Rose Diseases
Controlling Rose Diseases
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.
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.
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.
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
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.
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.
|
Resistant hybrid teas: |
Resistant floribundas/grandifloras: |
Resistant shrub roses: |
|---|---|---|
|
Carla |
Angel Face |
All that Jazz |
|
Cayenne |
Betty Prior |
Carefree Wonder |
|
Charlotte Armstrong |
Carousel | |
|
Chrysler Imperial |
Cathedral | |
|
Duet |
Europeana |
Resistant miniatures: |
|
Electron |
Fashion |
Baby Betsy McCall |
|
First Prize |
First Edition |
Gourmet Popcorn |
|
Forty Niner |
Gene Boerner |
Little Artist |
|
Granada |
Goldilocks Impatient |
Rainbow's End |
|
Miss All-American Beauty |
Ivory Fashion |
Rose Gilardi |
|
Mister Lincoln |
Love | |
|
Olympiad |
Mirandy | |
|
Pascali |
Montezuma |
Resistant Rugosa hybrid: |
|
Peace |
Pink Parfait |
F. J. Grookendorst |
|
Pink Peace |
Prominent |
Polyantha |
|
Portriat |
Queen Elizabeth |
The Fairy |
|
Pristine |
Razzle Dazzle | |
|
Proud Land |
Red Gold | |
|
Smooth Lady |
Rose Parade | |
|
Sutters Gold |
Sonia | |
|
Tiffany |
Sunsprite | |
|
Tropicana |
Black Spot of Roses
Black spot is the most important infectious disease of roses. It occurs only on roses (Rosa spp.), and is widespread among rose species and cultivars, although some of the shrub roses and rugosa roses show more resistance. Many hybrid tea roses are very susceptible. Lists of black spot-resistance hybrid tea roses often are variable due to localized races of the pathogen.
Diagnostic Symptoms
Round to irregular black splotches with fringed margins are quite obvious, mostly on upper leaf surfaces. Leaf yellowing develops around these black spots, with defoliation of these infected leaves common. Repeated defoliation weakens plants, leading to poorer blooming and greater sensitivity to other stresses. Occasionally symptoms are noted on petals (red dots, distortions), and on petioles, fruit and canes. Using a hand lens, the fungus can often be seen fruiting in the black spots.
Disease Cycle
The fungus overwinters on fallen leaves and diseased canes. Microscopic spores are then splashed to newly emerged leaves and stem tissue in the spring. Under ideal conditions of leaf wetness, humidity and temperature the spores can germinate and infect in 1 day, cause symptoms in 4 to 5 days, and produce new spores that can infect additional leaf, flower and cane tissue within 10 to 11 days. Spores are easily spread to new locations by air currents.
Control Hints
-
Keep foliage dry. Plant roses in sunny locations to encourage drying after rains. Avoid sites with dense surrounding vegetation, so that good air movement will dry leaves. Avoid overhead irrigation, especially late in the day. Black spot is most severe in summers with sustained rainy periods.
-
Sanitation. Remove all black spotted leaves from and around plants. This should be done throughout the season. Before winter, remove and clean up all diseased leaves and remove diseased canes where possible.
-
Disease resistance. Listing susceptibility and resistance of all roses would take volumes. In addition, the occurrence of local races of the pathogen often result in a particular cultivar being listed as susceptible in one area and resistant in another. However, some lists have general usefulness; see Table 1. Also, consult local Extension publications and books, consult local rosarians and garden center horticulturists, and make observations of relative disease incidence in local rose collections and your own gardens.
-
Preventive fungicide applications. Fungicide controls are not successful if cultural and sanitation practices listed above are not followed. For fungicides to work, applications must be made preventively, providing a protective fungicide barrier which kills germinating fungal spores that have landed on plant tissue.
If conditions for infection are present and a high level of control is desired, preventive spray programs often start as soon as rose foliage emerges in the spring and continue throughout the summer at frequent intervals (as frequently as every 7 to 10 days in wet weather). Frequently used fungicides for black spot control include triforine (Funginex), and phaltan.
|
Table 1. Rose varieties reported to have resistance to black spot. | ||
|---|---|---|
|
Resistant hybrid teas: |
Resistant floribundas/grandifloras: |
Resistant shrub roses: |
