The plant trade has led to many plants travelling around the world, but with them have come less welcome hitchhikers. Horta Jung and colleagues studied tiny but destructive organisms called Phytophthora (plant destroyers) in Portuguese forests, rivers, and tree nurseries between 2010-2015. These oomycetes, or water moulds, are microbes notorious for the damage they can do to plants. The research team collected samples from 70 forest areas, 34 rivers, and 13 tree nurseries across the country. They found these pathogens in nearly 90% of forests, 97% of rivers, and 92% of nurseries sampled, but most of these species appear to have originated outside of Portugal.
The most widespread pathogen, Phytophthora cinnamomi, was found in over half of forest sites and nurseries, causing severe damage to cork oak and chestnut trees. Sick trees were much more likely to harbour these pathogens than healthy ones – 65% versus 10%. The scientists discovered 15 species never before seen in Europe, including several completely new to science. Of the 48 Phytophthora species found, 34 (71%) were introduced from other parts of the world, likely through Portugal’s centuries of maritime trade and plant commerce.
Horta Jung and colleagues hunted for Phytophthora using three techniques. For forests, they collected soil from around tree roots and sections of diseased bark. For waterways, they floated oak and carob leaves as bait, which attracted swimming Phytophthora spores. In nurseries, they examined both sick and healthy-looking tree seedlings. Back in the laboratory, they used microscopes to study the physical features of any Phytophthora caught, and DNA analysis to precisely identify each species found.
Portugal’s role as a major seafaring nation since the 1400s made it a key entry point for plants and their pathogens from around the world. Ships returning from colonies in Brazil, Africa, and Asia brought exotic plants which enriched Portuguese gardens – today nearly 18% of Portugal’s plants come from overseas, the third highest proportion in Europe. The first signs of hidden hitchhikers were spotted in 1838, when Portuguese chestnut trees started dying from a mysterious “ink disease,” long before scientists understood what caused it. This study reveals that five centuries of plant trading has left a legacy of pathogens that now threaten Portugal’s native trees.
Horta Jung, M., Maia, C., Mora-Sala, B., Abad-Campos, P., Schena, L., Mosca, S., et al. (2024). High diversity of Phytophthora species in natural ecosystems and nurseries of Portugal: Detrimental side effect of plant introductions from the age of discovery to modern globalization. Plant Pathology. https://doi.org/10.1111/ppa.14022 (OA)
