The North American invasive fungal pathogen Heterobasidion irregulare is a new threat to European parklands and urban pinewoods

The North American pine-associated root rot agent Heterobasidion irregulare was introduced in central Italy during World War II and is currently distributed in forest stands along 103 km of coastline west of Rome. In this paper we review the pathways of introduction and invasion of the exotic fungus, and we provide evidence that its presence is not limited to monospecific Italian stone pine (Pinus pinea) plantations but encompasses urban parks and even small patches of pines. An analysis of the pathogenic airspora performed by combining the entrapment of Heterobasidion spores present in the air with a PCR-based diagnostic assay indicates the spore deposition rates (i.e. a proxy of the concentration of spores in the air) expressed as n. spores m2 h-1 to be similar in urban parks and in monospecific pine plantations (P>0.05). Observations and experiments conducted in the Fregene Monumental Pinewood suggest the fungus may occasionally infect pruning wounds. However, this infection court should be regarded as of minor importance compared to freshly cut stumps. The prompt removal of dead and declining trees in the absence of preventative treatments against the fungus may explain the significant presence of the pathogen in parklands and urban settings. As weakening of root systems leads to increased chance of windthrow, threats associated with H. irregulare are posed not only by tree mortality, but also by tree uprootings, which may be important in recreational sites. It should be noted that up to 2/3 of a root system may be decayed before symptoms appear in the crown, hence the disease may be undetectable at an early stage through visual analysis. Potential impacts are relevant and the exotic pathogen has been recently included in the Alert List of the European and Mediterranean Plant Protection Organisation (EPPO).