You've cut back all the rose canes with winter injury from the December 2000 temperature plummet to 20 degrees below zero. You've kept your plants watered through the recent heat and drought. You've even managed the Japanese beetle plague. Now you find some of your rose leaves have yellow mottling, yellow or white veins, banding of veins with various colors, yellow to light green blotches or lines in the leaf, ring patterns, distorted or puckered growth, or smaller than normal foliage. In other words, you suspect a virus. Like the other problems you have treated, this one needs to be stopped now before it gets out of hand.
Viral diseases of roses are generally diagnosed based on symptoms as just described. The diseases usually do not kill infected plants but may reduce plant vitality and the quality of flowers. Because the range of symptoms is wide, it would be helpful to find pictures of rose viruses to help determine whether a virus is really involved. Many horticulture publications have such pictures, and the Web is very helpful. Report on Plant Disease no. 632, "Rose Viruses," may help. The Compendium of Rose Diseases by APS Press has great detail for the avid rosarian or diagnostician, including some color photographs.
Virus particles will not grow in artificial media in a lab. The Plant Clinic cannot isolate a particular virus through culturing procedures. Viruses are too small to be seen with a light microscope, so thin sections of plant material cannot help pinpoint a virus. For those who want to know the exact virus involved, there is a private lab in Indiana, AGDIA, Inc. They can screen for eight rose viruses, as detailed on their Web site, http://www.agdia.com/. There is a fee for this service, so check the site before sending your samples.
Viral infections of rose are systemic, which means they are found in all plant parts. A few parts may remain without symptoms despite the infection. Plants do not have an immune system, and they retain the virus as long as they are alive (with the exception of heat treatment in commercial propagation, which is used to inactivate some mosaic viruses). The virus particles need a live host to replicate. Although it may appear that viruses are more intense in the spring and fall, they are present all year. The heat of summer inhibits virus activity.
Because the treatment of rose viruses is generally the same regardless of the specific virus involved, exact identification is not usually necessary. Still, you want to be sure you are dealing with a virus. Viruses can be confused with injury caused by growth-regulator chemicals. The pattern of injury and host range should help in diagnosis. Herbicide injury is most intense near the source and less intense moving with the wind or water away from the source. If broadleaf herbicides such as 2,4-D or dicamba are to blame, they should also affect other broadleaf plants in the area. Rose viruses (especially the mosaics) are spread primarily by budding and grafting techniques. It is rare to spread such viruses by insects, plant contact, or seed. Therefore, the pattern of infection in the planting would be very different from herbicide-drift injury.
If you would like help in diagnosing a rose virus problem, send a sample to a plant lab such as the University of Illinois Plant Clinic. Details on sending samples can be found on the Plant Clinic Web site: http://www.cropsci.uiuc.edu/research/clinic/clinic.html. We can help verify virus symptoms and eliminate other possible causes, but we cannot identify the specific virus. It is important to provide background information with samples–including symptoms, progression over time, number of plants infected, condition of nearby plants, chemicals used anywhere in the vicinity, time of application of chemicals, pattern in the garden, pattern on one plant, fertility practices, and any other cultural information available.
Because these viruses are spread almost entirely by budding and grafting techniques, the responsibility for control should lie with the nursery and commercial rose grower. If you purchase a plant that develops virus symptoms, contact your supplier and ask for a replacement. When buying plants in a retail center, examine the plant closely for possible virus symptoms. Buy only plants free of such symptoms.