How has Phytophthora become a problem in Sweden?

Photo: Mimmi Blomquist

The establishment of Phytophthora pathogens is primarily due to increased activity in international trade which result in the accidental introduction of diseased nursery stock. Sweden’s public parks are often planted with diseased nursery plants that are able to tolerate harsh conditions, such as drought and cold winters.

Sweden is a ‘hotspot’ for the introduction of many exotic pests and pathogens because of its proximity to Europe and the several ports of entry through which trade occurs.

Many Phytophthora species are newly introduced to Sweden (originating from elsewhere in the world), and because the trees lack a co-evolutionary history with these organisms and are therefore not adapted to fighting the pathogen, they are consequently very susceptible to infection.

In recent years, climate change has resulted in warmer and wetter summers, which have facilitated the advancement of Phytophthora, in some cases triggering disease development in areas that have never before been problematic.

Moreover, because most of the Phytophthora species we have in Sweden are primarily soil-borne (i.e. they are transmitted through saturated soils), the microscopic spores can be effortlessly and accidentally transmitted by hikers, mountain bikers, and machine operators that are moving from one part of a forest to another.

While Phytophthora has been researched in connection with diseased alder (work of Christer Olsson) and oak trees (work of Ulrika Jönsson), the newest concerns involving beech were sparked in 2010 and resulted in a public media frenzy in southern Sweden, particularly in city parks in Malmö, where several trees appeared to be failing and had to be removed.

Since then we have located the disease in protected forests, city parks, nature reservation areas, churchyards, nurseries, and across several urban landscapes throughout southern Sweden. Due to its sometimes, aggressive nature, there is a lot of concern from citizens and stakeholders to try and control this disease before it becomes an uncontrollable (rampant) issue.