Submitted to: Rice Technical Working Group Meeting Proceedings
Publication Type: Proceedings
Publication Acceptance Date: December 1, 2003
Publication Date: June 1, 2004
Citation: Shank, A.R., McClung, A.M., Fjellstrom, R.G. 2004. Development of improved methods for sheath blight evaluation. In: Rice Technical Working Group Meeting Proceedings, February 29-March 4, 2004, New Orleans, LA. 2004 CDROM. Technical Abstract: Sheath blight (SB) (caused by Rhizoctonia solani) is one of the most destructive diseases of rice in the world. Although screening nurseries are used by breeders to select for SB tolerant breeding lines, this requires a fair amount of seed and is usually performed only in advanced breeding generations. Development of molecular markers that are linked to SB resistance genes would allow marker assisted selection to be performed during earlier generations using a trivial amount of leaf material; as has been demonstrated with other disease resistance and cereal quality characters. For this technology to be developed, clear and repeatable screening methods are needed to accurately associate phenotypic resistant/susceptible ratings with marker polymorphisms. Previous research has shown that expression of SB disease symptoms is influenced by plant height and maturity. In order to circumvent interactions between adult-plant characters and the expression of sheath blight resistance under field conditions, alternative methods using greenhouse and laboratory evaluations of seedlings and detached plant parts were tested. Rice varieties that had been categorized based upon their field reaction to SB were evaluated: Susceptible - Dixiebelle, Lemont, and Rosemont; Intermediate - Saber, Priscilla, and Dragon Eyeball 100; and Resistant - TeQing, Jasmine 85, Pecos, and BR10 (PI 574667). SB inoculum was grown on PDA plates from which plugs or strips of mycelia were cut and placed at the base of the seedlings. Two-, four-, and six-week old seedlings that had been grown in flats in rows 5 cm apart were evaluated in a mist chamber after inoculation with the PDA strips. In addition, small clusters of 2-, 4- and 6-week old seedlings were evaluated in 10 cm pots that were covered by 1) 2-liter clear plastic soda bottles with the cap removed and the bottom cut off, 2) soda bottles with top and bottom cut off, and 3) tubular mylar cylinders. Detached plant parts were evaluated using Nunc bioassay and the mycelia-PDA plugs. Plant parts included 1) detached 2nd, 4th and 6th greenhouse-grown seedling leaves, 2) flag-2, flag-1, and flag field-grown leaves, and 3) detached stems from greenhouse-grown plants at the 6th leaf stage and from field-grown plants at the flowering stage. The most definitive results were obtained using seedlings grown under soda bottle containers that were evaluated seven days after inoculation for amount of necrotic lesions. Testing multiple cultivars from different disease classes under the same bottle covering resulted in confounded results due to leaf contact among the resistant and susceptible cultivars. However, differentiation was sufficient to discriminate levels of resistance/susceptibility among disease classes which suggests that this method could be valuable for scoring segregating populations.