HOW DO VIRUS DISEASES LOOK AND WHAT DAMAGE DO THEY CAUSE?

Viruses cause a wide range of symptoms and damage. Symptoms include occasional yellow spots or blotches, mosaics, vein yellowing, yellow line patterns, leaf margin yellowing and necrosis. Plants and roots may be stunted, deformed in shape, leaves may role or cup and blossoms and tubers may be suppressed in size and number. Often virus symptoms can be confused with mineral deficiencies, other cultural problems and insect damage. However, viruses very rarely rot or outright kill their host. Virus names often imply a specific type of disease symptom but in practice many different symptoms can be found with any virus.

Identification of specific viruses based on symptoms is difficult because of the variable nature of symptoms. Some of the reasons for symptom differences are:

1. There are multiple strains of most viruses. We have isolated 5 strains of DMV so far that cause anywhere from no symptoms to severe mosaics and ringspots.
2. Many dahlias appear to be infected with 2 or more viruses. Virus mixtures can affect symptoms from reduction in symptom severity to a synergistic effect.
3. Not all dahlia viruses have been identified or they have been miss-identified. For example, DMV has been considered a strain of Cauliflower mosaic virus but our DMV samples are not strains of the latter but a separate virus.
4. Specific dahlia cultivars can differ in their susceptibility to any one virus. A virus isolate that causes no symptoms in one dahlia may be devastating to some other dahlia.
5. Growth conditions (temperature, soil fertility, day length, moisture etc.) can influence symptom severity. Therefore, a range of symptoms can be found during the season on each infected plant.
6. Symptom severity and type are affected by stage of plant growth at time of initial infection. Generally, the earlier the initial infection the more severe the symptom. Initial infections that occur late in the season often are minimal but tubers may still carry the virus.