops cylindrica Host., Dasypyrum villosum (L.) P. Candargy], and tertiary gene pool (Hordeum vulgare L.) of wheat were screened for disease response to two isolates (C1 and D2) of R. solani AG-8 in controlled environment assays. Variation for magnitude of susceptibility to both isolates was detected, but no sources of resistance were identified among primary or tertiary gene pool members. Isolate D2 generally produced more severe disease than isolate C1, with the exception of the most susceptible accessions for which both isolates caused equally severe disease. All accessions of D. villosum exhibited some level of seedling resistance to isolate C1, and 73% also displayed a resistance response to isolate D2. All D. villosum/durum amphiploids, as well as Chinese Spring/D. villosum addition lines, were susceptible to both isolates. Differences in disease response between the two isolates suggest that varying levels of virulence, or aggressiveness, exist among R. solani AG-8 isolates.

Smith, J.D., K.K. Kidwell, M.A. Evans, R.J. Cook, and R.W. Smiley. 2003. Assessment of spring wheat genotypes for disease reaction to Rhizoctonia solani AG-8 in controlled environment and direct-seeded field evaluations. Crop Science 43:694-700.


Rhizoctonia root rot, caused by Rhizoctonia solani Kühn AG-8, is a yield limiting disease of direct-seeded cereal grains. Twenty-one adapted spring wheat (Triticum aestivum L.) genotypes from the U.S. Pacific Northwest were evaluated for two crop years in a split-plot field study with high and low levels of pressure from Rhizoctonia root rot. The high-inoculum treatment consisted of plots planted the previous fall with a 3:1 mixture of winter wheat and oat grains colonized by the pathogen, followed by a pre-plant, non-selective herbicide application 6 to 10 days before planting the following spring. The low-inoculum treatment was neither inoculated nor planted with a green bridge host the previous fall, and received the non-selective herbicide application in the fall, as well as prior to planting in the spring. Genotypes also were assayed for disease reaction in controlled environment (CE) assays. The objectives of this study were to (1) determine whether adapted spring wheat genotypes vary in susceptibility to Rhizoctonia root rot in inoculated field trials; and (2) evaluate whether disease ratings obtained by CE analyses are predictive of disease ratings or genotype performance in the field. Average disease ratings of field entries in the high-inoculum treatment were 2.1 times greater than those from the low-inoculum treatment (P< 0.05), however, disease pressure was low in both years. The yield average of entries in the high-inoculum treatment was 87% of that of entries in the low-inoculum treatment (P< 0.001). Adult plant heights also were 5 cm shorter (P< 0.001), heading date was slightly delayed (P< 0.05), and grain protein content decreased slightly (P< 0.05) in the high-inoculum compared to the low-inoculum treatment. However, test weight was not affected by inoculum level. A statistically significant, negative association (r=-0.6, P< 0.0001) between disease rating and grain yield was detected. Based on significant grain yield differences between entries in the high- and low-inoculum treatments, variation for reaction to R. solani exists among adapted spring wheat germplasm. Even though yield rankings among genotypes did not generally shift dramatically between inoculum treatments, several genotypes were more responsive to pathogen pressure than others. As a group, soft white genotypes appeared to be more severely impacted by the high-inoculum treatment than were hard red genotypes. Disease reaction differences were not detected among genotypes in CE assays, and field and CE disease ratings were not correlated (r=0.23, P =0.32), indicating that growth chamber assays were not prognostic of genotype performance in response to pressure from Rhizoctonia root rot in the field.

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