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Wheat resistance to head scab is an inheritable trait, but the number of genes controlling resistance is still controversial. Some investigators have hypothesized that resistance to WHS is controlled by many minor genes, but others have provided evidence for the existence of genes with major effects (Chen, 1983; Wu and Shen, et al., 1984; Liao and Yu, 1985; Li and Yu, 1988; Bai and Xiao et al., 1989; Chen, 1989; Bai and Zhou et al., 1990). Generation mean analysis of WHS resistance in crosses between resistant cultivars and susceptible cultivars indicated that, in some cultivars, WHS resistance is controlled mainly by additive gene effects, but non-additive gene effects might also be significant. Within the non-additive components, dominance appears to be the most important (Chen, 1983; Jiang and Wu, 1989; Chen and Zhang, 1989; Bai and Zhou et al., 1990; Wang and Wang et al., 1992). Heritability of WHS resistance has been reported from 28% to 86%, varying with crosses and experiments (Liao and Yu, 1985; Jiang and Wu, 1989; Chen and Zhang et al., 1989; Zhang and Wang et al., 1990; Wang and Wang, 1992).

Monosomic analysis assigned WHS resistance genes to different chromosomes in eight wheat cultivars (Table 1) (Liao and Yu, 1985, Yu, 1990, Yu, 1991). Among the eight cultivars analyzed, from two to nine chromosomes in each cultivar were reported to have WHS resistance (Table 1) (Yu, 1991). In most cases, when compared with moderately resistant cultivars by monosomic analysis, highly resistant cultivars seemed to have more resistance genes and fewer susceptibility genes (Yu, 1991).

^a Data from Yu (1991b)
^b WSB: Wangshuibai; SM3: Sumai 3; WZHHS: Wenzhouhongheshang; PHJZM: Pinghujianzimai; HUDTM: Honghudataibao; CYHM: Chongyanghongmai; YGFZ: Yangangfangzhu; WN2: Wannian2
^c R: resistance; S: susceptibility
^d (*) indicates significant difference at P=0.05; (**) indicates significance at P=0.01

Monosomic analysis is based on WHS resistance performance of F₂ plants. Since WHS resistance is a quantitative trait, WHS evaluation of a single plant may not always be repeatable. In addition, heterozygosity in F₂ plants may complicate WHS evaluations. Homozygous lines may provide more informative data than those from F₂ plants. A series of substitution lines with Sumai 3 as the donor parent and the cultivar Chinese Spring as the recipient parent has been developed. When tested in the field, the substitution line with chromosome 7A from Sumai 3 showed the same level of WHS resistance as Sumai 3 (Yao and Ge et al., 1997). Three substitution lines with chromosomes 2B, 3B, and 6B of Sumai 3 also had lower scab ratings than Chinese Spring, the recurrent parent. Chromosome 2D of Sumai 3 increased the head scab spread rate (Yao and Ge et al., 1997). To obtain more detailed information on chromosome location of WHS resistance genes, further research needs to be carried out with the aid of molecular mapping techniques.

WHS resistance is a complex quantitative trait, and the resistance of wheat to spread of head scab in a spike (Type II resistance) is the major component of resistance. Increasing experimental evidence indicates that Type II resistance is controlled by a few major genes (Gu, 1983; Bai and Xiao et al., 1989; Chen, 1989; Bai and Zhou et al., 1990). Inheritance of WHS resistance was investigated by evaluating Type II resistance in three Sumai 3 crosses. One major gene and some minor genes were found to control WHS resistance in Sumai 3 (Chen, 1989). In another study, six cultivars and their F₁, F₂, F₃, and backcross progenies were evaluated either by spreading F. graminearum infested wheat kernels on the ground at booting stage, or by spraying conidium suspensions on spikes during anthesis in the field (Bai and Xiao et al., 1989). Segregation of Type II resistance, as measured by percentage of scabbed spikelets in the F₂ progeny, showed a continuous distribution, but two peaks coincided with the resistant parents and with the susceptible parents, respectively. The ratios of resistant plants to susceptible plants within the peak areas suggested segregation of two or three genes (Bai and Xiao et al., 1989). Similar results were obtained following single floret inoculation (Bai and Zhou et al., 1990). Studies on substitution lines also demonstrated major gene effects on WHS resistance (Yao and Ge et al., 1997).

WHS resistant materials identified so far have many undesired agronomic traits. However, an unfavorable correlation between WHS resistance and yield traits has not been established (Yang and Zhao, 1995). A negative correlation between plant height and WHS severity was identified in some studies, but not in others (Zhou and Bai et al., 1988; Jiang and Wu, 1989; Lu and Liu et al., 1990; Yang and Zhao, 1995). Thus, it should be possible to combine high levels of WHS resistance with desired agronomic traits in WHS resistance breeding programs.

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