Effect of induced resistance on the efficiency of powdery mildew haustoria in wheat and barley

Resistance against Erysiphe graminis was induced in wheat and barley by treating the plants with metabolites from a selected isolate of Bacillus subtilis. The induced resistance did not affect conidial germination or the formation of appressoria, but it did reduce the formation of primary haustoria and the number of colonies which subsequently developed. The colonies formed were smaller with less mycelium per unit area of leaf than the colonies on untreated plants. Fewer haustoria and conidiophores were produced per unit area of colony, and numbers of conidia per conidiophore and per haustorium were also reduced. In terms of the quantity of fungal structures formed per haustorium, the efficiency of the haustoria was reduced by 46% by the induced resistance. A higher percentage of haustoria in the induced resistant plants showed an enlarged extrahaustorial matrix, although the structure of the haustoria themselves was not altered. The results suggest that the resistance induced by treatment with the microbial metabolites impairs the development of E. graminis in much the same way as genetically determined partial resistance.