4.2.2
INVESTIGATING THE DISTRIBUTION OF TARSPOT OF SYCAMORE ALONG A TRANSECT BASED ON GRADIENTS OF OZONE AND NITRIC OXIDE AND SULPHUR DIOXIDE CONCENTRATIONS

N JARRAUD1, I LEITH2, SAA ARCHER1, JNB BELL1 and D FOWLER2

1Department of Biology, Imperial College, Silwood Park, Ascot SL5 7PY, UK; 2Institute of Terrestrial Ecology, Bush Estate, Penicuick, Midlothian EH26 OQB, UK

Background and objectives
Rhytisma acerinum, which causes tarspot of sycamore, has been claimed to be sensitive to air pollution [1]. Moreover, a study in south-east Scotland [2] confirmed that tarspot is abundant in the rural areas but absent from Edinburgh city centre. However, it was unclear whether this was because the source of inoculum (i.e. infected leaf litter) was removed in city parks or because of the higher pollution levels within central Edinburgh. The aim of the present study was to establish a transect in the London area in order to make field observations relating to the distribution of the disease along a gradient of air pollution. In parallel with this, a transect was established in the Edinburgh area, and experiments involving the effect of ozone on the pathogen were carried out in open-top chambers (OTC) at the Institute of Terrestrial Ecology in Edinburgh.

Materials and methods
For the London transect, six sites were chosen ranging from rural to urban, the same applying to the Edinburgh transect. The tarspot index (TSI) was assessed in August 1997 for the London transect, and in September for the Edinburgh transect. In London, 50 leaves were sampled at random from each of 25 trees in each site, whilst in Edinburgh, only 10 trees per site were sampled. The TSI is the number of tarspots per 100 cm2 leaf area. The area of each leaf was determined using an electronic leaf-area meter. The TSI was then compared with pollution data obtained from the UK Departments of the Environment and Transport, and the regions' air pollution archives.

The OTC work involved the following treatments: control (no chamber used), filtered air chamber, ambient levels of ozone, 2x ambient, 4x ambient, 4x ambient with a UV filter.

Results and conclusions
Linear regression analysis indicated a significant decrease in TSI as a function of distance from central London (r2>0.9) and from central Edinburgh. However, regressions for NO, NO2 and NOx gave a similar trend but with very low r2, thus casting doubt on a causal relationship with these pollutants. There was a strong positive correlation between ozone levels and TSI. Other potential environmental factors were considered, such as temperature and rainfall, but the gradients from one end of the transect to the other were not steep enough to yield any significant information. The strong correlation between levels of ozone and TSI does not imply that ozone influences tarspot distribution. Indeed, the Edinburgh fumigation work has clearly shown that ozone has a deleterious effect on tarspot. In interpreting the transect, it is most likely that the correlation is an artefact generated by the fact that ozone levels respond to distance from London coincidentally with TSI. The explanation for the relationship between TSI and distance from London might be that one is dealing with a 'memory effect' (i.e. the tarspot was once regulated by a steep pollution gradient, such as SO2, which has since disappeared, but recolonization has not yet occurred). Other independent factors, such as the patchy distribution of the host in urban areas which would hinder the dissemination of inoculum, cannot be ruled out.

References
1. Bevan RJ, Greenhalgh GN, 1976. Environmental Pollution 10, 271-285.
2. Leith ID, Fowler D, 1987. New Phytologist 108, 175-181.