Most plant viruses are made up of nucleic acid (genetic material, usually RNA, more rarely DNA) in a protein coat. They are what is known as “obligate parasites”, in other words they can only gain nourishment from another living organism in order to grow and reproduce (a minority of fungal plant diseases such as rusts and powdery mildews are also obligate parasites). Once a virus has entered a plant, it is able to replicate by inserting its genetic material into plant cells and using them to produce more of itself.
Symptoms of viral infection may be latent (no visible affect on the plant), may reduce growth (common), or result in any of several other symptoms including visible marks on foliage or flowers such as distortion, yellowing, mosaics, spots, streaks, and mottling. There is generally no treatment available for an infected plant.
Some virus infected daylilies were recently (2005) discovered and removed from different nurseries by the Michigan Department of Agriculture. These plants had been imported for sale and it isn’t known whether other similarly infected plants have been available at nurseries or garden centers in other regions. If they have, then these plants present a risk to other daylilies and, perhaps, other plants growing in their vicinity.
The virus in question is the Tobacco Ringspot Virus (often abbreviated to TRSV), which can infect a number of different plants. However, there are different strains of this virus that may have variable host ranges and we do not, at this time, know which strain has infected these daylilies or whether it is a strain that already occurs in North America.
This virus is typically transmitted mechanically (such as during division of plants), and by vectors such as dagger nematodes, grasshoppers, melon/cotton and green peach aphids, onion thrips, spider mites and tobacco flea beetle, which introduce the virus by feeding on infected plants and then subsequently on healthy ones. Some of these are known pests of daylilies and could therefore spread the disease. In some plants, this virus can also be transmitted by pollenand seeds.
Daylily gardeners are advised to watch out for the yellow blotches of this virus as illustrated below, but be aware that other conditions like pest, disease and weather damage, nutrient imbalances, and chemical injury such as from pollution or sprays may resemble virus symptoms. Currently the only way to determine for sure if a plant has a virus is to have it tested by a diagnostic service.
If you suspect a daylily in your garden may have this disease, remember that once a plant is infected with a virus there is generally no treatment and the plant will likely be infected for the remainder of its lifetime. Any divisions taken from that plant are also likely to be infected, as are any other plants subsequently divided with the same tools if those tools are not adequately disinfected between plants. Also, there is the risk of pests carrying the disease to healthy daylilies. Until we know whether the virus is pollen and/or seed transmitted in daylilies, it is safest not to use any suspect plants for hybridizing.
The usual recommendation is to destroy virus infected plants. However, it can be difficult to differentiate virus symptoms from other disorders visually, plus we do not know at this time if any infected daylilies found their way to other nurseries, garden centers or gardens in North America. Expert advice is therefore recommended before determining a course of action regarding suspect plants. It is always prudent not to purchase any daylily that has an abnormal appearance.