Assessment of Spring Wheat Genotypes for Disease Reaction to AG-8 in Controlled Environment and Direct-Seeded Field Evaluations

Abstract

Rhizoctonia root rot, caused by Rhizoctonia solani Kühn AG-8 (Anastomosis Group 8), is a yield limiting disease of direct-seeded cereal grains. The objectives of this study were to (i) determine whether spring wheat (Triticum aestivum L.) genotypes vary in susceptibility to Rhizoctonia root rot in inoculated field trials; and (ii) evaluate whether disease ratings obtained by controlled environment (CE) analyses are predictive of disease ratings or genotype performance in the field. Twenty-one spring wheat genotypes were evaluated for two crop years in a split-plot field study with high and low levels of disease pressure. The high-inoculum (high) treatment consisted of plots planted the previous fall with a 3:1 mixture of winter wheat and oat (Avena sativa L.) grains colonized by the pathogen. The low-inoculum (low) treatment was neither inoculated nor planted with a green bridge host the previous fall. Genotypes also were assayed for disease reaction in controlled environment (CE) assays. Average disease ratings of field entries in the high treatment were 2.1 times greater than those from the low treatment (P < 0.05). The yield average of entries in the high treatment was 87% of that of entries in the low 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 compared with the low treatment. Test weight was not affected by inoculum level. A statistically significant, negative association (P < 0.0001) between disease rating and grain yield was detected, and variation for reaction to R. solani was detected among genotypes. Disease reaction differences were not detected among genotypes in CE assays, and field and CE disease ratings were not correlated (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.