4 ill., 2 tables; 14 ref. Summaries (De, En)

Wheat plants of four different growth stages (tillering, stem elongation, ear emergence and anthesis) were exposed to 80, 160 and 240 mig m(-3) ozone for 5 days (7 h per day) and to charcoal-filtered air. After exposure, plants were inoculated with conidia of Erysiphe graminis f. sp. tritici or uredospores of Puccinia recondita f. sp. tritici. The number of powdery mildew colonies and the amount of conidia formed on the two upper leaf positions was significantly increased on plants at young growth stages (GS 21 and 37) after moderate fumigations with ozone (80-160 mig m(-3)). However, at 240 mig m(-3) ozone, mildew attack was significantly reduced. On mature plants (GS 55), infection frequency and sporulation were decreased by all ozone treatments. Leaf rust was enhanced by ozone at all growth stages, the effects being stronger with increasing developmental stages. On younger plants (GS 29 and 51), however, the highest ozone variants reduced rust severity, whereas on older plants the two highest ozone levels induced the highest disease severity. These results were obtained equally for the number of rust pustules and the amount of produced uredospores. The latent period of both pathogens was not significantly affected by ozone. It is concluded that under elevated ozone pollution in the field, mildew epidemics may be accelerated in the early season, however associated with a premature breakdown due to adult-plant resistance, whereas leaf rust may be enhanced until maturity of the plants. Overall, under the influence of ozone a premature shifting within the pathogenic population from mildew to rust and to necrotrophic pathogens may be expected

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