Understanding the effect of elevated CO2 in disease resistance to powdery mildew in 150 year-old oaks

The research group where I worked this summer have discovered that elevated CO₂ evokes a change in the immune system of oak seedlings that causes them to become more susceptible to the pathogenic fungi causing powdery mildew. This fungus presents as a white film on the leaves of oak and other species, and is considered a bottleneck to woodland regeneration due to the devastating effect it has on oak saplings in particular. Usually however, mature oak trees are able to cope with the infection, and usually only succumb to the disease when also affected by other stressors.

What is not known is whether mature oak trees exhibit an increased susceptibility to elevated CO₂ in the same way as their seedlings. If they do, powdery mildew could be elevated from minor annoyance to major threat as carbon dioxide concentration in the atmosphere increases.

Thus my 10 week summer project began with the goal of answering this question. The true beginning however was five years previously in 2016 when the baseline samples started being collected from a mature, unmanaged forest in Staffordshire. This forest was to be the site of a free-air carbon dioxide enrichment (FACE) experiment.

The facility consists of several circular structures called arrays that allow carbon dioxide to be fumigated into the air. In this way the parts per million of CO₂ in determined areas can be controlled. Each structure, or array, is either at normal or elevated CO₂. Normal means the CO₂ concentration is at ambient CO₂ concentration, while elevated is 150ppm above normal. This is roughly the concentration of CO₂ that can be expected in 50 years’ time. The facility was switched on in April 2017.

Throughout the years, leaf samples have been taken from oak trees at normal and elevated CO₂ and frozen. If these samples could be scored for powdery mildew, elevated and normal conditions could be compared to decide if the mature oak trees in elevated conditions have more powdery mildew than their counterparts in normal CO₂.

The first step in this project was to defrost the leaves and scan them, so we had images of all sampled leaves. The second step was to develop an automated process for detecting the percentage coverage of powdery mildew on the leaves. We did this using imageJ. The final step was to use statistics and machine learning to identify patterns and important factors in the data.

So far, we learnt that there was no significant difference overall between the percentage cover of powdery mildew on leaves from normal vs elevated CO₂. However, some interesting patterns revealed themselves once broken down by year. For instance, in 2020, there seem to be differences between ambient and elevated CO2 but overall, the leaves had considerably less powdery mildew than other years. We analysed different meteorological patterns to see if there was correlation with the lower coverage of powdery mildew. Of particular interest was wind speed, which was significantly higher in 2020 than all other years. This, along with background information from the scientific literature allows us to form the hypothesis that due to spores being unable to settle on leaves, a higher wind speed causes a reduction in powdery mildew infection. As for our original question, the analysis continues to try to identify differences between trees, geographical locations and canopy location.

This project has been a great opportunity that has allowed me to learn more about the field of plant pathology and will inform my future career decisions. I would like to thank my supervisor, Estrella Luna Diez, PhD student Mark Raw, and finally the BSPP for funding the project.

Katie Hawkins

University of York