Version 7/31/2023
In nurseries that have successfully implemented clean nursery production BMPs, Phytophthora detections should not occur. From the discussion in section 2.1, it is obvious that random sampling is not an efficient way to detect rare disease cases in an otherwise clean nursery. The easiest way to increase the odds of finding diseased plants is by focusing first on plants that appear to have possible symptoms, including low vigor, off-color, stunting, wilting, dieback, basal stem cankers, and decayed roots. At early or even advanced stages of disease, any symptoms may be subtle. As noted in Best Management Practices for Producing Clean Nursery Stock and Phytophthora species in native plant nursery stock: issues and implications, drought-tolerant plants, such as many California native plants, may not show top symptoms under nursery conditions until root rot is severe. Hence, plants without obvious symptoms should also be monitored as part of your nursery inspection program.
In the absence of any apparent root rot symptoms, the next best criterion to use for selecting plants is their potential risk of contamination. Prioritizing plants or blocks of plants for testing based on their overall risk profile can help nursery staff determine how to most effectively use limited testing resources. Plants that have the greatest risk of being infected or being exposed to contamination should be monitored more frequently, but ideally, no set of plants should be categorically excluded from testing.
The factors in Table 1 should be considered when determining which plants have the greatest risk of being infected.
Table 1. Factors related to higher or lower risk of Phytophthora infection in nurseries following Best Management Practices (BMPs) for Producing Clean Nursery Stock
| Factor | Higher risk | Lower risk | Notes |
|---|---|---|---|
| Plant propagule source | nurseries not following Clean nursery BMPs or better | nurseries following Clean nursery BMPs or better | The Nursery Evaluation Form for Systems Approach to Clean Nursery Production(version 3.1) has been developed to help clients or qualified third party inspectors evaluate the practices of a nursery. |
| Plant propagule source | cultivated landscapes, e.g., gardens | healthy native stands in remote areas | Native stands near urban interfaces, in parks and other areas with high visitor use or a history of human use (e.g. forestry, homesteads, mining, etc.) are more likely to be contaminated. |
| Plant propagule type | propagules with soil contact, e.g., root divisions | clean seed, tip cuttings | See Propagating from field-grown plants for more details |
| BMP deviations and exceptions | deviations from BMPs have occurred in plant handling/production | no known deviations | With good record keeping, possible or known deviations from BMPs can be tracked; affected plant batches should be monitored more closely if they have been retained. Plants subjected to critical lapses in BMPs should be discarded. |
| Residence time in nursery | older plants (1 or more growing seasons) | young plants (less than full growing season) | The likelihood of accidental contamination increases the longer that a plant is held in the nursery. Large older plants can be less likely to show symptoms than young plants. |
| Location within nursery | adjacent to or close to potential sources of contamination, e.g., close to exterior fences, landscape plantings, watercourses, surface drainage channels | well-separated from potentially contaminated areas, e.g., in a clean greenhouse | In a nursery following clean production practices, the greatest chance of contamination may be from beyond the nursery. Benches located closer to nursery edges may have an elevated risk of contamination depending on which protections are in place to prevent contamination from water splash, movement of animals, etc. |
| Plant position within bench | adjacent to or closer to entrances, aisles, other potential sources of contamination | furthest from potential in-nursery sources of contamination | More opportunities for accidental contamination (handling, contact with people or equipment, accidental splash from water on the nursery floor) may occur in plants that are adjacent to high-traffic areas than those that are in the center of benches. |
| Plant species | most dicots, conifers, and many non-grass monocots | some grasses | Some Phytophthora spp. can infect grasses, so no species can be considered nonhosts. See additional discussion below. |
Nursery BMPs for the foliar pathogen Phytophthora ramorum emphasize sampling of known hosts, especially a few “high risk” host species that are highly susceptible to this pathogen. This tactic does not apply to the detection of root rotting Phytophthora species for several reasons. First, many different root-rotting Phytophthora species have been detected in nurseries, so it is not possible to pick out optimum detection hosts for this wide array of pathogen species. Furthermore, many root-rotting Phytophthora species have host ranges much wider than that of P. ramorum. Some of the more common root-rotting Phytophthora species in nurseries are known to infect hundreds of species in many plant families. Very few plant species have been tested to evaluate their susceptibility to a wide range of Phytophthora species, so many host-pathogen relationships are unknown. Even if a plant species has not been shown to be a host of one or more Phytophthora species, there is no guarantee that it will not be susceptible to other species.
Some nurseries that conducted extensive testing before converting to clean production under the nursery Phytophthora BMPs have identified some plant species that historically yielded more Phytophthora detections than others. Where such information is available for a given nursery, it may be used to help prioritize which plant species to emphasize in testing. However, due to differences that exist between nurseries with respect to growing conditions, plant species grown, potential sources of contamination, etc., the list of plants most likely to be infected with Phytophthora cannot be generalized across different nurseries. Furthermore, historical detections that occurred under very different production methods may not be especially relevant to clean production. Routes of accidental contamination and plant species that are more likely to become contaminated could change substantially with changes in production practices. In plants grown according to the nursery Phytophthora BMPs, contamination events should be very rare and the plant species affected are likely to be relatively random. Hence, for general disease monitoring purposes in nurseries, a variety of plant species should be included in the testing program.
Another important factor to consider when allocating time and resources for testing is where the nursery stock will be planted. Plants that will be installed in very limited habitats, near existing rare plant populations, or in other very highly sensitive sites need to meet the highest standard for being free of Phytophthora spp. Testing should be sufficient to provide high confidence that that no Phytophthora spp. are present in the stock. This can be accomplished by testing a high percentage (or all) of the stock and/or conducting repeated tests over time if feasible. If using leachate baiting tests, see section 2.4 for additional information on how to maximize Phytophthora detection efficency to increase overall confidence in test results.
For predelivery testing by clients, it is generally preferable to test all plant species being delivered, and to include testing of different batches within plant species, especially if production steps could have resulted in different risk profiles for the different batches (e.g., different source propagules, batches of potting media, location within nursery, etc.)