Cryptic Risks to Forest Biosecurity Associated with the Global Movement of Commercial Seed

Abstract

The import and export of tree seed carries with it risks of inadvertent introduction of pests and pathogens to hitherto unaffected regions. Although trade in seed of specified trees is regulated, phytosanitary requirements for most tree species are minimal, even those related to the most important forest tree species in a given region. A better understanding of the microbiome associated with seed intended for commercial production or ornamental use, and their potential risk with the transport from the source origin of distributors, will help regulatory agencies implement measures to safeguard seed health and avoid trade-related spread of potentially harmful pathogens. In this study we used high-throughput sequencing to show that highly diverse fungal communities were associated with seed of 14 different Pinus species obtained from seed banks (seed orchards) and retail sources (online distributors) in North America and Europe. Fungal diversity differed among the 23 seedlots tested. Community composition did not relate to the species of Pinus nor the country of origin. Assigned potential functions based on sequence identity using FUNGuild provided an overall understanding of the likely life strategies of fungal operational taxonomic units (OTUs). Of those sequences classified to a trophic level, 453 were plant pathogens, with the Dothideomycetes having the highest prevalence. The most common plant pathogens included Sydowia polyspora, Lasiodiplodia theobromae, Diplodia intermedia and Diplodia sapinea that were detected from the majority of Pinus species. The evidence presented here illustrates an urgent need for plant protection authorities, practitioners and the general public to recognize the potential risk of introducing harmful pathogens through innocent transport of seed.

First report of Sydowia polyspora causing disease on Pinus pinea shoots

“…In addition, Talgø et al () verified no symptom development in one of the three tests performed with the same strains, thus, suggesting that diverse abiotic and biotic factors can play an important role in the development of the disease, either by affecting the ecology of the pathogen or the susceptibility of the host. Moreover, Cleary et al () recently found S. polyspora in asymptomatic seeds of several Pinus spp. obtained across Europe and North America, including P. pinaster , P. radiata , P. strobus , P. sylvestris , P. mugo from Portugal and P. pinea from Turkey.…”

“…Sydowia polyspora is also considered endophytic, being present in asymptomatic plants and in seeds (Cleary et al, ; Ridout & Newcombe, ). However, some authors suggested that the fungus may become pathogenic in a colonized host, under the influence of certain abiotic and biotic factors (Ridout & Newcombe, ; Talgø et al, ).…”

Are traded forest tree seeds a potential source of nonnative pests?

“…, Cleary et al. , Liu and He ). Moreover, only pathogens that are transmitted to the seedling (vertical transmission) may represent a phytosanitary threat (Burgess and Wingfield ).…”

A worldwide perspective of the legislation and regulations governing sentinel plants

“…In most of the countries (69%) the import of plant parts for human or animal consumption is subject to different regulations than propagation material (Table 1). However, the EU has not banned the import of fruit and seeds of any forest tree species, with potential consequences on forest health (Cleary et al 2019). It is worth remembering that if infected materials (fruits and seeds) are composted and used as a fertilizer the risk of pest establishment persists, since some organisms can survive the composting processing (Noble and Roberts 2004).…”

“…Hence, to avoid damage to plants in post-entry quarantine and lower the risk of introducing pests on the imported sentinel plants, it may be preferable to import plant propagation material other than rooted plants, such as seeds, parts of plants or tissue cultures (Anonymous 1999). The phytosanitary risk associated with imported plant propagation material is presumed to be lower than the risk associated with rooted plants (Cleary et al 2019;Franić et al 2019) and the regulations for import of such material are often more relaxed than those for other types of plants for planting (Eschen et al 2015a).…”