Quantification of the Components of Resistance to Rice Sheath Blight Using a Detached Tiller Test Under Controlled Conditions

Abstract

Resistance of rice (Oryza sativa) to sheath blight, caused by Rhizoctonia solani, is quantitative and involves two mechanisms: physiological resistance and disease escape. The epidemiological concept of components of resistance was applied using a detached tiller method under controlled conditions, to specifically address physiological resistance to sheath blight in rice. A sclerotium was inserted below the leaf collar of individual rice tillers maintained in tubes filled with water. Different variables were measured after incubation: number of lesions, lesion length, vertical sheath colonization, presence or absence of dark margin at the edge of lesions, and survival duration of the leaf blade. Several rice varieties reported to have different levels of susceptibility to sheath blight were assessed, together with varieties that are cultivated over large areas. Although numerical differences between rice varieties were observed for all disease variables, only the number of lesions significantly differed among varieties tested in this study. The varieties Pecos and IR64 had the consistently lowest and highest disease intensities, respectively. This methodology may allow the detection of sources of resistance that specifically involve defense mechanisms. When combined with field assessment, this methodology should also enable to quantitatively assess the relative role of both mechanisms of resistance to sheath blight.

Phenotyping of Partial Physiological Resistance to Rice Sheath Blight

“…The following disease variables were measured for each tiller: number of lesions; disease severity was assessed according to a scale with the following grades: 0, 1, 5, 10, 20 … 80, 90 and 100% (Willocquet et al. ); vertical sheath colonization (VSC) (distance between the lowest and highest lesions on the sheaths); mean lesion length (mean length of all lesions). Stem length was also measured.…”

“…The number of lesions in Tetep was also the lowest when compared to TeQing, Dudruchi, Betichikon, Khato‐Chalani and Swarna, when several rice varieties were compared using the detached tiller test (Willocquet et al. ). QTLs resistant to sheath blight have also been identified in the population derived from resistant cultivar Tetep and susceptible cultivar HP2216 (Channamallikarjuna et al.…”

“…Twelve rice genotypes including four mega varieties with different levels of resistance/susceptibility were evaluated for quantitative measurement of partial physiological resistance to sheath blight under controlled conditions using detached tiller test (Willocquet et al. ). Genotypes Tetep (Sha et al.…”

A strategy to identify sources of quantitative resistance in pathosystems involving disease escape and physiological resistance: the case study of rice sheath blight

“…This isolate produces typical ShB symptoms on sheath and leaves, as well as typical mycelial growth and sclerotia production. LR‐1 has been used in previous studies and is representative of isolates originating from the Philippines (Willocquet et al ., , ).…”

“…These components are affected by quantitative host plant resistance, as well as by morphological traits, which can be associated with disease escape (Srinivasachary et al ., ; Willocquet et al ., ). Detached tillers assays were also used in order to measure the components of physiological resistance to the disease under conditions where plant or canopy morphology do not come into play (Willocquet et al ., ). The third component is complementary multivariate and logistic regression analyses to: (i) identify groups of genotypes according to different traits related to quantitative resistance (disease intensification, disease spread, physiological resistance and morphology), (ii) assess the relationships between the different traits measured, (iii) identify predictors for quantitative resistance, and (iv) identify genotypes which can be used as sources of quantitative resistance.…”