1Department of Plant Biology and Biogeochemistry, PBK-301, Risø National Laboratory, DK-4000 Roskilde, Denmark; 2Institute of Grassland and Environmental Research, Aberystwyth, Ceredigion SY23 3EB, UK (present address)

Background and objectives
In barley with mlo5 resistance against powdery mildew (Erysiphe graminis DC f. sp. hordei Em. Marchal), fungal penetration of epidermal cell walls is effectively prevented and associated with the rapid formation of large cell-wall appositions at the penetration site. Although mlo5 resistance is highly effective it allows development of occasional colonies, mainly originating in the less-resistant stomatal subsidiary cells. Recently, two unique powdery mildew isolates were described with increased infection efficiency on barley lines with different mlo resistant alleles [1], and the ability to infect all epidermal cell types. This raises questions about the effectiveness and race non-specificity of mlo resistance in barley [2]. Here we present data on infection, growth and spore production of a powdery mildew isolate with increased penetration efficiency (mlo virulent) on mlo5 resistant barley.

Materials and methods
A time-course study was conducted using near-isogenic barley lines, with or without mlo5 resistance, and two near-isogenic powdery mildew isolates, showing high (virulent) or low (avirulent) frequency of haustorium formation (infection efficiency) on the mlo5 resistant barley line. The parameters measured were: infection efficiency at 24 and 48 h after inoculation (h.a.i.); length of secondary hyphae at 48 h.a.i., generations of haustoria at 72 and 96 h.a.i. and spore production per day until 16 days after inoculation. Observations up to 96 h.a.i. were made for long and short epidermal cells and for stomatal subsidiary cells on the adaxial surface of seedling leaves.

Results and conclusions
On the susceptible barley line, the two isolates produced the same number of haustoria. The amount of haustoria increased up to 48 h.a.i. and different cell types were infected with different efficiency. In general, both isolates formed fewer successful infections in long epidermal cells. On the mlo5 resistant barley line, the avirulent isolate produced very few haustoria and the majority of these (90%) were in stomatal subsidiary cells. By contrast, the mlo virulent isolate infected all cell types in leaves of the mlo5 resistant line, with approximately half the efficiency found on the susceptible line. Successful infections from the two isolates on both the mlo5 resistant and the susceptible line developed and grew to equal-sized colonies at the same rate. Individual colonies from the mlo virulent and the avirulent isolate produced a similar amount of spores on the same barley line. However, the spore production by individual colonies was reduced by more than 50% on the mlo5 resistant line compared to the susceptible. With such a low reproduction rate by the mlo virulent isolate on the resistant line, the contribution to the aerial spore pool may never reach significant levels during a growing season. This may be the main reason for the lack of, or slow evolution of mlo virulence in the field. However, the high infection efficiency of the mlo virulent isolate shows that powdery mildew has the potential to overcome, at least partially, mlo-based resistance.

1. Lyngkjaer MF, Jensen HP, Østergård H, 1995. Plant Pathology 44, 786-790.
2. Lyngkjaer MF, Østergård H, 1997. Plant Pathology (in press).