Factors associated with grasshopper resistance in four spring wheat crosses

In an earlier paper strong negative correlation coefficients were obtained between the amount of mechanical tissue in the upper part of the culm and susceptibility to grasshoppers, Melanoplus bivittatus (Say). The number of heads clipped was used as the measure of resistance. Although the mechanical tissue measurement eliminated the need for artificial infestations of grasshoppers in the early generations of selection, it proved too laborious for more than a few observations. It was reasoned that mechanical tissue could be estimated by the strength and the diameter of the culm section. Such proved to be the case, with r values of 0.64** and 0.42** for diameter and breaking strength, respectively. The diameter and breaking strength of the dried culm sections taken just below the peduncle were studied in four crosses involving resistant and susceptible parents: Clarendon (C.I. 8462) X Thatcher (C.I. 10003); Thatcher X Triunfo (F.P.I. 104138); Reward (C.I. 8182) X Thatcher; and Clarendon X Reward. Clarendon and Triunfo have been previously shown to be resistant to M. bivittatus (Say), while Thatcher and Reward were considered susceptible. The F3 means of the resistant X susceptible crosses exceeded the means of the susceptible X susceptible cross by a significant difference (P<0.01) for both the diameter and breaking strength measurements. Two resistant X susceptible crosses were carried in the F4 and significant correlations were established between the means of the F3 and F4 families for diameter and breaking strength. As a final test a total of 20 F5 families from two resistant X susceptible crosses were subjected to grasshoppers of the species M. differentialis (Thomas). A correlation coefficient of -0.48 (p<0.05) was obtained between the breaking strength of the dried culm and the percentage of heads clipped by grasshoppers. No relationship was found (r = -0.06) between the diameter of culm and grasshopper damage. The possibility of this lack of relationship being due to the influence of leaf rust, Puccinia rubigo-vera tritici (Erikss, and Henn.) Carl., was discussed.