The damage virus infection causes dahlias ranges from no observable symptoms to very severe. However, it is very rare that a virus outright kills its host. Typically, viruses stunt the plant, reduce the number of blooms produced and cause leaf distortion such as leaf rolling. Often, there will be green or yellow mosaic or mottling of the leaves. Viruses may also cause premature leaf senesence. It is not completely known how viruses damage plants but viruses can alter uptake of nutrients, affect metabolism, degrade cellular structures and alter plant hormone metabolism and activity required for normal plant development. The type and severity of the damage is dependendent on 1) the genetic composition of the plant, 2) the genetics of the virus (virus type and type of virus strains present), and 3) the environment (day length, temperature, moisture levels, nutrient levels). Initial symptoms are often initially very mild and easy to miss but they tend to intensify with time. Therefore, virus detection and identification if often difficult in the early stages of plant growth before the virus has had a chance to spread to other plants. Because disease symptoms are affected by natural environmental or seasonal changes, disease symptoms can seem to appear suddenly and later somewhat disappear giving the impression that plants recovered. However, plants can not recover from virus infection as we do.
It should be noted that the disease symptom pictures shown here are of plants grown in the greenhouse at Washington State University. Greenhouses are used because we can work on the virus diseases continuously including winter. However, generally the symptoms are more severe on outdoor grown dahlias. Although our greenhouses have temperature control and good lights, they do not duplicate natural conditions. It will be interesting to see what environmental condition is lacking in the greenhouse. It could be light intensity, day length, frequency of fertilization or watering or other factors less obvious.