Marssonina leafspot disease of poplar under elevated ozone: pre-fumigated host and in vitro studies

The effects of O3 on leafspot disease of poplar were assessed for the pathogen (Marssonina tremulae Lib.) in isolation and combined with the host Populus trichocarpa T&G×Populus balsamifera Duroi non L. in enclosed chamber fumigation experiments. O3 exposure of 150 nl l−1, 8 h day−1, for 7 days and 200 nl l−1, for 8 h, had no effect on colony growth in vitro, although O3 enhanced conidiospore germination (150/200 nl l−1, over 8/5–6.5 h) and increased significantly the numbers of germtubes produced (150 nl l−1, over 8 h). Pre-fumigation of host plants (100 nl l−1, 8 h day−1, for 30 days) increased significantly the size of lesions on old leaves, but decreased these significantly on young leaves. Spore germination was significantly increased on leaves of all ages after a 15-day chronic exposure (100 nl l−1, 8 h day−1), but decreased significantly after acute exposure to 200 nl l−1, over 8 h.