Testing Procedures
for BMPs for Producing Clean Nursery Stock

2.3.  Testing strategy

Use the individual plant sampling/baiting (3.2) protocols to test individual symptomatic plants. When performed correctly, baiting is a sensitive test for many species of Phytophthora and can provide definitive positive results. However, false negative results can be obtained if infection levels are very low. If plants are already dead, use direct baiting of the entire plant root system, which is a destructive test, instead of the leaching or flooding protocols. Immunoassay test strips (3.1) may be used to get an indication of whether Phytophthora is present in a symptomatic plant, but this test can generate both false negatives (from insufficient pathogen antigen in the small sample of roots tested) and false positives (due to cross reaction with Pythium and Phytopythium species in the sample. Hence, a positive detection via baiting is definitive, whereas a positive reaction in an immunoassay test is not.

Use bench testing of leachate (3.4) to monitor batches of container-grown plants for quality control purposes. Prioritize your testing to emphasize higher-risk plants, considering the risk factors in Table 1. For example, 2 year-old Salvia plants grown from root divisions that originated in cultivated landscapes would have a very high risk (and should not be found in a nursery following the BMPs), compared with 6 month-old grasses grown from clean seed.

If the purpose of baiting is to detect infected plants, the development of (typical Phytophthora lesions on pear baits ) is sufficient to indicate that Phytophthora is present in the tested plants. If pear baits develop atypical lesions that do not match typical Phytophthora or Pythium lesions, it will be necessary to submit baits to a qualified lab for confirmation. To identify Phytophthora species present, symptomatic baits or cultures isolated from lesions on baits should be submitted to a qualified lab that can conduct DNA barcoding to match the isolate to known species.