Relative tolerances of wild and cultivated barleys to infection by Blumeria graminis f.sp. hordei (Syn. Erysiphe graminis f.sp. hordei). I. The effects of infection on growth and development

The two lines of wild barley, B19909 and I-17-40 and the cultivated barley, cv. Prisma used in this investigation were found to be the most susceptible to infection of 25 wild and four cultivated barley lines when exposed to the local population of Blumeria graminis f.sp. hordei. All three lines were susceptible during the early stages of growth but expressed some level of adult plant resistance although this level of resistance was significantly higher in line I-17-40 than in either of the other two.The relative tolerances of the lines to the mildew were determined by comparing the effects of infection on their growth and development in growth cabinet experiments. Mildew developed more slowly on line I-17-40 than on the other two lines and by the third week after inoculation, when mildew cover on B19909 and cv. Prisma had reached about 27%, only about 15% of the green leaf area of line I-17-40 was covered. Mildew continued to increase on line B19909 and cv. Prisma so that 6 weeks after inoculation it covered 40% of their leaf-blades. On line I-17-40 30% of the green leaf area was colonised by 4 weeks after inoculation but because of adult plant resistance coupled with the loss of the earlier infected leaves through senescence mildew cover then reduced falling to 15% by 6 weeks. Although total mildew biomass, measured as conidial production was higher on line B19909 than on cv. Prisma all its growth parameters were reduced less indicating that it was the more tolerant line. Conidial production on the lower susceptible leaves of line I-17-40 was slightly lower than on cv. Prisma yet the reaction of these leaves to infection was the same on both lines indicating that tissues of I-17-40 were slightly less tolerant than those of the cultivated barley. However, during the later stages of growth when its upper leaves expressed high levels of ‘adult plant resistance’ dry matter production in this line increased to levels higher even than in the controls. This capacity for compensatory photosynthesis ensured that by the end of growth few differences in any of the measured growth parameters between infected and uninfected plants of line I-17-40 were significant.The greater tolerance of line B19909 over the other two lines and of cv. Prisma over line I-17-40 during the early stages of growth appears to be due to a lower sensitivity to infection of those processes which regulate dry matter accumulation and its distribution around the plant.