Symptoms of virus infection include necrotic spots, abnormal dark green and light green mosaic and mottling of leaves, growth distortion, stunting, ring patterns or bumps on plant foliage, and abnormal flower coloration and formation. Plants may also be infected with more than one virus or viral strain. Plants infected with multiple viruses and can show a combination of symptoms or more severe symptoms as a result of the multiple infections.
Plant viruses differ from all other plant pathogens. They are subcellular and can only be visualized with the aid of an electron microscope. Plant virus names usually refer to the first plant in which they were discovered. Viruses are composed of small pieces of either DNA or RNA encapsulated inside a protein coat. The protein coat can be of different sizes and is usually either sphere- or rod-shaped. Viruses can consist of a single particle that contains all the genetic information needed for the virus to function, or may be split among several particles.
Viruses reproduce inside living cells and can spread within infected plant tissues. Viruses do not divide or produce any kind of reproductive structures such as spores. Instead, viruses take over the host cells replication mechanisms and force them to manufacture more virus particles.
Vegetative propagation, sap, seed, pollen, insects, mites, nematodes, fungi, and parasitic plants can spread viruses. Viruses can be spread relatively long distances in insect vectors such as aphids and thrips carried on the prevailing winds. Handling infected plants spreads some viruses. Viruses present in plant sap can contaminate hands or tools and then infect healthy plants during normal handling.
In the Maryland IPM program, the most commonly encountered viruses in perennials are cucumber mosaic virus and impatiens necrotic spot virus. However, many other viruses are probably also present but have not been confirmed. Additional viruses that have been confirmed include alfalfa mosaic virus, arabis mosaic virus, tobacco etch virus, tomato spotted wilt virus, tobacco ringspot virus, and tomato ringspot virus. Most plant viruses cause systemic infections and there is no cure for infected plants. Viruses disrupt normal cellular processes and eventually cause abnormal cellular function.
Cucumber mosaic virus (CMV)
Infects many different plants and is a worldwide problem in perennials. CMV has many strains and typically causes stunting, distortion, and a mosaic pattern on leaves and flowers. CMV is primarily spread by aphids from plant to plant and infects many weeds that serve as reservoir hosts for infection of perennials. CMV can also be spread in infected sap by people handling infected and healthy plants simultaneously. Plants infected with CMV include asclepias, cimifugia, coreopsis, chrysanthemum, echinops, leonitis, lobelia, oenothera, phlox, polemonium, styokesia, and trollius.
Tobacco mosaic virus (TMV)
Is very common and also worldwide in distribution. A discussion of TMV is included because of its wide host range and high potential for spread in production facilities. It infects more than 150 plant genera that include many ornamentals. The most common symptom of TMV is a mosaic pattern of dark green and light green areas on infected plant leaves. Infected sap, grafting, dodder and seed spread TMV. Insects do not spread TMV. The most common means of transmission is by people handling infected and healthy plants simultaneously.
Impatiens necrotic spot virus (INSV)
Is currently the most common tospovirus found in greenhouses. INSV was described as a distinct virus in the tospovirus group in 1991. It is related to another tospovirus called tomato spotted wilt virus (TSWV). Both viruses have a very wide host range that includes many popular ornamentals. Symptoms can include necrotic streaks, spots, rings and lines on leaves and stems, distorted flowers, stems and leaves, general stunting and bud drop. Black, brown, reddish, or yellowish concentric rings, although not always present, are also symptoms of virus infection. Both viruses are transmitted by at least five species of thrips. Western flower thrips is currently the most common species on ornamentals. Only immature larval thrips can acquire the virus from an infected plant, and are then able to transmit the virus for the life of the thrips (12-44 days) as larvae and adults. Virus spread occurs by movement of infected thrips to healthy plants by crawling, jumping or on air currents within greenhouses or fields. Infected stock plants used for propagation and weeds that remain between crop cycles are possible sources of virus. To date, plants infected with INSV include aconitum, aquilegia, campanula, coreopsis, delphinium, dianthus, echinacea, gallardia, hebe, hosta, impatiens, lamium, lathyrus, lobelia, malva, monarda, peony, phlox, polemonium, primula, ranuculus, senecio, verbena, and annual vinca.
Tobacco ringspot and tomato ringspot viruses
Are transmitted by nematodes in the genera Longidorus, Paralongidorus and Xiphinema. These viruses can also be transmitted by infected sap. These nematodes acquire the virus by feeding on infected plants for a few hours, frequently weed species growing next to the crop, and then can continue to transmit the virus for several months. In some cases, infected seeds can also transmit viruses. Initially, virus infected plants show severe symptoms of necrosis, mosaic, and ring spots on the foliage. However, on perennial hosts that survive to the following growing season, foliar symptoms become less obvious or may disappear entirely.
Management
There is no cure for infected plants. Remove and discard infected plants. Weed control and sanitation are very important since weeds and plant debris can serve as sources for viral infections. Eliminate weeds that can act as reservoirs of the viruses where aphids, larval thrips, and nematodes can acquire viruses. People who use tobacco products should wash their hands with a phosphate detergent or soap before handling plants. Sanitation is especially important for controlling TMV since it is very heat stable and remains infective for long periods of time in sap and plant debris.