This page provides images and information from OCRS Master Rosarian Tom Cooney on common rose problems in Southern California gardens. You can always obtain extra help by contacting one of the OCRS Consulting Rosarians by e-mail or by using the form on the "Contact" page. Extra information, expert advice and tips for rose care also are included in our award-winning newsletter, which you can receive by becoming an OCRS member.
Symptoms: White growth of the fungus appears as patches on leaves and stems. Young tissue is most susceptible.
Conditions: Night temperatures of 60 degrees F and relative humidity of 90 - 99%; day temperature of 80 degrees F and 40 - 70 % RH. It is a warm season disease and does not require free moisture for spore germination.
Control: Wash off every few days with a hose or use the Cornell formula: 1 tsp ivory dish detergent, 1 tsp white vinegar, 2 tbsp corn or canola oil, 1 tbsp baking soda. Mix all into a one gallon container with water. Sodium and potassium bicarbonate have shown fungicidal activity. Also, stronger chemicals like Ortho Rose Pride, Banner Max or Compass may be used. Spray with Liqui Kop dormant spray after winter pruning.
ANTHRACNOSE (on leaf)
Control: Control Anthracnose by dormant pruning and removal of attached leaves. Cleaning up old leaves under bushes can do much to reduce the over wintering inoculum.
Also, reduce the incidence of free water on the foliage for long periods of time.
For chemical control, use Compass, Banner Max or Neem Oil (100% Neem for best results). Also use LiquiKop dormant spray after winter pruning.
Symptoms: Grayish brown mycelial growth is very characteristic of this fungus. It is a pest of stored and refrigerated roses, rose buds that can't open, cut flowers and cuttings used for propagation. It also can be found on canes as a secondary low level pathogen.
Conditions: Cool temperatures, high humidity and moisture. It needs free moisture.
Control: Prevention is the best means of control. This can be accomplished through intense sanitation procedures and eliminating dead plant material. Good ventilation also is essential in reducing disease incidence. Some sprays may give short term relief but the fungus usually becomes resistant. In most cases, removal of infected plant parts and protection of wounds by chemicals is all than can be done until warmer and dryer conditions arrive.
Symptoms: For downy mildew on leaves, notice the purplish irregular blotches as well as the necrotic area sometimes attributed to pesticide burn.
Control: The general practice for control of downy mildew is good sanitation. Sanitation in the garden will reduce the primary source of inoculum. Where damp humid conditions exist, systemic fungicide containing metalaxyl will give some control. Other fungicides also may be used but the persistence of the fungicides through wet periods or rain is important to maintain protection. Some chemical controls for downy mildew include Aliette and Agriphos.
Symptoms: Powdery pustules of orange to orange-red spores on the underside of the leaves in mid spring through fall are symptomatic of rust. Black pustules also may be present during winter months.
Conditions: The optimal conditions for disease development are temperatures of 64 - 70 degrees and continuous moisture for two to four hours. Rust needs free moisture.
Control: Dormant pruning, removal of attached leaves, and clean up of old leaves under the rose bushes. Dormant spray with copper/oil after winter pruning. Also a source of chemical control is Ortho Rose Pride (triforene).
ROSE MOSAIC VIRUS
Symptoms: Since there is no cure for viral disease in roses, it is important to purchase only quality materials with no symptoms of the disease. Some pathologists suspect that mosaic may be pollen transmitted, and in some exhibition gardens the disease can be common. Propagation of buds from infected roses will probably result in transmission of the disease.
Control: Part of above; roses might be removed if uninfected roses in garden are valuable and not already infected.
Symptoms: Aphids generally all small green or brownish buds which sometimes can appear by the 100s on buds or canes on developing roses.
Control: There are many options for controlling aphids. The simplest is by blasting them with a strong spray of water to get them off the plants. Safer insecticidal soup is another option, as are chemical controls such as Merit, Orthene or any good insecticide. It also can help to control ants in the garden.
Symptoms: Rose slugs are the immature stage of wasps called sawflies. They eat through the leaves causing leaves to appear as in the top two photos above.
Control: Rose slugs look like caterpillars but they are not, consequently some insecticides such as Bacillus Thuringiensis will not kill them. If there are only a few rose bushes infested with the rose slugs, pull off the leaves and kill any larvae found on the upper and lower surfaces of the leaves. Any contact or systemic insecticide labeled for use on roses will kill the rose slugs. The key thing is to spray thoroughly to make sure that the spray covers the upper and lower leaf surfaces. Also, spray under the rose bush as the larvae pupate in the soil prior to overwintering. For a less toxic approach, Monterey Garden Insect Spray has shown to be effective. The active ingredient is spinosad.
Symptoms: Spider mites generally are found on the underside of leaves and between leaves, and they can cause the leave to feel gritty.
Control: The first defense against spider mites is a strong stream of water to the underside of the rose bush every few days. Means of chemical control include Avid, Mavrik, Floramite SC, and Akari, which also stops the laying of eggs with residual control in 21 - 28 days.
Abiotic disease-like symptoms also may be caused by physiological problems, a nutritional imbalance, water, pH imbalance, environmental extremes, herbicide drift, air pollutants, pesticide injury and/or incorrect drainage. Contact OCRS rosarians for further assistance/information in identifying these problems.