Sensitivity of wheat genotypes to a toxic fraction produced by Cephalosporium gramineum and correlation with disease susceptibility

Cephalosporium stripe is an important disease of winter wheat (Triticum aes tivum) in several areas of the world, especially where stubble mulch and ea rly seeding are practiced to maintain soil moisture and prevent erosion. We developed a procedure to mass-produce a toxic fraction produced by Cephalo sporium gramineum through a modification of the method of Kobayashi and Ui. Exposure of excised wheat leaves to a concentration of 60 mul/ml of the to xic fraction for 72 h produced distinct wilting symptoms that allowed us to distinguish toxin-sensitive wheat genotypes in a repeatable manner. Twenty wheat genotypes belonging to four distinct germ plasm groups (common, club , durum. and synthetic) were evaluated. Variation in toxin sensitivity of w heat genotypes was mostly at the level of the germ plasm group, and all dif ferences among the four germ plasm groups were highly significant (P < 0.00 1) based on linear contrasts. Seventeen winter wheat genotypes representing the common, club, and durum germ plasm groups were planted in C. gramineum -infested fields at two locations. The logarithm of the percentage of tille rs showing whitehead symptoms at each of the two locations was significantl y (P < 0.0001) correlated with wilting symptoms measured by the toxin assay (r = 0.80 and 0.84). The common wheat genotypes were all sensitive to the toxic fraction, but showed a substantial range of disease reactions in the field. However, we found no case of a toxin-insensitive genotype being susc eptible in the field. These results suggest that toxin insensitivity may be an important mechanism of resistance to Cephalosporium stripe, but that ot her mechanisms are operative as well. The toxin assay may be useful as an i nitial screening procedure to reduce the number of genotypes to be tested i n the field.