Propagating from field-grown plants: How clean can my nursery stock be?

Propagating from field-grown plants

Phytosanitary tiers for nursery production of field-collected propagules with emphasis on Phytophthora contamination

Ted Swiecki and Elizabeth Bernhardt, Phytosphere Research
Update 2/15/21

Native plant nurseries typically propagate plants from field-collected material, including seeds, cuttings, divisions, runners, tubers, stolons, and so forth. One of the cornerstones of producing healthy nursery plants is starting with clean propagating materials. So can we really produce “disease-free” material from field-collected propagules, even when following clean production best management practices?

The plant pathogens of greatest concern in restoration are exotic pathogens that are primarily or exclusively introduced via infected or infested nursery-grown material, soil movement, or other avoidable activities. Many root-rotting Phytophthora species and pathogenic fungi fall into this category.

Note that a pathogen should be considered exotic to a site if it does not occur there, even if it occurs in other portions of the host’s range. In general, it is a bad idea to introduce a pathogen or other destructive agent into a plant population or site that was previously free of the agent. If the pathogen is introduced in efforts aimed at habitat restoration, we have violated a primary principle of restoration: first do no harm.

The likelihood that exotic pathogens will be introduced into a nursery with propagative material will vary by the type of propagules used. In the table below, we have grouped field-collected plant propagules into phytosanitary tiers. These tiers have significant implications for nursery layout, management, and work flow. In general, plants in a cleaner tier (lower number) tier should be segregated from those in tiers with greater chance of contamination (higher number) to maintain the best possible phytosanitary status for each tier.

Even within a given phytosanitary tier, further segregation of plant material may be needed to prevent inadvertent spread of pathogens between native plant populations, especially for rare plant species and populations. For example, native or naturalized pathogens that are disseminated mainly by water splash (as opposed to dry windblown spores) have limited potential for long range dispersal and may not be present across disjunct plant populations. A splash-dispersed fungal pathogen that occurs on plant population "A", may not occur in population "B" if the populations are separated by distances greater than a few km or miles, especially if plants come from different ecological zones (e.g., coastal vs. inland). To avoid inadvertent spread of such pathogens, related plants from widely-separated populations that may share such pathogens should be adequately separated in the nursery.

Alternatively, especially when dealing with rare or threatened plant species, related species from multiple geographic locations should not be grown in the nursery at the same time, especially if they are in phytosanitary tier 1B or higher risk tiers. Because various pathogens (fungi, bacteria, viruses) can be seedborne, these precautions can apply even to seed-propagated (Tier 1A) material unless preplant seed treatments are effective against the pathogens of concern. Restoration specialists should consider consulting with qualified plant pathologists to determine what additional precautions may be needed.

Phytosanitary tiers for nursery production of field-collected propagules with emphasis on Phytophthora contamination

Tier	Tier 1A (cleanest)
Propagule type	Seed collected from upland plants not subject to inundation
Some applicable BMPs for collection and handling	-do not collect from known or likely diseased plants or areas -do not collect from ground or within splash zone (within about 1 m of soil or water surface) if feasible -avoid collecting seed during wet conditions or when soil is muddy -remove debris before storage and planting -sanitize (disinfest) seed surfaces if appropriate before storage and sowing -cull symptomatic seeds (most obvious for large seeds) before planting -if feasible and appropriate, use seed treatments to eradicate pathogens (e.g., heat, chemicals)
Pathogen risks	-Phytophthora (soil-borne) – Least potential for contamination. Soil-borne Phytophthora spp. can contaminate seed in only uncommon situations such as when seed is mixed with contaminated soil, which should not occur if all applicable BMPs are observed -Phytophthora (aerial) – very low, but possible for some hosts under high disease pressure -Other pathogens – some seed-borne fungal, bacterial, or viral pathogens may be present unless eliminated by effective seed treatments
Notes	Clean seed provides the best starting point for producing disease free plants. Many diseases that are readily transmitted via vegetative propagation are not transmissible via seed. Some diseases are seed-borne but many can be reduced or eliminated by various treatments. Phytophthora and many seed-borne pathogens require wet conditions to sporulate and infect flowers, seed heads, or fruit. In California’s Mediterranean climate, little or no precipitation occurs between flowering and seed maturity for many native species, so seed from these species has an extremely low risk of being contaminated by Phytophthora . However, plants that are periodically inundated by high stream flows or are low to the ground (within the splash zone from soil or water) have a higher risk of contamination and should be handled as tier 2 materials.

Tier	Tier 1B
Propagule type	Shoot tip cuttings (from above splash zone or produced during the dry season if lower)
Some applicable BMPs for collecting and handling propagules	-do not collect from known or likely diseased plants or areas -do not collect from within splash zone (about 1 m of soil or water surface) -if possible, use shoots produced during the dry season that are not subject to fog drip from overhead vegetation -do not collect material with apparent disease symptoms -avoid collecting seed during wet conditions or when soil is muddy -sanitize (disinfest) plant surfaces before rooting -cull symptomatic material before and during rooting period -maintain separation to prevent disease spread during and after rooting -if feasible and appropriate, use heat treatment to eradicate pathogens
Pathogen risks	-Phytophthora (soil-borne) – very low -Phytophthora (aerial) –low, possible if asymptomatic infections present -Other pathogens – fungal, bacterial, or viral pathogens may be present
Notes	Healthy tip cuttings, especially those produced during the dry season and collected under applicable BMPs, have low risk of contamination if source populations are free of Phytophthora and other pathogens of concern.

Tier	Tier 2
Propagule type	-Soil-contact propagules (e.g., runners, divisions, roots, bulbs, rhizomes, tubers, etc.) that can be completely cleaned of soil -low stem cuttings (near ground or extending below ground) -seed or cuttings collected from plant parts subject to inundation or splash from soil or water.
Applicable BMPs for collecting and handling propagules	-do not collect from known or likely diseased plants or areas -do not collect material with apparent disease symptoms -remove all surface soil; if this is not possible the propagule becomes tier 3 -sanitize (disinfest) plant surfaces before propagation -cull symptomatic material before and during rooting period -maintain separation to prevent disease spread during and after rooting -if feasible and appropriate, use heat treatment to eradicate pathogens
Pathogen risks	-Phytophthora (soil-borne) – possible from infections that will not be eliminated by surface disinfestation -Phytophthora (aerial) – possible for structures with any part above ground; less likely for propagules that are entirely below ground unless systemic or root infections also occur -Other pathogens – fungal, bacterial, or viral pathogens may be present
Notes	Effective surface disinfestation of propagules can eliminate external contamination but will not free propagules of established infections. For some pathogens, heat or other treatments may reduce or eliminate limited infections. If the collection source is free of Phytophthora and other pathogens of concern, clean material can be produced from tier 2 propagules. Propagative material may also be tested (baiting, direct isolation, molecular methods) to check for the presence of Phytophthora to decide if material is suitable for use. Regardless, material should be segregated from all lower-risk and higher-risk tiers.

Tier	Tier 3 (high likelihood of being infected or infested)
Propagule type	Soil-contact propagules that cannot be completely cleaned of soil and do not tolerate surface sterilization
Some applicable BMPs for collecting and handling propagules	-do not collect from known or likely diseased plants or areas -remove surface soil to maximum degree possible -use drying, heat treatment, or other methods to reduce inoculum that may be present -cull symptomatic material before and during rooting -maintain separation to prevent disease spread during and after rooting
Pathogen risks	-Phytophthora (soil-borne) – for field-collected material, highest likelihood of being infected or infested from external contamination. -Phytophthora (aerial) – possible for structures with any part above ground; may also be present in infested soil, and if systemic or root infections occur -Other pathogens – fungal, bacterial, or viral pathogens may be present
Notes	As with other vegetative propagules, risk is low if the collection source is free of Phytophthora and other pathogens of concern. Propagative material may be tested (baiting, direct isolation, molecular methods) to check for the presence of Phytophthora to decide if material is suitable for use. However, even with testing, the presence of Phytophthora and other pathogens may be difficult to ascertain with confidence. Therefore, plants originating from tier 3 propagules should be handled under the assumption that they may be contaminated with pathogens and segregated from all lower-risk and higher-risk tiers.

Tier	Tier 4 = Non-BMP nursery plants (highest overall likelihood of being infected or infested)
Propagule type	Any
Some applicable BMPs for collecting and handling propagules	Non-BMP stock should not be brought into clean production areas of the nursery
Pathogen risks	-Phytophthora (soil-borne) – possible to likely depending on source and nursery practices -Phytophthora (aerial) – possible to likely depending on source and nursery practices -Other pathogens – fungal, bacterial, or viral pathogens may be present
Notes	Plants that have not been produced and maintained in full compliance with these nursery Phytophthora BMPs have a high likelihood of having some level of infection. Possible exceptions may be Phytophthora non-hosts, but it is not clear which plant species are never colonized by any Phytophthora species. Also, Phytophthora contamination can still be present in containers of non-BMP plants that are not hosts. Because of their high risk, non-BMP material should be excluded from all clean areas of the nursery.

2/15/21 – Some additional notes and Tier 4 added.

Phytophthora in nursery stock and restoration plantings