Fifty years ago, the large cities of the UK were still affected by severe winter smogs and high concentrations of sulphur dioxide (SO2) from industry and coal burning in the home. At that time, the black spots on Sycamore leaves caused by the tar spot fungus were almost entirely absent from cities and industrial areas. Research showed that this was directly related to the high levels of sulphur dioxide in the atmosphere.
Today, the levels of sulphur dioxide in our cities are very low. If sulphur dioxide concentration was the only environmental factor limiting the growth of the tar spot fungus (Rhytisma acernium), we would have expected the black tar spot symptoms to be found throughout our cities. However, recent observations suggest that tar spots are still less frequent closer to city centres.
Why is this?
We designed the OPAL tar spot survey to test two hypotheses that have been suggested to explain the current distribution of tar spot symptoms:
- Greater cleaning and sweeping of streets, especially within our cities, removes an important source of infection. Sycamores are deciduous trees and shed their leaves in autumn. The fungus that causes tar spots (Rhytisma acernium) then overwinters on these fallen leaves and its spores re-infect the new leaves that are formed in the spring. Therefore, if there are fewer dead leaves around the trees, we would expect fewer tar spot symptoms on them. We asked you to record the level of fallen leaves, on a 0/1/2 scale, to allow us to test this hypothesis. Very few trees fell into the intermediate category, so we created two categories for analysis – trees with no or few fallen leaves and trees with many fallen leaves.
- Air pollution in a different form is still inhibiting tar spot formation. Air pollution is still a significant health and environmental problem, but it comes from very different sources compared to 50 years ago. Road traffic is now the main source of air pollution in our cities, and one particular pollutant connected to road traffic may be affecting tar spot – nitrogen dioxide (NO2). To explore this hypothesis we compared the numbers of tar spots that you reported at your survey locations with concentrations of nitrogen dioxide at that location, derived from a national database of NO2 levels.
What did we find?
After collating the survey results that you submitted, we first checked the data to make sure no incorrect or inappropriate records were included in our analyses.
We then calculated a tar spot index for each surveyed tree, by dividing the number of tar spots that you recorded by the length of the leaf that you recorded.
We then applied a series of statistical tests to assess whether the tar spot index was associated with the two possible causal factors: fallen leaves and nitrogen dioxide levels. We focussed this analysis on records from England up to June 2014 – records for a total of 1627 trees in total. The map in Figure 1 shows the distribution of the records that we used.
Figure 1: The location of the OPAL Air Survey results that were analysed.
We then related the tar spot index to the levels of nitrogen dioxide and the number of fallen leaves, to test the two hypotheses described above. Figure 2 below shows the results.
Figure 2: The average (mean) tar spot index for each of five NO2 concentration (µg m-3) categories and two fallen leaf status categories. The unshaded bars represent values with no or few fallen leaves, and the shaded bars represent values for many fallen leaves.
We divide the nitrogen dioxide (NO2) concentrations into five categories, along the x axis of the graph. There seemed to be no effect at concentrations below 20 µg m-3, but at the two highest concentrations, and especially at concentrations above 25 µg m-3, the tar spot index begins to fall.
At each NO2 concentration, we also found that the tar spot index was lower for tress with no or few fallen leaves around them (the white bars in the figure) than when there were lots of fallen leaves around them (the grey bars in the figure).
In summary, we were able to show that both our original hypotheses were right – both a low number of fallen leaves and a high level of nitrogen dioxide reduce tar spot symptoms. This is the first time that these links have been proved, and it has only been possible with the large number of records that you all submitted – so thank you for your help!
These results were recently published in the scientific journal Environmental Pollution.To view this article click here.