Comparative Studies on Callose Formation in Powdery Mildew Compatible and Incompatible Barley

Callose formation in barley mutants, lines and varieties with different genes for resistance to powdery mildew in seven different loci was compared. Only barley with resistance genes in the ml‐o locus showed so carly a callose formation passing off at such a high rate that it prevented fungal penetration. Ml‐(La) resistant varieties and near‐isogenic lines in‘Manchuria’with resistance genes in five other loci showed only a tendency to a larger callose formation than their susceptible counterparts after inoculation with avirulent as well as virulent powdery mildew. The callose formation in ml‐o resistant barley was independent of the powdery mildew culture applied. This supports the hypothesis set forth as to why the ml‐o mutants are resistant against all known cultures or races of barley powdery mildew, and why this resistance may be more durable than other powdery mildew resistances. Further, this is the first case where the effect of callose refers to the action of a specific gene. Six susceptible Japanese varieties formed very large appositions but they were initiated as late as in other susceptible varieties, and their colour was plaer than in other barleys. Nine Hordeum species fell in one group with small appositions and another with appositions of the most common size in barley. Wheat reacts strongly with deep coloured appositions, rye with haloes with a somewhat diffuse margin, and oats large, central spots (papillae) nearly without any halo.