Combined whole-season effects of elevated ozone and carbon dioxide concentrations on a simulated wheat leaf rust (Puccinia recondita f. sp. tritici) epidemic

A complete growth season with the physical climate and ozone pollution from 1 April to 31 July as recorded at a field site in Northern Germany, averag ed over several years, was simulated in climate chambers and combined eithe r with a current (370-400 mu l l(-1)) or enriched (620-650 mu l l(-1)) CO2 atmosphere. Wheat, grown from seedling emergence to maturity under the diff erent physico-chemical climates, was inoculated with leaf rust (Puccinia re condita f. sp. tritici) at tillering stage and a rust epidemic was induced by repeated re-inoculations with the newly Formed inoculum. Ozone significa ntly reduced disease severity, uredospore production and increased the late nt period of leaf rust on young planes, consequently inhibiting the epidemi c spread on upper leaves of mature plants. Inhibiting effects of ozone on l eaf rust development were not reflected by the early infection stages such as spore germination, germ tube growth, formation of infection hyphae, haus torial mother cells and haustoria, which remained largely unaffected by the ozone treatments. However, ozone induced a significantly higher extent of hypersensitive responses of the infected leaf tissue. Additionally, plants exposed to elevated ozone turned senescent much earlier than plants without this stress which prematurely degraded the growth conditions for the funga l pathogen. Enrichment with CO2 increased the rotal carbohydrate content in leaves but this had only minor effects on the disease. Thus, elevated CO2 only poorly compensated for the disease-inhibiting effects of ozone. The co mpensation of ozone effects on wheat leaf rust by elevated CO2 is much smal ler than known compensatory effects of both gases on plans productivity.