What is a “clean” plant? What process does the Clean Plant Center Northwest use to ensure fruit tree, grape, and hop plants (or root stocks) are healthy?

6DSC_0013healthy cluster 01Nice apples

At the Clean Plant Center Northwest (CPCNW), we follow a rigorous testing, propagative, maintenance, and retention process to ensure our plant materials are free from virus-like pathogens. In addition to verifying the health of our plants, we also verify they are true-to-type through genotyping.

The stages of the clean plant process are as follows:

  1. Introduction: A specific cultivar is submitted to the CPCNW for testing (either a piece of budwood or rootstock, or plant) and labelled with a propagation number, plant identification number, and variety name to ensure each piece of plant material can be tracked as it moves through the clean plant process.
  1. Initial screening and therapy: For each of the commodities we work with, the pathogen screening (and subsequent therapy if needed) varies slightly. We test the plant material for an array of viruses and virus-like pathogens. If pathogens are detected, then the cleanup process begins.
    • In the fruit trees program, we use heat and cold therapy techniques.
    • In the grapes program, we employ microtip tissue culture processes to propagate a clean plant.
    • In the hops program, we also use several different tissue culture processes, including meristem culture, heat therapy and meristem culture, chemotherapy and meristem culture, and cold therapy and meristem culture.

Plants are grown under strict biosecurity conditions in growth chambers, greenhouses, and screen houses. For all commodities, the pathogen testing conducted at screening is repeated and if the plant is determined to be clean, it will move forward in the process.

  1. Selection of ‘mother plants’: We identify a healthy, true-to-type plant to serve as the ‘mother’ plant; propagative materials are derived from this plant and prepared for release to the public (typically nurseries and growers).
  1. Release: Depending on the plant type (fruit tree, grape, or hop) we release healthy plant materials (budwood, dormant cuttings, propagative green cuttings or whole plants) through an ordering and distribution process that meets all phytosanitary requirements.
  1. Maintenance: Mother plants undergo annual testing to verify their pathogen-free status.
  1. Retention: We retain one to three plants per cultivar in our greenhouses, screen houses, and field blocks.

Why use tissue culture methods?

The meristematic region of the shoot tips has high levels of metabolic activity, and high hormone concentration. Most pathogens do not like these conditions, making the meristematic region a pathogen-free zone. Rapid division of the cells and the lack of well-formed vascular connections are the other factors which stops the viruses and other pathogens from getting into the meristem. The chances of producing a clean plant are high, if the plants can be generated from this specialized group of tissue found in the shoot tips.

This technique has been used to generate clean plants in many plant species since 1952. In grapes, the success rate for tissue culture methods in generating clean plants is higher compared to other methods. However, tissue culture requires a great deal of skill. The microtip must be dissected precisely to ensure it is located and sized correctly. Then the microtip must be placed in the right nutrient and hormonal media combination as it transforms into a plantlet.

Further details are available regarding our grapevine virus testing and tissue culture therapy and hop virus testing and tissue culture therapy techniques.