Canopy openings may prevent fir broom rust (Melampsorella caryophyllacearum) infections

Melampsorella caryophyllacearum is a heteroecious fungus that produces disease on silver fir resulting in galls that depreciate the value of trees. Currently, little is known about its epidemiology. The purpose of this study was to identify those site or management factors associated with fir broom rust infection. A 29-plot systematic sampling was carried out throughout all silver fir forests in the Spanish Pyrenees. In every sampling plot, we examined each tree for the presence of M. caryophyllacearum galls or witch's brooms. We differentiated their position as either on the main stem or on the branches of trees. We identified the most susceptible stands by relating a compendium of tree and stand variables with the presence and severity of symptoms. Disease symptoms were observed in 11.8% (95% CI: 5.8-17.9%) of trees. Fir broom rust-infected trees tended to be those of larger diameters. The rust infection was observed in 55.2% (95% CI: 35.7-73.6%) of the stands. We identified a wide range of severities with 5 of 29 stands having more than 30% of trees infected. Fir broom rust appeared to be a widespread disease, although we observed important differences in terms of severity. The presence and severity of the rust in stands was associated with understories composed of more shade tolerant plant species and with longer periods without thinnings. Canopy opening should be experimentally tested as a possible control method.