Plants with possible disease symptoms (low vigor, off-color, stunting, wilting, dieback, decayed roots) are the first priority for testing. However, because Phytophthora-infected plants may not develop obvious symptoms in the nursery, plants without obvious symptoms should be monitored as part of your nursery inspection program.
If you are following clean nursery production BMPs, Phytophthora detections should occur rarely, if ever. Because resources for testing are limited, you should prioritize the plants or blocks of plants for testing based on their overall risk profile. Plants that have the greatest risk of being infected 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||In the absence of a certification system, it is difficult to 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 associated with 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.|
|Plant species||most dicots, conifers, and many non-grass monocots||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, often including hundreds of species in many plant families. Finally, few California native species have been tested for Phytophthora susceptibility, and these tests commonly include only one to a few Phytophthora species. The host ranges of Phytophthora species on California natives are largely unknown. Hence, for general disease monitoring purposes, a variety of plant species should be tested.