Some background information is important to understanding the strategies that will be recommended for managing -- not controlling or eliminating -- viruses in dahlia gardens. All viruses are obligate parasites. (Greenhouse plant viruses) This means they cannot survive outside of their host (the plant or certain insects [vectors]). They cannot survive in soil, compost, or other garden related materials and substances even though fungal and bacterial organisms can.
Viruses can be carried from one plant to another by insect feeding, vegetative propagation, and human mechanical inoculations. Insect feeding is, by far, the most common means of transmission. Some viruses, impatiens necrotic spot virus for example, can only be transmitted by one vector, the western flower thrips. Other viruses can be transmitted by up to 50 different insects. According to Westcott's Plant Disease Handbook, dahlias are known to be host to four viruses: Dahlia Mosaic (vector--aphids); Tomato Spotted Wilt (vector--onion, flower, and three other thrips); Cucumber Mosaic (vector--aphids and seeds); Tobacco Ring Spot (vector--nematodes).
In the case of vegetative propagation, viruses can be transmitted in two ways. Cuttings may be taken from a contaminated plant in which case the cutting is already infected. Secondly, the blades used to take the cuttings from a contaminated plant may be immediately used to take cuttings from a plant that is not contaminated, transferring juices containing viruses from the infected plant to the non-contaminated plant. This later is actually a case of mechanical inoculation.
Human mechanical inoculation occurs when juices from a contaminated plant are transferred by mechanical means to a non-contaminated plant (i.e. during activities such as dividing tubers, taking cuttings, disbudding, topping, trimming, etc.). The probabilities of transmitting virus by mechanical inoculation are thought to be much lower than by the other two means. Nevertheless, the possibility of such transmission is there and must be considered in developing strategies for managing viruses. A crude analogy might be "are you going to practice safe sex or not"?
Managing viruses in dahlias by observing virus symptoms and destroying plants with such symptoms is not possible. First, symptoms that correlate with virus also correlate with fungal and bacterial diseases, environmental stresses, and cultural practices. Thus, without a serological test, the diagnosis may be wrong and plants not having virus may be destroyed needlessly.
More seriously, however, is the fact that dahlias can be infected with virus and show no symptoms whatsoever (Greenhouse plant viruses). Thus one may destroy all dahlias showing symptoms of virus and still unknowingly and with high probability keep plants and tubers infected with virus.
The only sure way to keep viruses out of ones dahlias would be to start with all new stock, have all new stock tested prior to usage, and prevent any of the vectors (aphids, or other piercing, sucking insects, such as thrips, planthoppers, and leafhoppers) from reaching your dahlias. While this may sound easy, it is financially impractical and almost certainly infeasible.
Remove from the proximity of the garden and destroy (compost* or trash can), all plants (not just dahlias) bearing symptoms of virus even though they may not have virus. Since stunting is the most common symptom of virus disease, give special attention to removing stunted dahlias and other stunted plants (including peony, columbine, delphinium, iris, impatiens, aster, gaillardia, tomato, cucumber) (Diseases of Annuals and Perrenials Page 29).
* If you choose to compost, make sure you do it right. While viruses cannot survive when the plant is finally dead, fungal and bacterial pathogens can. The composting temperature must reach 160 degrees fahrenheit to kill most pathogens.
Vectors (insects) include: aphids, thrips, planthoppers, and leafhoppers. While some viruses are transmitted by only one type of vector (i.e. Tomato Spotted Wilt Virus and Impatiens Necrotic Spot Virus are transmitted only by Thrips), dahlias can be infected with several types of viruses some of which are transmitted by other vectors. Thus, when applying chemical controls, it is necessary to control for all of those vectors. Use chemicals wisely. Follow the instructions on the label for the particular insects. In most cases, the chemicals will need to be applied at 5 or 7 day intervals. Rotate insecticide chemicals every two life cycles to prevent the insects from developing immunity to the chemical.
While the probabilities are low, viruses can be transmitted from one plant to another by means of cellular juices. A virus can survive in those juices (until they are dried up) for a short period. Blades or other instruments used in dividing tubers, taking cuttings, disbudding etc., can carry those juices from one plant to the next. Dipping such instruments in alcohol may help. While alcohol will not kill virus, it may wash away and/or dilute the infected juices, reducing the probability of transmission considerably.
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