Ohio State University Extension Fact Sheet

Ohio State University Extension Fact Sheet

Plant Pathology

2021 Coffey Road, Columbus, OH 43210-1087


Controlling Rose Diseases

HYG-3063-96

Stephen Nameth
Jim Chatfield

For many years, roses have been a favorite of home gardeners and professional growers alike. Unfortunately, there are several diseases that commonly occur on hybrid teas, floribundas, moss roses, or whatever type of rose you are growing. To produce top quality roses, these diseases must first be identified and controlled.

Powdery Mildew

Identifying Symptoms

Powdery mildew, caused by the fungus Sphaerotheca pannosa, appears as a white powdery growth on rose leaves, stems, buds, or flowers. It usually first appears on new growth in periods of warm, dry days followed by cool, damp nights. The new leaves may become curled or twisted and the shoots may look badly deformed. The fungus may also infect older leaves. Often, the upper surface of the leaves appears normal, but there is extensive fungus growth on the underside of the leaf.

Figure 1. Powdery mildew on rose leaves. Figure 2. Powdery mildew on rose flower. Figure 3. Black spot on rose leaves.

Control

Although detailed information is lacking, some rose varieties are more resistant to powdery mildew. When planting new roses, find out from local rosarians which varieties are most resistant. The planting site can also be made less conducive to powdery mildew development. Do not plant roses in shaded spots, especially those areas that tend to dry out slowly in the mornings. Surrounding hedges or shrubs should be pruned or thinned to allow for more air movement over the roses. Finally, a regular, preventive spray program with fungicides should be carried out (see below).

Black Spot

Identifying Symptoms

The fungus, Diplocarpon rosae, produces round black spots with fringed margins on leaves or stems. On some varieties, yellowing may show up around the spots. These symptoms are often seen on the lower leaves first. Infected leaves will drop off and may leave the plant almost completely defoliated except for a few leaves that have recently grown at the tip of the canes. Such plants are badly weakened and may die over the winter.

Control

As with powdery mildew, some varieties of rose are less susceptible. Select and plant resistant varieties whenever possible. The fungus survives the winter in fallen leaves. Raking and removing these leaves each fall may provide some control. Avoid watering the plants by splashing water on or about the leaves. Plant in sunny locations where plants will dry quickly after watering, rains, or damp nights (dew). Begin a fungicide spray program as soon as new leaves appear in the early spring (see below).

Rust

Identifying Symptoms

Rose rust caused by the fungus, Phragmidium sp., appears as orange or rust colored growth on the underside of the leaves. Older leaves tend to show symptoms before younger leaves. Under favorable conditions rust can cover the entire leaf and stem of the rose plant. Severe infections can cause premature defoliation.

Figure 5. Rust symptoms on a rose cane. Figure 6. Rust symptoms on a leaves.

Control

Whenever possible plant resistant varieties. Any practice that prevents the leaves from remaining wet for extended periods of time is beneficial for control. Never spray leaves with water in the evening so that the leaf surface is wet over night. Plant roses in areas that have full sun and allow air to flow freely around the plants. As a last resort, fungicides should be used (see below).

Stem or Cane Cankers

Identifying Symptoms

Several fungi cause stem cankers on roses. The different fungi cause slightly different looking cankers, but they usually produce brown, oval shaped, sunken or shriveled areas anywhere on the cane. When the canker completely surrounds or girdles the cane, the cane dies and the leaves wilt from that point outward. Sometimes small black specks of fungus spore forming structures can be seen erupting on the cane surface within the cankered area.

Figure 7. Cane canker on rose.

Control

Always plant disease free material. Each year, prune out and destroy all diseased canes, making sure to cut well below the obviously cankered areas. Protect the plants from cold or freeze injury in the winter. This can be done either with mulch or another kind of cover. Keep the plants vigorous with proper fertilization, good watering practices, and black spot and powdery mildew disease control programs.

Mosaic

Figure 8. Symptoms of rose mosaic virus on leaves. Figure 9. Symptoms of rose mosaic virus on leaves.

Identifying Symptoms

Rose mosaic is caused by a virus. Bright yellow patterns made up of wavy lines may appear on the leaves of some varieties. Other varieties may show no yellow lines, but may be stunted and weak due to virus infection.

Control

Virus infected plants cannot be cured. Plant virus resistant roses if possible. Try to control insects, particularly aphids, since they help spread the virus. If you are pruning virus-infected plants, don't prune healthy plants unless you have disinfested your pruners. Pruners can be disinfested by dipping in a 10% solution of chlorine bleach in water. Severely infected plants should be removed and destroyed.

Rosette and Witches Broom

Identifying Symptoms

Rapid stem elongation may be an early symptom of this disease. Later on, certain branches of the plant will develop thickened, thorny stems. Many short, deformed shoots will form, often with red pigmentation and tiny misshapen leaves. These shoots give the appearance of a witches broom. Plants die within one to two years as symptoms spread from branch to branch.

Figure 10. Symptoms of rosette on rose.

Control

The exact cause of this disease is unknown. Infected plants cannot be cured. Try to control insects, particularly leaf hoppers and plant hoppers. To protect other plants against the possibility of an infectious agent, symptomatic plants should be dug and discarded as soon as the disease is noticed.

Crown Gall

Identifying Symptoms

Irregularly shaped, bulbous masses of tissue (galls) appear on stems near the soil line. These can appear as small swellings, or be several inches across. Severely infected plants become stunted and fail to produce acceptable flowers.

Control

Avoid buying infected material with suspicious swellings or galls on lower stems or crowns. Protect plants from injuries on the stems. Maintain vigor with fertilization and watering. Pull and destroy badly infected plants. There is no chemical control for this disease.

Fungicide Spray Programs

Fungicides generally recommended for powdery mildew control include: Triadimefon (Bayleton, Strike); Triforine (Funginex), Thiophanate-methyl (Cleary's 3336, Domain). Propiconazole (Banner) and sulfur fungicides. Frequently used fungicides for black spot include Folpet (Phaltan), Captan, Mancozeb (Fore), Chlorothalonil (Daconil) and Triforine (Funginex). For rust, Triforine (Funginex), Mancozeb (Fore) and Chlorothalonil (Daconil) are effective. Follow labeled instructions regarding dose and frequency of application. It is important to spray on a regular schedule.

Follow all labels carefully. Be sure and spray both surfaces of all leaves thoroughly. With some wettable powder materials, it may be well to add about a teaspoon per gallon of a spreader-sticker to properly wet the rose foliage.



Table 1. Rose varieties reported to have resistance to black spot.
Resistant hybrid teas: Resistant floribundas/grandifloras: Resistant shrub roses:
Carla Angel Face All that Jazz
CayenneBetty PriorCarefree Wonder
Charlotte ArmstrongCarousel
Chrysler ImperialCathedral
DuetEuropeanaResistant miniatures:
ElectronFashionBaby Betsy McCall
First PrizeFirst EditionGourmet Popcorn
Forty NinerGene BoernerLittle Artist
GranadaGoldilocks ImpatientRainbow's End
Miss All-American BeautyIvory FashionRose Gilardi
Mister LincolnLove
OlympiadMirandy
PascaliMontezumaResistant Rugosa hybrid:
PeacePink ParfaitF. J. Grookendorst
Pink PeaceProminentPolyantha
PortriatQueen ElizabethThe Fairy
PristineRazzle Dazzle
Proud LandRed Gold
Smooth LadyRose Parade
Sutters GoldSonia
TiffanySunsprite
Tropicana

Information taken from Pest Resistant Ornamental Plants by Deborah C. Smith-Fiola. Rutgers Cooperative Extension.


This publication contains pesticide recommendations that are subject to change at any time. These recommendations are provided only as a guide. It is always the pesticide applicator's responsibility, by law, to read and follow all current label directions for the specific pesticide being used. Due to constantly changing labels and product registration, some of the recommendations given in this writing may no longer be legal by the time you read them. If any information in these recommendations disagrees with the label, the recommendation must be disregarded. No endorsement is intended for products mentioned, nor is criticism meant for products not mentioned. The author, The Ohio State University and Ohio State University Extension assume no liability resulting from the use of these recommendations.

All educational programs conducted by Ohio State University Extension are available to clientele on a nondiscriminatory basis without regard to race, color, creed, religion, sexual orientation, national origin, gender, age, disability or Vietnam-era veteran status.

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All educational programs conducted by Ohio State University Extension are available to clientele on a nondiscriminatory basis without regard to race, color, creed, religion, sexual orientation, national origin, gender, age, disability or Vietnam-era veteran status.

Keith L. Smith, Associate Vice President for Ag. Adm. and Director, OSU Extension.

TDD No. 800-589-8292 (Ohio only) or 614-292-6181



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