Screening Oryza Species Plants for Rice Sheath Blight Resistance

Sheath blight of rice (Oryza sativa L.) is a major rice disease throughout the world including the southern United States. This disease, which is caused by Rhizoctonia solani Kühn (teleomorph: Thanatephorus cucumeris (Frank) Donk), was first reported in 1910 and now occurs throughout temperate and tropical production areas, being most prominent where rice is grown under intense, high-fertility production systems. Initial symptoms consist of lesions on the sheaths of lower leaves at late tillering or early internode elongation growth stages. Under favorable conditions of low sunlight, high humidity (≥5%), and warm temperature (28 to 32°C), the infection spreads rapidly by means of runner hyphae to upper plant parts, including leaf blades and adjacent plants. Lesions may coalesce to encompass the entire leaf sheath and stem (13,21). The fungus affecting rice belongs to anastomosis group AG-1, intraspecific group I-A of R. solani (21). R. solani isolates used in screening for reaction to sheath blight are obtained from infected rice plants, and differences in virulence of R. solani isolates have been noted by measuring the lesion size, but no accepted system of differentiating the isolates pathogenic to rice has been reported to date (4). Molecular differences in isolates have been characterized using both restriction fragment length polymorphism (RFLP) (14) and polymerase chain reaction (PCR)based markers (20). To date, only partial resistance to rice sheath blight has been identified, as evidenced by a survey of 6,000 rice cultivars from 40 countries from which no cultivar exhibiting a major gene for rice sheath blight resistance was identified (8). More recently, quantitative trait loci (QTL) analysis identified six QTLs associated with sheath blight resistance on 6 of the 12 rice chromosomes, but only one QTL appeared to be independent of plant height, a morphological trait associated with sheath blight resistance (14). Additional research suggests it is feasible to identify major genes conferring high levels of partial resistance (19), pyramid these genes, and achieve nearly complete sheath blight resistance. Sheath blight resistance also was identified in the Oryza spp., O. minuta J.S. Presl. ex C.B. Presl. (1) and O. officinalis Wall ex Watt (12), and transferred into cultivated rice through backcrossing. These studies indicate Oryza spp. are an important source of sheath blight resistance genes, and transferring these genes into rice cultivars adapted to the production area in the southern United States is an important disease management strategy. Most testing for sheath blight resistance is done in the field for rice breeding programs. Reported methods of evaluating rice sheath blight resistance are based on field ratings done at internode elongation or later, when the plants are in the reproductive growth stage (23). However, this is problematic for most wild relatives of rice (Oryza spp.) because these species often are considered weeds and some are noxious weeds in the United States (26). In addition, most wild Oryza spp. drop mature seed to the soil and may have an “open” growth habit or morphology not conducive to sheath blight infection under field conditions. Also, few progeny are obtained from crosses between cultivated rice and wild Oryza sp.; therefore ratoon tillers of individual plants need to be evaluated in early generations. Ratoon tillers, tillers which develop after the original tillers are cut off, are used so the panicle morphology can be observed and additional backcrosses can be made. For these reasons, evaluating the Oryza spp. and their early generation progeny is more practical in the growth chamber (or greenhouse). The objectives of this study were to (i) determine how well sheath blight disease ratings from a growth chamber inoculation method compared with field ratings of southern rice cultivars using primary or secondary tillers and ratoon tillers and (ii) evaluate a growth chamber method for assessing sheath blight resistance using ratoon tillers of Oryza spp. and their F1 progeny from crosses with cultivated rice.