Plant Selection Methods

Risk-Based Plant Selection for Nursery Testing

Mimulus bifidus plants in a tray
Asymtomatic Mimulus bifidus infested with Phytophthora nicotianae, detected via leachate test

Plant testing is a crucial check on BMP compliance through each step of nursery production. While the bench leachate test is capable of screening several arrays of plants at once, it is not feasible to attempt to test each and every plant present in a nursery. Consequently, even the most rigorous testing schedules risk missing Phytophthora infections in the nursery. 

Despite these testing limitations, various strategies can be used to greatly increase the likelihood of detecting potential Phytophthora infestation in the plants that we do test. We therefore recommend using biased sampling based on risk-based plant selection when selecting batches of plants for leachate testing. This targeted, structured approach will allow us to test those plants that are most likely to harbor Phytophthora infection, increasing our confidence that the nursery is free of detectable soilborne Phytopthora pathogens. 

Plant sampling strategies based on risk-based plant selection require a robust scouting protocol to search for plants showing visual symptoms of Phytophthora root rot and a consideration of relative risk between batches. Please consider the following criteria, listed in order of importance, when conducting plant selection:

Plant Selection Criteria

  1. Plants showing potential signs of Phytophthora root rot should be prioritized for plant testing. Symptoms include:
    • Root rot
    • Wilting
    • Stunting
    • Leaf browning blade margins
    • Twig and branch dieback
    • General decline or loss of vigor
    • Death (avoid testing completely dead plants, as it is likely that the pathogen will no longer be present in dead tissue)

Plants infected with Phytophthora can remain symptomatic for long periods of time, and may only exhibit symptoms once exposed to stress, often after outplanting.

  1. Older plants held in the nursery for longer than one growing season. The longer a plant is in the nursery, the greater the risk of acquiring Phytophthora infection.
A tray of ceanothus seedlings, with some healthy and some dead. Dead plants are scattered in an irregular pattern across the tray
Symptomatic Ceanothus 'Dark Star' infected with both Phytophthora citrophthora and P. niederhauserii. Plant death in an irregular or clustered pattern may be indicative of infection.
  1. Plants that die or decline during unusual times in the production cycle.
    • Some degree of plant death may be expected after transplant, however established plantings suddenly exhibiting decline symptoms or that have quickly died should be considered for testing.
  2. Plants propagated from high-risk propagules, including rhizomes, tubers, bulbs, collection sites in riparian zones, or tips/seeds collected low to the ground within water splash zones. See Phytosanitary Tiers for more information.
    • Tier 2 materials (runners, divisions, etc.) should be prioritized over Tier 1a (clean seed).
  3. Native plants that are known to be particularly susceptible to Phytophthora infection, such as Arctostaphylos, Ceanothus, Mimulus/Diplacus, etc.
    • High risk plants tend to be flower plants (dicots) such as shrubs, trees, and bedding plants.
    • Low risk plants tend to be grasses (monocots) and ferns.
  4. Plant batches physically located in areas of the nursery closest to sources of contamination. This may include border plants on benches next to a driveway with higher watersplash potential, plants in high-traffic areas, or batches bordering potentially symptomatic plants.
  5. Plants destined for planting in high risk, highly sensitive areas where Phytophthora introduction would have a devastating impact on native plant communities. These include very rare habitats, pristine locations where Phytophthora has not been previously detected, and sites containing or close to sensitive species.
    • These plants should be tested at the highest rate possible. 

Concluding Remarks

Due to the nature of plant testing and nursery production, there is no set number of plants that any particular nursery should test. Testing requirements will depend on the sampling method used, the number of plants produced, Phytophthora detection history, and the projected impact of Phytophthora introduction for a given project. 

Wherever possible, ideally 100% of smaller plant batches should be included in plant testing. Where this is not feasible, risk-based selection is a reliable method of directing testing efforts for the highest likelihood of detecting Phytophthora in a subset of larger plant batches.