Viruses on Roses

Philip Brierley.

The rose is a leading crop in commercial floriculture. Firms that produce roses under glass usually are specialists, for those roses require well lighted and ventilated greenhouses and higher growing temperatures than many other flower crops.

Some producers propagate roses by cuttings, but most greenhouse roses are grafted or budded on Manetti, a special understock variety (Rosa noisettiana var. manetti) that is grown and propagated chiefly in the Pacific Coast States and Texas. Manetti stock for grafting is shipped from nurseries in Oregon, Washington, the Netherlands, England, and France to the producers who do their own grafting in the greenhouse with scions from their own flowering varieties. Nurserymen also bud Manetti in the field, inserting buds of varieties from a greenhouse or from their own field-grown roses.

Buds that are set early in the season may be forced into growth the same year by removing the top of the under-stock. Such plants are marketed in the fall as started buds. Buds set later in the season are not forced into growth, but on removal of the top of the under-stock at maturity in the fall may be sold as dormant buds.

The greenhouse producer replaces some of his flowering plants each year, generally using his selection of greenhouse varieties budded or grafted on Manetti. Hybrid tea roses dominate the greenhouse business, but a few hybrid perpetuals, ramblers, and baby ramblers, also purchased as budded plants, are grown in pots for Easter and for Mother's Day.

The nurseryman thus has an important role in supplying rose plants for greenhouse production.

Perhaps equally important is his production of roses for garden planting primarily hybrid teas but also hybrid perpetuals, climbers of the rambler and large-flowered classes, singles, and species roses. Nurseries in New York, Pennsylvania, New Jersey, Texas, California, and other States propagate roses for garden planting. The common understock used for outdoor roses is Rosa multiflora and its varieties, but Ragged Robin (Gloire des Rosomanes) is preferred in California. A few types of roses are grown on their own roots. Budded rose plants for the garden are marketed only after one or two seasons of growth in the nursery.

Many diseases afflict roses, both under glass and in the garden. The worst ones are of fungus or bacterial origin. No virus disease of major importance has yet appeared on roses in this country.

Viruses diseases of the mosaic class and rose streak are present here. Rose mosaic was believed to be serious when it was first recognized in the late 1920's. Rose wilt occurs in Australia, and it or a similar one has been reported in Italy. Because reports from Australia and Italy indicate that the virus disease is much more damaging than the ones we already have in this country, roses may ma not be imported from Australia and Italy to the United States.

ROSE MOSAIC is characterized by yellowish areas in the rose leaflet, usually next to the midrib and feathering away from it. Growth is retarded in the chlorotic yellowish--patches, and some distortion of the leaflet results. In varieties like Ophelia, Mme. Butterfly, Radiance, Rapture, and Templar, both chlorosis and distortion are conspicuous, and the disease is easily recognized, although not every leaf shows symptoms. In Better Times, Briarcliff, Columbia, Talisman, and others, the mosaic symptoms are similar but milder and more easily overlooked. In Rosa odorata, formerly used to some extent as a greenhouse under-stock, symptoms are also mild. In Manetti they are mild and erratic.

A second symptom is water marking, an intricate pattern of fine grayish lines in leaf surfaces with little or no distortion. Water marking is the typical expression in Blaze, Conrad Ferdinand Meyer, Duchess of Wellington, Joanna Hill, Kaiserin Auguste Viktoria, Paul's Scarlet Climber, R. hugonis, R. wichuraiana, Souvenir de Claudius Pernet, Ulrich Brunner, and others. Ring patterns of fine grayish lines also occur.

Roses affected with mosaic are less vigorous than normal plants and production is lower. The degree of inferiority varies with the intensity of the symptoms. In some greenhouse varieties, among them Better Times and Peters' Briarcliff, production seems to be only slightly inferior, and plants with mosaic frequently pass unnoticed.

Soon after rose mosaic was first recognized, observers agreed that no natural spread occurred in greenhouses. Its appearance in new varieties of recent seed origin and the occasional appearance of high percentages of mosaic in nurseries, suggested,' however, that natural spread was taking place in the field.

Floyd F. Smith and I made test plantings of healthy roses in Oregon, New York, Virginia, and Maryland, but we failed to detect any natural spread of rose mosaic. No mosaic resulted from 229 transmission tests made by Smith with insects commonly found on roses in Eastern States. The insects in the tests included 42 species in the families Cicadellidae, Cercopidae, Membracidae, Araeopidae, Fulgoridae, Aphiidae, and Coccidae and in the order Homoptera, and one species of thrips (order Thysanoptera). In further experiments, roses were not infected with strawberry crinkle by the strawberry aphid, nor with red raspberry mosaic by the raspberry aphid, nor with aster yellows by the six-spotted leafhopper.

Various methods of grafting and budding serve to transmit rose mosaic, the symptoms appearing 20 to 49 days or longer after such inoculation. No transmission takes place if the bud is removed before tissue union is effected. No evidence of seed transmission was found. We failed to transmit the rose mosaic virus from rose to rose by mechanical means.

Robert W. Fulton at the Wisconsin Agricultural Experiment Station in 1952 transmitted rose mosaic virus by mechanical inoculation from rose to cucumber and cowpea. From cow-pea he transmitted it to 25 other plant species in 7 plant families. The virus proved unstable and was extremely difficult to inoculate into rose even from the most favorable source, cowpea.

Floyd Smith and I concluded that simulated natural spread of rose mosaic resulted from the nursery practice of taking cuttings from the tops of the understock of field-budded plants after such tops had been removed to force the inserted buds. By this practice mosaic could be introduced in either understock or budwood and transmitted to additional understocks and top varieties in succeeding years. When new rose varieties were supplied as budwood, the disease appeared in these new sorts and suggested natural spread of mosaic. The weak and erratic expression of mosaic symptoms in Manetti permitted this process to go on undetected, so that natural spread of the disease was inferred. Although the origin of rose mosaic is unknown, such nursery practices very likely account for the distribution of the disease in this country.

A NUMBER OF VARIANTS or distinct mosaic diseases of rose have been reported. They differ from the typical rose mosaic in developing patterns of a brighter yellow, or more severe leaf distortions, or some other characteristic symptom.

Transmission of a chlorotic disorder by grafting from rose to rose was reported by M. Vibert in France as early as 1863. A. Christoff in Bulgaria found a virus of Rosa gallica transmissible by grafting to apple and pear and a virus of apple transmissible to Rosa gallica and to pear. I. Kovatchevsky also reported a rose virus disease in Bulgaria. C. Blattny found a vein mosaic of Rosa caning in Czechoslovakia.

H. Earl Thomas and L. M. Massey found two mosaic diseases of rose in California that differed sufficiently from the typical rose mosaic to be designated as rose mosaics 2 and 3. They also infected rose with the virus of apple mosaic and with another from peach affected with the disease now known as yellow bud mosaic. The diseases were shown to be distinct from each other by distinctive responses in suitable rose test varieties. Their rose mosaic 3 proved infectious to apple.

L. C. Cochran found two roses in California naturally infected with the virus of peach ring spot.

Floyd Smith and I collected five yellow mosaics of rose that differed from the typical mosaic in having brighter and lighter yellow areas. Each of the collections was from a single affected rose one from a greenhouse and the other four from garden varieties. Comparison of the five collections in five test varieties of roses showed that no two were identical in expression of symptoms, but we tentatively accepted them as strain variants of rose mosaic.

M. Kramer studied rose mosaics in Brazil and assigned one of his collections to rose mosaic 3 of Thomas and Massey and another to our rose yellow mosaic. I. Klastersky in 1949 reported from Czechoslovakia a morphogenic virosis of Rosa lucida, R. rugosa, and the moss rose Gloire de Mousseuses. It had abnormal cornet-shaped leaves. Manual inoculation with crushed leaves reproduced those symptoms in Rosa arvensis and R. moyesi. Klastersky's reports in 1949 and 1951 indicate that the morphogenic virosis generally causes little injury to roses. I cannot form even a tentative conclusion as to how many rose virus diseases are involved in the several reports I have mentioned. The several workers have made use of different varieties or species of roses as test plants, so that even the reported symptoms cannot be compared. The facts that some (but not all) rose viruses are infectious to apple and pear and that roses can be infected by viruses from apple and from peach show that a number of separate entities are involved. Further study of the interrelationship of the viruses of rosaceous plants is clearly needed. It is a fortunate fact that all the rose viruses known in North America can be transmitted from rose to rose by tissue union only that is, by budding or grafting, and not by manual methods nor (as far as we know) by insects. In this important respect they are similar to the typical rose mosaic, and should be subject to control by measures effective against the type disease.

Symptoms suggestive of virus disease but never proved to be of virus origin are common in garden roses and particularly in understock varieties. Manetti canes sometimes bear zones of severely dwarfed and crinkled leaves, alternating with zones of nearly normal leaves, a condition called rattlesnake by western nurserymen. Similar zoning appears in the less commonly grown Rosa odorata.

The rattlesnake symptoms are far more conspicuous than rose mosaic symptoms in those understocks. Smith and I were unable to get evidence that a virus is involved, for affected Manetti induced no symptoms in hybrid tea roses when grafted or budded into them, and we found no Manetti that did not sometimes develop the rattlesnake symptoms spontaneously. The nature of this abnormality remains unknown, but it is certain that confusion of rattlesnake symptoms with rose mosaic led to high estimates of mosaic prevalence in early surveys. Similarly, some early claims that rose mosaic had lethal effects now appear to have been based on confusion of rose mosaic with the effects of immaturity of the Manetti understock. Such immaturity is now avoided by use of a starch test, which shows when the understock is ripe enough for digging and shipping.