Effects of air pollution and water stress on leaf blight and twig cankers of London planes [ Platanus × acerifolia (Ait.) Willd.] caused by Apiognomonia veneta (Sacc. & Speg.) Höhn.

summary Potted 2‐year‐old cuttings of London plane trees [ Platanus × acerifolia (Ait.) Willd.] were raised under well‐watered (WW) or water stress (WS) conditions in two semi‐open‐top chambers and fumigated with ambient air (AA) and filtered air (FA). In the FA chamber, charcoal and Reox‐Purafil † filters removed 65–100% of O 3 , 50–90% of NO x , and 80–95 % of SO 2 . After leaf inoculations in spring with conidia of Apiognomonia veneta (Sacc. & Speg.) Höhn., significantly fewer and smaller lesions appeared on AA than on FA trees. The WS treatment led to an additional, highly significant suppression of leaf blight. In winter, WW trees inoculated with mycelium of A. veneta developed significantly larger cankers in the AA than in the FA chamber. Under WS conditions, canker formation was somewhat reduced in AA and was similar in both chambers. The production of Apiognomonia conidia on necrotic bark in spring, however, was stimulated by the WS treatment, whereas no differences in conidiation were found between AA and FA. Germination and growth of A. veneta in vitro were both significantly suppressed in the AA chamber compared with the FA. In the laboratory, 50 nl O 3 /l significantly reduced the germination of A. veneta but the addition of an equal concentration of NO 2 (up to 100 nl/l) had no effect. After one year of exposure, initial shoot growth of WW London planes was significantly inhibited under AA conditions and a delay of bud burst was also observed. It is concluded that the longer duration of tree dormancy in AA positively affected the development of Apiognomonia cankers. We suggest that ambient air pollutants suppressed leaf blight through a direct effect on the germinating conidia of A. veneta but that the enhancement of twig canker formation was rather due to effects on the host tree.