Phytophthora in nursery stock and restoration plantings

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


Ted Swiecki and Elizabeth Bernhardt, Phytosphere Research

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 higher (cleaner) tier should be segregated from those in lower (greater chance of contamination) tiers to maintain the highest possible phytosanitary status for each tier.

Even within a given phytosanitary tier, further segregation of plant material may be needed. 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. Hence, a splash-dispersed fungal pathogen that occurs on plant population "A", may not occur in population "B" that may be separated by tens or hundreds of miles. To avoid inadvertent spread of such pathogens, related plants 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 propagated in the nursery at the same time, especially if they are in phytosanitary tier 1B or lower.

 

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

Tier

Tier 1A (cleanest)

Propagule type

Seed

Applicable BMPs for collecting and handling propagules

-do not collect from known or likely diseased plants or areas
-do not collect from ground or within splash zone (within about 0.5 m of soil surface if feasible
-avoid collecting seed during wet conditions or when soil is muddy
-surface sterilize before storage and sowing
-cull symptomatic seeds (most obvious for large seeds) before planting
-use seed treatments to eradicate pathogens (e.g., heat, chemicals)

Pathogen risks

Phytophthora (soil-borne) – essentially zero; soil-borne Phytophthora spp. can contaminate seed in only uncommon situations such as when seed is mixed with contaminated soil, which is precluded when following good technique
Phytophthora (aerial) – very low, but possible for some hosts under high disease pressure
Other pathogens – 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.  Some diseases are seed-borne but many can be reduced or eliminated by various treatments.  Many diseases that are readily transmitted via vegetative propagation are not transmissible via seed. 

 

Tier

Tier 1B

Propagule type

Shoot tip cuttings

Applicable BMPs for collecting and handling propagules

-do not collect from known or likely diseased plants or areas
-do not collect from within about 0.5 m of soil surface
-avoid collecting seed during wet conditions or when soil is muddy
-surface sterilize
-cull symptomatic material before and during rooting
-maintain separation to prevent disease spread during and after rooting

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

If source populations are free of Phytophthora and other pathogens of concern, tip cuttings are a viable option for producing clean plants.

 

Tier

Tier 2

Propagule type

Belowground structures (roots, crown divisions, bulbs, rhizome, tubers, etc.) completely cleaned of soil, low stem cuttings (near ground or extending below ground)

Applicable BMPs for collecting and handling propagules

-do not collect from known or likely diseased plants or areas
-surface sterilize
-cull symptomatic material before and during rooting
-maintain separation to prevent disease spread during and after rooting

Pathogen risks

Phytophthora (soil-borne) – possible from infections that will not be eliminated by surface sterilization
Phytophthora (aerial) –possible for stem cuttings; lower for belowground propagules unless systemic or root infections also occur
Other pathogens – fungal, bacterial, or viral pathogens may be present

Notes

Effective surface sterilization 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, acceptably clean material can be produced.

 

Tier

Tier 3 (highest likelihood of being infected or infested)

Propagule type

Belowground structures that cannot be completely cleaned of soil and do not tolerate surface sterilization

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) – possible from both external contamination and internal infections
Phytophthora (aerial) –possible through both soil contamination and systemic or root infections
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.  However, because this may be difficult to ascertain with confidence, materials propagated should be handled under the assumption that they may be contaminated with pathogens and should be segregated accordingly from all higher tiers.