Immunology of the pathogen virulence and phytotoxin production in relation to disease severity: a case study in sheath blight of rice.

Polyclonal antibodies against purified Rhizoctonia solani toxin obtained from infected rice sheath tissues (sheath blight toxin, SBT) and culture filtrates (culture filtrate toxin, CFT) were developed in rabbit and chicken. The IgG was isolated from serum and egg yolk of rabbit and chicken, respectively, and their specificity was investigated by indirect ELISA. Antibodies developed against CFT and SBT in rabbits exhibited relatively higher titer values when compared to chicken antibodies. Positive correlation was observed between the degree of sheath blighting and the levels of antigens induced by each isolate during sheath blight symptom development as detected by rabbit SBT antibody and the isolate RS7 was identified as most virulent. Optimization of incubation period for maximum toxin production in liquid medium and rice sheaths indicated that the production of CFT and SBT is maximum after 15 d and 6 d of pathogen inoculation. Studies of the possible translocation of RS-toxin in rice plants upon inoculation with R. solani showed downward translocation as detected by rabbit/chicken SBT antibodies. Since plant inoculation required a higher concentration of inoculum and maintenance of plants, serological assay by ELISA is more sensitive than whole-plant assays in detecting RS-toxin, with the advantage that ELISA also allows rapid determination of RS-toxin production.