Four Diseases of Garden Roses

L. M. Massey.

Of the numerous diseases of roses, four are of chief importance in plantings in North America: Black spot, powdery mildew, rust, and the brown canker.

Because of its wide occurrence and destructiveness, black spot is the worst disease of roses in this country. It causes unsightly spots on the leaves. Yellowing and premature dropping generally follow.

Black spot is confined to the rose. Probably all classes of roses are affected but not equally so. The hybrid perpetuals, introduced in 1843; the hybrid teas, introduced in 1867; and the Pernetianas, which appeared in 18go, Were more susceptible than their wild or almost wild forerunners. Most of the present-day popular varieties are sufficiently susceptible to black spot to require protection by fungicides.

But more attention is now being given to the development of resistant or immune varieties through the use of such parents as Rosa wichuraiana, R. multiflora, R. cinnamomea, and R. pendulina, which tests have shown to be resistant or immune. A start has been made, and progress is sure to be made. Some varieties Radiance, for example while possessing no true resistance regularly have but little blackspot because the foliage is not easily wetted and therefore escapes infection.No one knows when or where black spot started, but by the time the early botanists knew something of diseases and began to collect and identify fungi, they found the roses affected with the disease. Early reports of it came from Sweden in 1815, France in 1822, Germany in 1833, England in 1840, and Holland in 1844. The disease was noted in North America in 1831 and has now been reported in practically all States and Canada and South America. It probably occurs throughout the temperate and tropical regions of the world wherever roses are grown. The disease is most serious in areas of heavy rainfall during the growing season.

Upper: Enlarged cysts of golden nematode of potato. The cysts are naturally about the size of a pinhead. This destructive pest has recently come to us from abroad. Lower: Rose black spot. The ragged margin of the spots, yellowing, and leaf drop are characteristic symptoms of this serious and widely distributed rose disease.

All aerial parts of the plant may be affected. The spots on the leaves are conspicuous and often are half an inch or more across. They are black and have margins marked by rays and tiny fibrils. A number of spots may merge to form larger ones and cover nearly the whole leaf. The spots may be on either surface usually the upper.

The leaf tissue next to the black spots becomes yellowish. Sometimes the entire leaf becomes yellow before defoliation occurs. The leaflets may turn yellow in spots, or the yellow area may be limited to a band outside the black spot.

Premature loss of leaves is one of the most pronounced characteristics of the disease. Plants affected by black spot generally show naked stems except for a few young leaves at the tips. Defoliated plants are less vigorous than healthy ones and produce fewer and less desirable blossoms. Also, defoliated plants often send out a late, second crop of leaves; in doing so they fail to mature and develop normal resistance to low temperatures.

The sizes and shapes of the black spots and the rapidity and extent of yellowing and defoliation vary with the varieties as well as the conditions under which the plants are growing.

Spots on the canes look much like those on the leaves. Spots on the petioles and stipules are relatively inconspicuous, as are those on the flower receptacles, sepals, and petals. Discoloration and distortion of the flower may occur.

Black spot of the rose is caused by the fungus Diplocarpon rosae.

In sections where the rose loses its leaves in the autumn and becomes dormant, the fungus hibernates in the old leaves on the ground and in the lesions on the stems. If the rose is in leaf throughout the year, as in greenhouses and in the warmer areas, the fungus lives the year around in the infected leaves on the plants. In any event, spores will be produced in the spring to initiate infections on the season's new leaves. The spores will be either ascospores or conidia if they are produced in the old leaves on the ground. They will be conidia if they are produced in the spots on the stems. The ascospores are forcibly discharged from the asci (little sacs), which are produced in fruit bodies, called apothecia. The ascospores are disseminated by wind. The conidia are liberated only if water is present. They are disseminated by splashing and dripping water. Spores; conidia produced in the spots on the overwintered stems probably are the most important in initiating the infection in the garden following dormancy.

The spores must have water for germination and infection. They will germinate in a humid atmosphere if they have been previously wet. It takes about 9 hours for germination to occur. Infection will be assured if favorable temperatures and humidities continue for at least 6 or 7 hours. Penetration takes place through the cuticle. The black spots become visible in 3 to 10 days, depending on temperature and moisture. Within a day or two after the appearance of the spots on the leaves, mature fruiting bodies (acervuli) are present within the spots. The conidia will be found in these fruiting bodies, which are barely visible to the unaided eye. The spores can be liberated only in the presence of water and are disseminated by splashing rain, by wind-borne rain, by dew, and by water from the hose.

The spores of D. rosae can germinate arid produce infection immediately after dissemination. The increase in the amount of black spot in the garden depends on the frequency of rains, which favor dissemination and germination, and in the greenhouse on the frequency of syringing. The development of the disease is favored by temperatures around 70 F. and by wet foliage, or at least a saturated atmosphere.

Black spot is controlled through the use of sprays and dusts applied at the proper times to the foliage to protect it against infection.

Wettable-sulfur sprays have replaced lime-sulfur. The "fixed" coppers, such as tribasic copper sulfate, copper oxychloride, and copper oxychloride-sulfate have largely replaced bordeaux mixture. A number of the newer fungicides, including several salts of dithiocarbamic acid gave promising results in tests in 1953.

Fungicides in dust form are as effective as those applied as sprays, and the method has advantages in ease and speed of application. A sulfur dust of at least 325-mesh fineness is often used with success. A dust mixture of 90 percent 325-mesh sulfur and to percent fixed copper has been found to be especially effective in Texas.

As sulfur in any form will cause injury to the foliage when the temperatures get into the 90's and above, many gardeners find advantage in turning to the use of a fixed copper, as spray or dust, during periods of high temperature. Copper fungicides are more injurious during cool, wet weather.