Effects of host resistance on the development of haustoria and colonies of oat mildew

SUMMARY Colony development of Erysiphe graminis f.sp. avenae race 2 was studied on detached leaf segments of a range of Avena hosts with different levels of resistance, none of which possesses known specific gene resistance to this race. Resistance affected the length attained by mature primary haustoria, and also colony size as assessed by numbers of haustoria and conidiophores produced per colony 5 days after inoculation. A more accurate assessment of the size of mature haustoria was provided by the total length of their digitate processes. At the primary haustorium stage resistance affected not only haustorial size, but also haustorial efficiency measured as colony growth per unit size. Adult plant resistance of some hosts decreased haustorial size and/or efficiency in colonies on the fifth in comparison with the first formed leaf.