Best Management Practices for Producing Clean Nursery Stock

2. Clean planting materials

Objective: Start with propagative material that is free from infection or external contamination by Phytophthora species as well as other possible pathogens.

What you need to know:  Phytophthora propagules can move in infested soil and water.  Water splash can disperse spores in still air at least up to a height of about 0.6 m (2 ft) and a horizontal distance of 1.5 m (5 ft) (see Basic Rule of Thumb 6 here).  Spores splashed from the soil surface can initiate above-ground infections of flowers, leaves, and stems.  Seeds lying on infested soil can become infested or infected, especially under moist conditions. Phytophthora can also be present in surface waters (creeks, ponds, storm runoff, etc.) and plant parts that come in contact with these waters many become contaminated or infected.

Infested or infected plant propagules can introduce Phytophthora into the nursery, where the pathogen can develop and spread. Tools, footwear, and other items can also pick up Phytophthora contamination from field sites and introduce them into the nursery. Conversely, Phytophthora from nurseries, landscaped areas, or other contaminated sites can be inadvertently introduced into seed collection areas via these same items.

Best practices:

2.1.  Don’t collect plant materials for propagation in areas known or likely to be infested with Phytophthora.  Only a small percentage of the Phytophthora-infested areas that exist have been documented.  As a general rule, assume that nurseries and cultivated landscapes (yards, gardens, parks, botanical gardens, arboreta, etc.) have the potential to be infested.  Some natural areas, including portions of regional or state parks, open spaces, ecological reserves, and private lands may also infested, especially in areas where nursery stock has been planted.

In the unusual situation where collection must occur in Phytophthora infested areas (e.g., for rare plant populations), seed or tip cuttings may be collected, from a height of 1 m (3 ft)or more above the ground if possible. Material propagated from such sources should be kept segregated from plant material propagated from pathogen-free areas.

2.2.  Collect seeds, cuttings, or other propagules only from plants and fruit that appear healthy. Do not collect or store seeds or other propagules with apparent disease symptoms such as decay, atypical discoloration, or fungal fruiting bodies. 

2.3. Don't collect seed from the ground.  Collect seeds and cuttings as high above the ground as possible, preferably at least 1 m (3 ft) above the soil surface. Seed can be knocked onto clean tarps placed on the ground or collected using seed traps. For low growing species (less than 1 m) avoiding collecting seed or fruit that has been contaminated with soil or flooded during development. In general, seed that sets after the rainy season has ended has a low risk of being contaminated if collected before fall rains begin.

2.4.  Don't collect seeds or other propagules during wet or muddy conditions to minimize potential for contaminating propagules or spreading contaminated soil.

2.5. Collect propagules with clean hands/gloves and equipment (pruning shears, etc.) and place them in new bags/envelopes and new or clean containers. Sanitize gloves, hands, and tools immediately if they come in contact with soil or other potential sources of contamination. Sanitize cutting tools frequently.

2.6. To avoid introducing Phytophthora into seed collection areas.  Make sure your equipment, vehicle, and footwear are clean.  Clean and sanitize your footwear and tools between locations.

2.7. Conduct all processing of seeds or cuttings in a clean work area with clean equipment and clean hands or gloves.  Discard or sanitize any seed or propagule that is dropped on the ground or comes in contact with contaminated surfaces or materials.

2.8. Clean seed as soon as possible after collection to remove any debris before storage and stratification.  Inspect stored seeds or other propagules regularly and discard materials that develop symptoms in storage. 

2.9. Where compatible with seed storage and germination requirements, treat seed using heat or appropriate disinfecting chemicals to eliminate seed-borne pathogens or external contamination. Seed treatment may be omitted for species where it is impractical or the risk of seed-borne or contaminating pathogens is negligible.

2.10. Do not bring potentially infected or contaminated plant material into clean production areas of the nursery.  Properly collected seed and tip cuttings (described above) will normally be free of Phytophthora.

2.11. Plant propagules that have been in contact with the soil, such as runners, divisions, tubers, rhizomes, and bulbs (phytosanitary tiers 2 and 3), have an elevated risk of being infected or contaminated with Phytophthora or other soilborne pathogens.  Plant stock originating from such propagules should be segregated from planting material started from cleaner sources, such as seed or cuttings, and from other vegetatively propagated material from different locations.  See Propagating from field-grown plants for additional information about plant propagule phytosanitary tiers for managing Phytophthora contamination risk.

2.12. Plant propagules that have been in contact with the soil, such as runners, divisions, tubers, rhizomes, and bulbs (phytosanitary tiers 2 and 3), should be thoroughly cleaned to remove soil and inspected.  Discard propagules that show evidence of decay.  Surface contamination can be removed with treatments such as diluted bleach dips, but surface treatments will not eliminate internal infections.  Internal infections can only be eliminated by heat treatments, but not all plant propagules will tolerate temperatures needed to kill Phytophthora infections (see Phytosanitary Procedures for BMPs for Producing Clean Nursery Stock). See Propagating from field-grown plants for additional information about plant propagule phytosanitary tiers for managing Phytophthora contamination risk.

2.13. Assume nursery stock from others is infested and do not bring such material into clean production areas of the nursery. Stock from another nursery may be clean if it was produced entirely under a well-documented and certifiable clean production system that is substantially the same as these BMPs. If such material is accepted, maintain it in quarantine for at least 3-4 months for observation and repeated testing for Phytophthora . Continue to keep such material isolated (at least 1 m or separated with a barrier) from stock produced by your nursery under clean production practices.

2/15/2021 - minor updates and numbering corrected.