Silent invaders: the hidden threat of asymptomatic phytobiomes to forest biosecurity

Populations of diverse, unknown, and potentially pathogenic fungi and fungus-like organisms are continuously introduced into new locations via asymptomatic infections (e.g. as endophytes or latent pathogens) within internationally traded live plants. Interactions between these asymptomatic fungi and novel recipient host trees can be unpredictable, and urban introductions may act as bridgeheads into natural and managed forests. Historical examples of novel, highly destructive forest tree diseases highlight the potential threat of this pathway. As the trade in live plants continues to expand, the likelihood of high-impact incursions increases. This has led to calls for more proactive management, including more stringent treatment and regulatory standards, and even the phasing out of trade in plants determined to be an untenable risk to forest ecosystems. In this review, we discuss how biosecurity systems should consider advances in understanding the diversity and ecology of phytobiomes associated with asymptomatic plants and what measures can be considered to reduce this threat to global forest health.