It is useful, say plant pathologists at Washington State University, to know the identity of the virus affecting a dahlia because they differ in terms of the vectors responsible for their spread, the type of damage the virus causes, and inoculum sources and control procedures. Only about a dozen viruses, they say, have been reported to infect dahlias, and only a few of these known viruses are commonly found in dahlias. Most virus transmission seems to be associated with wounds, mostly those made by chewing and sucking insects; the viruses then spread in vegetative plant parts, such as cuttings or divided corms. While they are easily transmitted by vector insects, they are organism-dependent and do not survive in plant waste, compost or soil.
Viruses can slow plant growth and affect the appearance of foliage and flowers. Virus-infected leaves can become spotted, streaked, mottled, distorted or stunted. The veins in leaves may lose color or develop growths. Flowers may become smaller, deformed, streaked or faded in color, or fail to develop color and develop into leaflike structures. Symptoms can be similar in various viruses, according to WSU's Dr. Hanu Pappu, who holds the President Sam Smith Distinguished Professorship in Plant Virology. Some of his research is supported by an endowment created by the American Dahlia Society at the land-grant university in Pullman, Washington.
Probably the most common of the dahlia viruses is dahlia mosaic, most often associated with the vector aphids. Of diseased plants brought to WSU Cooperative Extension sites for testing, more than half turned out to be infected with dahlia mosaic virus (DMV). Symptoms include the yellowing and stunting of new growth, according to Dr. Pappu. Additional symptoms can include mosaic (alternating islands of light and dark green coloring on leaves), yellowish spots splashed across leaves, and malformation of young shoots.
Thrips is one of those strange words that's both singular and plural, so one bug is a thrips, as is an infestation of hundreds. Thrips are tiny, slender insects of the order Thysanoptera, so small that they're hard to see, mostly under 1/20 inch in length, with fringed wings. They're poor fliers but can catch and float on the wind.
Aphids, which are small, soft-bodied insects with long, slender mouth parts, live on the fluids they suck from plants. Almost every plant has one or more aphid species which occasionally feed on it. They are considered among the most destructive of insects among garden and market plants.
Both thrips and aphids feed on the undersides of leaves and thrips also attack the buds of late-season dahlias, causing rot. Controlling these insects will help prevent the spread of viruses in dahlias.
At this time, viruses cannot be controlled with pesticides. Dr. Hannu and others in the plant pathology field are looking at possibilities, and at Washington State University, there is discussion of starting a program which would involve treating virus-infected plants with chemotherapy. However, for the time being, getting rid of a virus-infested dahlia plant is the only way to prevent the spread of the viruses.