Propagating plants vegetatively which clonal material has one great disadvantage: clones can become infected with systemic viruses and mycoplasma-like organisms that are passed along to the daughter plants during asexual propagation procedures. In time, all clonal members may become infected with viruses. Some viruses are latent in particular nonsusceptible clonal material, but if this material is used in a graft combination where the viruses can move through the graft union to the graft partner-which is susceptible-then the entire grafted plant will be killed by the virus. On the other hand, virus-fee seedlings can be obtained in many species by seed propagation because the virus is not transmitted through the embryo.
Some viruses can be removed from clonal material by heat treatment. The virus-infected plant material, perhaps a small nursery tree growing in a container, is held at 37oC to 38oC for two to four weeks or longer. This combination of time and temperature eliminates the virus. After treatment, cuttings can be taken for rooting, or buds may be taken for budding into virus-clean seedling root-stocks.
Another procedure to eliminate viruses from clones is shoot-tip culture. In virus-infected plants the terminal growing point is often free of the virus. By excising this shoot apex aseptically and growing it on a sterile medium, roots will often develop, producing a new plant free of the virus. Here again, a starting point becomes available for continued propagation of the clone but without the virus. This method has succeeded with many herbaceous plants such as carnation, chrysanthemum, hops, garlic, rhubarb, orchid, and strawberry. Sometimes, in strawberry plants for example, both procedures are required-heat treatment of the plant, followed by excision and culture of the shoot tip-to free the plant of known viruses.
In recent years certification programs have been established by government agencies in many states in the United States and in other countries to provide nurseries with source of true-to-name, pathogen-free propagation material. Elaborate programs, for example, have been established for citrus in Florida and California and for deciduous tree fruits, grapes, strawberries, potatoes, and certain ornamentals in many states.
In such programs, mature flowering or fruiting plants known to be true-to-name and true-to-type are selected as mother plants. These are “indexed” by certain grafting procedures to be sure that no known viruses or other diseases are present. If no pathogen-free source plants can be located. Once a “clean” source is obtained, it must then be maintained under isolated and sanitary conditions, with periodic inspection and testing to ensure that it does not again become infected. Sometimes it is necessary to grow the plants in insect-proof screenhouses or greenhouses or in isolated areas far from commercial production fields.
J.mcmahon, Margaret, et.al,. 2002. Hartman’s plant science. New jersey: pearson eduction.