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August, September and October are the months when the RHS Pathology Department receives the most enquiries about this disease. The Pathology staff have also noticed an increase in enquiries over the past ten years. The two organisations believe that now is therefore a good time to get the public involved.

“We are not sure whether this increase in enquiries is due to suitable weather conditions caused by environmental changes, an increased awareness of the fungus or perhaps it is just part of the natural cycle of this disease,” says Béatrice Henricot, RHS Principal Scientist, Plant Pathology. “Even though this fungus causes striking bright orange spots, our knowledge of the distribution of it is limited. So I am really pleased that we are working with TreeWatch to find out more.”

Pear rust fungus attacks both pear trees and junipers. On pears, the spores produced on the underside of infected leaves cannot re-infect pear, but instead are wind-dispersed and infect several juniper species, causing perennial stem infections. In spring new spores are produced that then re-infect pear trees. One of the recommendations therefore, to help control infections, has been to remove infected junipers close to pear trees.

New to the amateur market in 2010 the fungicide difenoconazole is labelled for the control of pear rust. The use of fungicides recommended for the control of pear scab also seems to give incidental control of this rust. Myclobutanil is approved for scab control by gardeners.

“Our knowledge of how far this rust is distributed within the UK is limited,” says Gabriel Hemery, Chief Executive of the Sylva Foundation, which runs TreeWatch. “We also don’t know if some cultivars of pear trees are more likely to be infected or may be immune. We hope that lots of people will adopt a tree and join the survey, which is available on a dedicated website www.TreeWatch.com. August, September and October are the months when we see most infections so this is the best time for monitoring the distribution of these infections.”

The RHS and TreeWatch believe that this survey will help improve the understanding of this fungus. It is thought that pear tree rust was probably a rare fungus that has become more prevalent. Why this is scientists don’t know. Analysing data to show where the disease is and what pear trees it infects will help build up information and knowledge so that gardeners can be better informed how to control the effects on an infection.