Diagnosis and Control of Rose Diseases
by Ethel Dutky
Department of Entomology, University of Maryland, College Park
A – Fundamentals
The incidence and severity of diseases can be reduced using the following fundamentals:
1. Select varieties with resistance to diseases and pests.
2. Sanitation--Start clean and stay clean. Purchase healthy roses. Remove diseased canes and foliage from the planting periodically.
3. Provide good cultural conditions. Roses require a well-drained, moderately fertile soil. Do a soil test before you apply fertilizer. Roses are sensitive to salt and can be damaged by excessive fertilizer. Most foliar diseases and canker require a film of water on the plant surface for infection. If you can irrigate your roses using a trickle or soaker method rather than overhead sprinklers, you can reduce many diseases.
B – Root Problems
Provided the drainage is adequate, roses have few serious root diseases.
Nematodes in the soil can cause a failure of the plant to establish and thrive. The most common plant parasitic nematode in home gardens is the root knot nematode (Meloidogyne species). Several other nematodes can stunt roses, if present in high populations. There are no chemicals registered for home gardens that can eradicate nematodes in plants or in the soil. Have a soil test for nematodes done before you plant. You can use crop rotations or heat treatments (solarization) to reduce nematode populations to below damaging levels prior to planting.
Crown gall is a disease of the roots and crown caused by the bacterium Agrobacterium tumefasciens. Do not purchase plants with galls on the roots. There is a therapy for infected plants. If you notice crown gall on a thriving plant, you can ignore it. Often a plant can have crown gall and show no other ill effects. The biological control Galltrol will prevent crown gall.
C - Stems
A variety of canker diseases are found on roses. All canker fungi invade small wounds. Stressed plants are much more susceptible, and more canker and dieback often is seen following severe winters and/or severe drought.
Control: Prune out all shoots showing cankers. Promote good vigor by irrigation during drought and proper fertilization.
D – Foliage
There are several important foliar diseases. Most “disease resistant” roses have resistance to some of these major foliar diseases. Foliar diseases are ugly. They damage the plant by causing leaves to fall, depriving the plants of the energy manufactured by the foliage.
Black Spot (fungus Diplocarpon rosae) is the single most common and destructive leaf spot disease of roses. The fungus survives over the winter in little spots on stems and on fallen leaves. Wet conditions are required for infection.
Control: Roses that are susceptible to black spot must be sprayed on a regular basis with a fungicide.
Powdery mildew (fungus Sphaerotheca pannosa var. rosae primarily) is easy to diagnose by the white spots on stems, flowers and both upper and lower leaf surfaces. Foliage may be distorted when infected early. Powdery mildew can also cause premature leaf drop.
Control: Select varieties with resistance. Susceptible varieties will need regular applications of fungicide or alternative chemical. Some fungicides registered to control powdery mildew are Rubigan, Funginex, Systane, Strike, Heritage, Compass and Banner Maxx. Alternatives to fungicides include horticultural oil sprays, Triact (neem oils and waxes) and MilStop (potassium bicarbonate, formerly called First Step).
Rusts are not very common on roses in the mid-Atlantic region. Caused by at least nine species in the genus Phargmidium, they are easy to diagnose by the orange powder pustules on lower leaf surfaces. As this is a minor disease of roses in our region, no control is needed. Fungicide sprays provide good control where rust is common. Fungicides listed for powdery mildew also control rusts.
Downy mildew (fungus Peronospora sparsa) is another very destructive rose disease that is not common in the mid-Atlantic region. In over 20 years I have only seen downy mildew on roses shipped in from the West Coast or Texas, already infected. Our spring weather is too warm for downy mildew to persist and spread in Maryland gardens. Most downy mildews produce profuse white or purplish sporulation on lower leaf surfaces. The rose downy mildew fungus is unusual in that it produces very sparse sporulation. Symptoms consist of leaf distortions, and some reddening, but the damage comes from the rapid drop of infected foliage = defoliation.
Control: Fungicide sprays are highly effective to prevent infection by downy mildew in regions where this disease is common. Chlorothalonil, coppers, and mancozeb prevent. Stature contains both mancozeb and the systemic dimethomorph, which is specific to “water mold” fungi such as downy mildews, Phytophthora and Pythium. Heritage and Compass are also labeled for downy mildew.
Viruses in roses make conspicuous symptoms. Bright yellow spotting, yellow veins, mosaic, yellow rings or yellow green mottle are all symptoms that indicate the plant is infected with one or more viruses. Symptoms may appear on the first flush, and then later flushes may be free of symptoms. They remain a problem in rose production because roses can be infected but show no symptoms; allowing virus-infected plants to be used for propagation. Another source is virus-infected rootstock. Most viruses found in roses are difficult to transmit, so plant-to-plant spread is rare in the garden.
Control: There is no control for virus. Infected plants should be destroyed. However, as these viruses do not spread out readily in the garden, it is not urgent that you destroy a plant that you like just because it shows virus symptoms. I would urge you not to purchase roses showing virus symptoms.
Rose Rosette Disease (assumed to be a virus, vector is an Eriphyid mite). This old disease is new to the mid-Atlantic region, but is certainly here now. Unfortunately once a rose shows symptoms it will die and cannot be saved. Symptoms seen include red foliage, excessive thorns, excessive stem elongation, foliage crinkle, decline and death. The tiny mite vector is carried on air currents from infected plants, especially multiflora rose thickets. Mites can be found in early summer and fall. Miticide sprays to control the mite vectors can help protect cultivated roses. A good web site on Rose Rosette is http://web.ntown.net/~apeck. This web site is done by Ann Peck.
E – Flowers
The most important disease attacking flowers is Botrytis. This fungus can cause the buds to “ball” and fail to open. Botrytis also attacks the petals, causing spotting and progressive blight. Wet conditions are needed for Botrytis infection.
Control: Remove blighted flowers from the garden (you can compost them). I do not advise fungicide spays for Botrytis in landscape plants.
1 – Compendium of Rose Diseases 1983. APS Press. Call 1-800-328-7560 to order.
2 – Easy Care Roses. 1995. Brooklyn Botanic Garden Series.