No such disease of roses is known in other countries, but the limited distribution we know of suggests that the streak disease was imported in one or more of the many rose species and varieties assembled from all over the Northern Hemisphere for breeding purposes. Since streak was not recognized as a disease by the plant breeders, it was evidently propagated by budding or grafting and probably sent to a few other areas in named varieties or in understock roses. The disease is now static because it has no natural means of spread in this country and affected plants are seldom propagated.

Some 60 varieties of rose have been found affected with streak. Among them are teas, hybrid teas, hybrid perpetuals, hybrid multifloras, hybrid wichuraianas, hybrid rugosas, hybrid Bengals, Noisettes, Chinas, polyanthus, and the understocks Manetti and multiflora.

Symptoms vary greatly in different types of roses. Most distinctive patterns are brownish rings and brown vein banding in fully expanded leaves, accompanied by brownish or greenish rings in canes, as shown by Rosa odorata, Silver Moon, and other roses. Green vein banding patterns persist in old leaves that have otherwise lost their green color and are later prematurely abscised. A yellowish-green vein banding appears in the leaves of some hybrid multifloras, usually accompanied by greenish or brownish rings in the canes. The characteristic symptoms in these systemically infected outdoor roses are expressed in leaves and canes as they approach maturity in the fall. No distinctive effects of streak are recognizable in young leaves or young canes of such plants.

Some hybrid tea roses Briarcliff, Mme. Butterfly, Ophelia, Radiance have not been found affected in nature. When such a variety is experimentally infected with streak by budding, a black, dead lesion develops in the bark next to the inserted bud. It soon girdles the cane and kills the bud and all parts distal to it. The virus usually remains localized in the black lesion about the inserted bud, but a secondary lesion of similar appearance may appear in an adjacent rapidly growing shoot. Such lesions are construed as local infections in canes. That symptom has been confused with cane canker and other fungus diseases. The black lesions resemble fungus cankers. They can be eliminated by pruning. The uninvaded parts of the same bush remain free of virus. The streak virus has never been successfully sub-transferred from hybrid teas that express this local necrotic reaction. Buds taken from the discolored areas die without uniting with a test variety; buds from adjacent normal tissues unite but carry no virus.

The rose streak virus was not found to be transmissible mechanically. No evidence of transmission by seed was found. An intensive search for insect vectors failed to reveal any insect capable of transmitting the virus. Transmission by budding is uniformly successful if the bud is from a systemically infected rose and if union is accomplished.

Symptoms appear after 18 to 40 days or occasionally longer. As with the rose mosaic virus, transmission fails if the streak-affected bud fails to unite or if it is removed before union has taken place. The rose streak virus invades roots as well as canes and can be recovered from roots by inserting a sliver of root in the cane of a suitable test rose just as a bud would be inserted. When roots are inoculated with the virus, the upward movement of the virus into the canes of the inoculated plant may be delayed 2 years or more. Hybrid teas like Briarcliff or Mme. Butterfly develop the typical local lesions of streak even when previously infected systemically with rose mosaic. That affords some evidence that the viruses of rose mosaic and rose streak are not closely related.

ROSE WILT was first recognized in Australia in 1908. B. J. Grieve began to study it at the University of Melbourne in 1929. The appearance of the disease at first suggested a bacterial disorder, and rose wilt was tentatively attributed to bacteria until Grieve established it as a virosis. It is known in Victoria, New South Wales, South Australia, Tasmania, and in New Zealand. A similar disease occurs in Italy.

Rose wilt is extremely serious in some seasons, but may lapse into milder form for a few years, only to reappear with original virulence. Pernetiana roses, as Golden Emblem and Ville de Paris, seem most susceptible. Hybrid teas are somewhat less susceptible, but rose wilt has appeared in severe form in several, among them Dame Edith Helen, Sunburst, Mme. Abel Chatenay, Columbia, and Etoile de Hollande. Tea roses are relatively resistant; Grieve had not seen definite examples in this class when he wrote about the disease in 1942. No plants other than roses are known to be susceptible to rose wilt.

The first symptoms of rose wilt are downward curling and brittleness of young leaves. Defoliation of young shoots then proceeds from the tip downward. About a day later the young shoot tips become discolored and begin to die back 1 to 2 inches. The rest of the cane then takes on a yellowish-green appearance before it turns brownish black. Leaf buds often remain green after nearby tissues are darkened. Later the whole stem may die back to older wood. The leaf buds at that stage also turn brown and rot away. Apparent recovery may follow: The plants grow normally for a season or two, but eventually the symptoms reappear.

Grieve found gummy deposits in and around the vessels of young affected stems, and necrosis of cortex, medullary rays, and phloem. Later there was swelling of cell walls, with distortion and collapse of phloem elements and the formation of intercellular spaces. In older leaves that persisted on the canes, circular or irregular red-brown lesions sometimes appeared. Parenchyma cells near such necrotic areas were depleted of starch and yellowed and contained gumlike deposits. Spherical or oval intracellular inclusions were present in palisade cells near the areas of necrosis and gum formation.

Unlike the rose viruses known in North America, that of rose wilt is mechanically transmissible from rose to rose. Extracted sap is infectious even after passing a Seitz filter. Transmission by budding succeeded if the bud lived, but many diseased buds died without uniting. Symptoms appeared 10 to 20 days after plants were inoculated. R. Gigante in Italy found the Italian rose virus was transmitted by mechanical inoculation, and four of seven roses exposed to aphids of the genus Macrosiphum became infected. Grieve recorded a single transmission of the rose wilt virus by rose aphids, but he did not claim significance for the single infection. Although no direct comparison of Gigante's rose disease with rose wilt has been made, Grieve has expressed the opinion that the two viruses are the same.

No CURE IS KNOWN for any virus disease of roses. Workers in British Columbia, California, and Indiana attempted to cure roses affected with mosaic diseases with heat treatments, but the viruses were not inactivated at any temperature that rose tissues endured.

Control therefore must rest on choosing only healthy roses for propagation. That would seem to be the effective way, for none of the rose viruses present in this country is known to be transmitted by any means other than by propagation practices. It is hard to select healthy roses, however, because the virus symptoms are often poorly defined, or erratically expressed, or obscured by other diseases or by weather or insect damage. The problem is doubly difficult because most flowering roses are propagated on understocks; unless both understock and scion variety are free of disease, the program is a failure. The flowering roses that supply budwood or scions generally are more easily diagnosed than the understocks. Symptoms are expressed more clearly in greenhouses and there are fewer obscuring factors under glass. There is an advantage, therefore, in choosing budwood and scions from roses grown under favorable greenhouse conditions. Even then it is advisable to mark diseased plants plainly whenever they are detected, lest symptoms be masked or overlooked when budwood is chosen.

understock roses generally express ill-defined virus symptoms. Grown in the field, they are exposed to many diseases and injuries that obscure the signs of virus. Thus selection of virus-free understocks by direct inspection seldom is practicable. Some nurserymen propagate Rosa multiflora by seeds, thus ensuring virus-free understocks, for the rose viruses are not seed-borne. But, the Welch variety of R. multora, which is prized in Texas for thornlessness and for resistance to black spot, and Manetti, which sets no seed, must be propagated vegetatively. Floyd Smith and I suggested indexing such vegetatively propagated understocks by budding them into good test varieties, preferably under favorable conditions in greenhouses. The hybrid tea roses Ophelia, Mme. Butterfly, Rapture, and Radiance express mosaic, yellow mosaics, and streak. Under-stock plants that proved virus-free on such indexing could then be lined out as a foundation block to supply cuttings for commercial propagation. Virus-free understocks would afford an important advantage in that a virus introduced into the nursery in a diseased bud variety would remain confined to that variety. The mechanism for contaminating additional varieties would be lacking.

THE NURSERY practice of utilizing the tops of field-budded Manetti as a source of cuttings for the following season persists even though it affords little opportunity for detecting rose viruses and it provides a means of transmitting them to other plants. Economic considerations favor this practice. The tops are a crop byproduct, available in good condition to supply cuttings at a time when cuttings are in demand. Such cuttings are available without extra cost.

PHILIP BRIERLEY is a senior pathologist in the Bureau of Plant Industry, Soils, and Agricultural Engineering. He is a native of New Hampshire and was trained in plant pathology at the University of Minnesota and Cornell University. Dr. Brierley has been with the Department of Agriculture since 1922 and has studied diseases of Irish potatoes, onions, and various ornamental plants.