Linda Benedict, Valverde, Rodrigo A.
Rodrigo A. Valverde
Plant viruses are biological entities made of RNA or DNA. They are disseminated by way of vectors, usually insects, although in many cases they are transmitted through seed, cuttings from infected plants or mechanical contact. In general, plant viruses have been studied because they cause disease and economic loss in crops. The most common symptoms that result from virus infections include foliar mosaic, yellow vein, leaf malformation and plant stunting.
In many cases, ornamental plants with variegated colors in their leaves, such as white or yellow, have been propagated and commercialized as distinct varieties. This practice has been conducted also with flowers exhibiting desirable aesthetic value in which the petals are variegated because of the irregular distribution of pigments. Most foliar or flower variegations are the product of genetic factors that affect the production of pigments that give the colors to leaves and flowers. Examples of this are the variegated flowers of Red and Yellow tulip and the variegated leaves of Shell ginger. However, some plant viruses can cause these conditions in plants, and these viruses have been used by ornamental horticulturists to enhance the commercial value of plants. Tulip breaking virus in tulips is a classic example of a virus used to increase the beauty of a flower.
Tulips with beautifully variegated flowers are referred to as Rembrandt tulips because they were a favorite subject in many paintings by the Dutch Masters. In the past, tulips producing variegated flowers were desired and often sold at high prices, especially during the early 17th century. At that time, the virus was deliberately transmitted to healthy plants by some growers, particularly in Holland. Today, tulips with the same variegation patterns are widely available. Although often sold as Rembrandt tulips, most of them are actually virusfree, and the variegation is the product of genetic factors and not a virus.
In the 1940s, LSU AgCenter researchers showed that a virus named Camellia yellow mottle virus was the cause of the attractive color variation of some camellia varieties commonly grown in Louisiana. Today, we know that this virus does not appear to significantly affect the vigor or flowering of camellias. Nevertheless, as in the case of variegated tulips, the flower variegation of some camellia varieties is of genetic origin, while in others it is the result of viral infections.
In Louisiana, during the past few years, there has been an increase in the number of landscape ornamental plants showing a variety of attractive exotic foliar variegations. Laboratory testing at the LSU AgCenter revealed that for some of them, viral infections were the cause of the variegation and not genetic factors. Some of these ornamental plants include cannas, which are popular landscape plants grown worldwide. Varieties such as Bengal Tiger and Pretoria show attractive yellow stripes on their foliage, which has been associated with Canna yellow mottle virus.
Nandina, also called heavenly bamboo, is one of the most common ornamental plants in Louisiana landscapes. Plants infected with Nandina stem pitting virus show leaf curling, reddening and stunting, which can be desired characteristics. The virus does not have any known vector, and the only identified host is nandina.
Flowering maple infected with abutilon mosaic virus has been commercialized throughout the world since the late 1800s. The virus has negligible effects on plant growth, vigor and flowering. Several plant viruses have been associated with Golden Ginger mint, an attractive mint variety exhibiting yellow veins on green leaves.
Recently, LSU AgCenter scientists discovered that the attractive yellow variegation of Dancing Flame salvia leaves is caused by a plant virus.
All the virus-infected plants described are available through nursery catalogs and at garden centers across the United States. They are considered safe because they are already widely established. The viruses have very narrow host ranges and have lost the ability to be transmitted by vectors but can be transmitted by grafting in species normally propagated vegetatively.
It is important to have in mind that some of these viruses may cause disease in crops. Therefore, it is recommended that in-depth studies on the biological properties of these viruses be conducted before commercializing their use.
Rodrigo A. Valverde is a professor in the Department of Plant Pathology.
(This article was published in the winter 2013 issue of Louisiana Agriculture magazine.)