Virus diseases of annuals and perennials are quite common. And, although some viruses can cause serious damage to plants, others are so mild that they may be overlooked. Symptoms may include overall chlorosis (yellowing) or chlorotic mottling; vein clearing (yellow or white color of the veins); green or dark bands of color parallel to the veins; yellowish or light green spots or blotches on the leaves; watermarks or rings on the foliage; leaves that are blistered or puckered, dwarfed, curled, wrinkled, and cupped downward; or plants with low vitality, small leaves, and shorter, bushier growth than normal.
Some of these symptoms are very similar to symptoms caused by some of the growth-regulator herbicides. Look closely at the pattern in the planting. Herbicide injury will be more intense near the source of the herbicide and progressively less intense moving away from the source. Other broadleaf plants will likely show symptoms as well, and all at about the same time. Viruses are more likely to occur on scattered plants and to spread slowly during the season. Viruses tend to be fairly specific to one type of plant species.
Once infected, plants remain so for life. The virus particle needs a live plant cell in which to multiply and spread. It cannot be cultured, extracted, or induced to sporulate in a lab, so we are not often very helpful in confirming a virus disease. There are private labs that specialize in serological techniques for confirming some viruses, so there is help for commercial growers or those willing to invest some time and money in tracking down a specific virus.
Viruses are spread by many vectors, including mechanical transmission, insects, seed, grafting, nematodes, vegetative propagation, and fungi. If you can identify the virus disease, you will probably be able to learn how the disease is spread, and this information will help you determine disease control practices.
You cannot kill or inhibit virus particles with sprays. Try to purchase only disease-free plants or those that appear healthy, without odd colors or leaf patterns. Remove plants that have damaging virus symptoms. Insect control is also a vital part of virus control in greenhouse or polyhouse situations.
If you are interested in detailed information on viruses, try the virus on-line Web site at http://biology.anu.edu.au/Groups/MES/vide/. The University of Illinois has several Reports on Plant Disease that discuss virus diseases of annuals and perennials: No. 608, Geranium; No. 612, Gladiolus; No. 614, Orchids; No. 632, Rose; No. 634, Tulip; No. 654, Iris; and No. 665, Tomato Spotted Wilt Virus (on many floral crops).