Best Management Practices for Producing Clean Nursery Stock

6.6. Inspection and testing

Objective:  identify potentially diseased material at the earliest possible stage so it can be culled in a timely manner to prevent further spread in the nursery.  Note that with clean inputs and clean production practices, the need for testing will be minimized and tests should show no Phytophthora detections.  If Phytophthora contamination is detected in the nursery, you will need to reevaluate your practices (see “7. Record Keeping” below) and look for possible avenues of contamination.

What you need to know:  Most of the Phytophthora species currently circulating in nurseries cause decay of fine feeder roots, though some may progress to cause lethal basal cankers.  Decay of fine roots alone can result in stunting and drought stress symptoms, because affected plants have reduced capacity to absorb water and nutrients.  In nursery situations, plants commonly get plenty of water and nutrients and have low evaporative demand due to shading and high humidity.  Under these nursery conditions, plants that are drought tolerant, like many California natives, can survive and show no obvious top symptoms even if they have very few functioning roots.  Although visual inspection of the tops is of limited use for detecting root rots, close attention to plant appearance (e.g., off-color) and patterns of growth (e.g., stunting, low vigor) in a batch of plants can help you spot plants or sets of plants that should be tested.  Various Phytophthora species, including many that mainly cause root rot, can also cause foliar blighting or stem cankers if they are splashed onto shoots and leaves.  These shoot and leaf symptoms can be detected by careful inspection, though testing is needed to confirm whether such symptoms are due to Phytophthora.

Routine testing for Phytophthora in production areas should be conducted as a quality control measure. Individual plant tests are warranted when symptoms suggest that plants may be infected.  Bench-level tests are needed to follow up on detections and are useful for quality assurance.  Not all test protocols are equally useful for all situations.  Most tests will identify highly-infected plants, but to detect low levels of disease in asymptomatic plants, the proverbial “needle in a haystack”, tests that depend on small samples from one or a few plants will be very inefficient and result in false negatives.  See Phytophthora Testing Procedures for BMPs for Producing Clean Nursery Stock for more information.

Best practices:

6.6.1.Visually inspect all plants regularly and frequently (at least weekly) for poor plant growth or appearance.  You can use photos from a fixed point to help compare appearance over time. Symptoms in Phytophthora-infected plants may include (in order of increasing severity): low vigor, stunting, off color, intermittent wilting or water stress symptoms, leaf dieback or blighting, root collar and/or stem cankers, whole plant wilting or necrosis.

6.6.2. Look for patterns of symptoms in the block that may suggest spread from one or more infected plants.  This will not be possible if you rearrange plants on the bench.

6.6.3. Remove suspected diseased plants from the clean production area in a manner that will prevent contamination of other remaining plants.  In particular, don't let water or potting media from removed containers fall into other containers or onto clean surfaces.  Place pots directly in a plastic bag or clean container before moving. 

6.6.4. The positions of culled plants on a bench should remain unoccupied at least until testing has been completed.  Quarantine adjacent plants (hold in place without selling or moving) within 2 m (6.5 ft) ft of suspected diseased plants until testing is completed. 

6.6.5. Remove suspected diseased plants from the clean area to a contained, cleanable surface to inspect the root system.  Look at as much of the root system as possible by separating roots from the potting media; rinse with water if needed to see roots more clearly. Roots with severe Phytophthora root rot may have roots that appear discolored, mushy or decorticated (outer soft tissues slough off, leaving only the woody vascular tissues).  In less decayed roots, you many only see decay or discoloration of small side roots or newly-emerging root tips and overall root growth may be less than expected.  There may be areas of apparently healthy roots and others that show decay.  At early stages of disease or in some species it may be difficult to see any clear symptoms of disease.

6.6.6. Plants with possible disease symptoms and surrounding plants should be tested for the presence of Phytophthora, or other pathogens if appropriate (see Phytophthora Testing Procedures for BMPs for Producing Clean Nursery Stock.

6.6.7. If Phytophthora is detected within a block of plants, the entire block should be considered to be contaminated. Further testing should be conducted to determine whether the contamination is associated with one or more batches or is associated with the block (see definitions). Use bench level testing or other methods to test the suspect block and other plants from the same batch that are in other blocks.

6.6.8. Dispose of all plants in contiguous blocks that test positive for Phytophthora.  Quarantine and continue to test blocks within splash distance (typically 1.5 m [5 ft]) of any known positive block until you determine the limits of the infected plant material.  Plants adjacent to a detection can be considered uninfected if no detections are made in two successive tests conducted at least 2 weeks apart under suitable test conditions. If the detected pathogen is under quarantine or regulation (e.g., P. ramorum) additional rules and protocols may apply.

6.6.9. Thoroughly sanitize bench surfaces, irrigation equipment, and other items and surfaces that may have been in contact with Phytophthora-infected plants.

6.6.10. Seek diagnostic help from qualified experts if you encounter unfamiliar pests or disease symptoms.

6.6.11. Bench level testing can be used to test for the presence of Phytophthora in blocks of plants that do not show obvious symptoms as a quality control check.  Emphasize testing of higher risk material (e.g., older plants, plants from tier 2 or 3 propagules), and test material before upsizing to larger containers. See Sample size and sample selection for additional details on selecting plants for testing.

3/1/2021 - minor edits for clarity.
6.6.4 – Increased recommended quarantine area around suspected Phytophthoraem>-infected plants from 1 m to 2 m.
6.6.7. Additional details.
6.6.8. Additional details.
6.6.11. Additional details.