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 O3, 50–90% of NOx, and 80–95 % of SO2. 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 O3/l significantly reduced the germination of A. veneta but the addition of an equal concentration of NO2 (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.

Young potted silver firs were treated with six different mixtures of N.iCl and CaCK. Growtli was measured as well as the fohar contents of Cl, Na and Ca and tlic development of chlorosis and necrosis was recorded. At both concentration levels used, growth inhibition and needle necrosis were more pronounced in test conditions rich in CaCl2, a fact which is prtibably duo to Cl toxicity. AbstractRing characteristics of pine trees grown in polluted air showed up distinctly as a decrease in newly devised synthetic indicators such as product of W (relative ring width) and DD (relative difference between maximum and minimum density within a ring) or product of W and MD (relative mean density). Intensity of combined air pollution was estimated by the total of daily maximum concenu-ation of SO,, ozone, NO, and NO2. Since synthetic indicators show a significant correlation, at the over 98 % confidence level, with the intensity of combined air pollution, phytotoxicity of air to pine growth must be attributable at least in part to combined effect of air pollutants.

SUMMARY Three types of artificial mist (acid = A, pH 3.0, 1400 μequiv. total ionic strength l−1; NH4+‐enriched = N, pH 5.4, 1400 μequiv. l−1; and control = C, pH 4.9–5.1, 10–15 μequiv. l−1) were used to detect the effects of wet deposition on the fungus Apiognomonia veneta (Sacc. & Speg.) Höhn. and on the susceptibility of its host tree, the London plane [Platanus×acerifolia (Ait.) Willd.], to leaf blight or anthracnose. On a nutrient‐poor agar, germination of conidia of A. veneta was strongly inhibited by mist A. Germination in mists N and C was higher, being 22 and 25 %, respectively. In autumn and spring, London plane seedlings were sprayed 15 times with each of mists A, N or C. Thereafter, leaves were inoculated with conidia. At this time, the pH of all leaf surfaces was similar. Inoculation in spring led to typical leaf blight symptoms. However, infection was light and no significant differences in symptom development were detected between mist treatments. After the C pre‐treatment, the germination of conidia was significantly inhibited on leaf surfaces, and in leaf washings, when compared with the A and N pre‐treatments. More spores germinated in a water blank than in leachates from C‐misted trees. A fungistatic, phenolic compound was found in leachates and isolated by TLC. It is suggested that this compound was involved in the inhibitory effect of the leaves treated with a control mist, but was removed or inactivated by mist A and N. We conclude that constitutive resistance of London plane leaves is affected adversely by acid and/or nutrient‐enriched mist.