Sources and mechanisms of resistanc

Sources and mechanisms of resistance: Several sources, levels, and mechanisms of resistance in sorghum to the sugarcane aphid comprising germplasm accessions, parental lines (A/B and R lines), agronomic elite lines, hybrids, varieties, and locals have been reported from different countries ( Table 3, Table 4, Table 5 and Table 6). Resistance to insects has been characterized into three components as antixenosis (nonpreference), antibiosis, and tolerance ( Painter, 1951; Horber, 1980; Smith, 1989; Smith et al., 1994). However, antixenosis and antibiosis resistance to M. sacchari in sorghum do not differ with plant age ( Teetes, 1980). Both alate and apterous virginoparous adults showed a stronger tendency of preference to S. bicolor and S. halepense rather than M. sinensis. Tolerance to the sugarcane aphid injury in sorghum increases greatly with slight increase in plant height, and has inherent advantage over antibiosis and antixenosis in that it does not impose selection pressure on aphid populations, and thus may have greater permanence. Setokuchi (1988) reported that M. sacchari failed to establish on resistant lines under field conditions although resistant sources served as suitable hosts in confinement studies, which indicate the involvement of both antixenosis and antibiosis mechanisms. Antixenosis for adult colonization was noticed in TAM 428, IS 1144C, IS 1366C, IS 1598C, IS 6416C, IS 6426C, IS 12661C, and IS 12664C. Among them, IS 1144C and IS 12664C were preferred less than the resistant check, TAM 428. High levels of antibiosis expressed in TAM 428, IS 1144C, IS 5188C, IS 12609C, and IS 12664C for least number of days to reproduction; TAM 428, IS 12609C, and IS 12664C for greater mortality and shorter longevity of adults, and production of fewer or no nymphs ( Teetes et al., 1995).