Elimination of Chrysanthemum Virus B to Produce Virus Free Chrysanthemums Stock
by Alan Petravich
Chrysanthemums play a lead role in the Autumn’s Colors Festival at Longwood. Thousands of plants are produced from cuttings taken from stock plants, which are kept indefinitely in Longwood’s production facilities. Chrysanthemum stock plants are held on property since many of the exotic cultivars are not readily available, commercially, in the United States. It is critical that these plants be free from virus diseases that can cause stunting, chlorosis (yellowing of parts of leaves), and lack of vigor. Plants exhibiting these symptoms are not suitable for display.
Chrysanthemums are easily infected with virus in the greenhouse. Insects such as aphids and thrips spread the disease as they feed and move from plant to plant. Cutting instruments that have not been sanitized with bleach or alcohol can also spread virus. Once a plant has contracted a virus, it is spread systemically to nearly every plant part. The virus invades the very cells of the plant. Cuttings taken from a virus infected stock plant will also contain the virus. To remedy the virus issues, a virus elimination process has been implemented in Longwood’s research facilities.
Virus elimination is accomplished by harvesting the apical meristem, the tissue occurring at the vegetative shoot tip. A meristem is where new cells are made. Meristem cells are often virus-free, possibly because new cells are made faster than the virus can invade them. To begin the process, a vegetative apical shoot of a virus-infected plant is dissected under a microscope. Leaves are removed from the shoot one-by-one with a scalpel. Each leaf is smaller than the previous one. It is similar to opening a matryoshhka Russian stacking doll. Eventually, the apical dome containing the meristem, is revealed. It resembles a shiny drop of green jelly. The dome and surrounding area are carefully removed with a scalpel. The resulting speck on the tip of the scalpel is barely visible to the naked eye. The tissue is placed in a test tube on a specialized medium containing mineral nutrients, sugar, plant hormones, and a gelling agent. The tissue grows on this meristem-isolation-medium for 6 weeks.
During this time the cells differentiate into micro-shoots with leaves and stems looking the same as a normal plant except on a miniature scale. Once a micro-shoot is large enough, it is removed from the isolation media and placed on multiplication medium (similar to isolation medium, but with higher mineral content). After a few weeks the micro-shoot can be harvested and rooted like a conventional cutting. Once the plant is established and actively growing, leaf samples from the plant are tested for presence of virus. We have successful at recovering virus-free plants from over 20 Chrysanthemum cultivars including ‘Kokka Senkin’, ‘Icy Isle’, and ‘Western Brushfire’. These new virus-free plants provide the stock plants to produce all the new display plants for the future. Virus-free cultures are also maintained in case of re-infection of stock plants.