Introduction 
Powdery mildew is one of the most common foliar diseases of roses. It is caused by the fungus Podosphaera pannosa. The conspicuous white growth can affect all aerial parts of the plant, but mainly new soft growth – producing microscopic spores that spread the disease. High humidity is favourable for infection, as well as plants growing in areas where air movement is poor or on plants that are grown in too much shade.
Description
A white, powdery fungal growth on the leaves and shoots. Both leaf surfaces can be affected.
There may be discolouration (yellow, reddish or purple) of the affected parts of the leaf, and heavily infected young leaves can be curled and distorted.
Mildew growth may also be found on the stems, flower stalks, calyces and petals.
Damage
Powdery mildew not only causes the foliage to curl and distort making it unsightly but the fungus also lowers photosynthetic efficiency that results in reduced plant growth and vigour.
The growing tips and flower buds may be malformed but the death of an entire plant is rare. Plants can be severely stunted if they are heavily infected early in the growing season. Rose tissue becomes more resistant to infection as it ages.
Severely infected foliage can prematurely fall off.
Environmental conditions
High relative humidity is favourable for infection. Temperatures bewteen 16-27 degrees Celsius make conditions favourable for the development and spread of the Fungus.
Plants growing in shaded areas or where air movement is poor or the soil is dry can be prone to Powdery Mildew.
Unlike many other fungal diseases, extended periods of leaf wetness are not required in order for the spores to germinate. This means that powdery mildew is often a problem during dry summers.
Life Cycle, Survival and Dispersal
All powdery mildew fungi require living plant tissue to grow. On perennial hosts such as roses, powdery mildew survives from one season to the next as vegetative strands in buds or as spherical fruiting bodies, called chasmothecia, on the bark of branches and stems.
The powdery mildew fungus overwinters as dormant mycelium in bud scales and rudimentary leaves within the dormant buds. (That is why we recommend that all the leaves are removed after winter pruning so that the leave axil does not harbor the dormant spores.)
Infected buds break open in the spring and develop into systemically infected shoots. The fungus sporulates on these shoots, producing large numbers of microscopic spores (conidia) in chains that are carried by the wind or other means to healthy rose tissue where they infect the upper and lower leaf surfaces, thus initiating a new disease cycle.
Rose powdery mildew spreads during the growing season by means of microscopic, air-borne spores produced on the powdery growth.
Treatment and Prevention
Natural prevention
Plant roses in full sun. They should receive a full six to eight hours of sun daily.
Plants will grow more robustly and be able to resist powdery mildew better. Shade causes slower moisture evaporation thus creating a breeding zone for powdery mildew and other fungal diseases.
Plant roses in an area with good air circulation and space them well. Moisture evaporates faster. In addition the breeze will dry off the foliage.
Aerate the soil in winter. The roots of roses need an aerated soil; plants are stressed if water logging occurs and stunt new growth, thus being more susceptible to powdery mildew.
Water correctly. Plants that do not receive enough are more prone to fungal infection. Deep soakings, 3 times a week in the hot summer months will suffice.
Choose resistant varieties. Roses vary in their resistance to this disease. Use resistant varieties for low maintenance plantings.
The method of picking off diseased leaves to prevent spreading has become an old fashioned method due to the availability of new, disease tolerant roses and effective pesticides that should be used for major infestations.
Pesticides
Spot checks and preventative spraying are essential. Effective fungicides should be on the shelf in regions where this disease is prevalent. Protecting the leaves by spraying is effective.
During ideal “powdery mildew” weather conditions, spraying on a fortnightly basis is essential. The following fungicides are effective to a degree in preventing the spores to enter the leaves as well as killing spores on the leaves. The most common group contains the active ingredient Mancozeb. Of these are many fungicides registered under various trade names. Several fungicides are registered for control of powdery mildew. Because of the waxy nature of rose leaves, a spreader added to the spray will give better coverage.
We strongly recommend ‘CHRONOS’, a suspension concentrate fungicide with the active ingredient: Prochloraz zinc complex (imidazole) & Prochloraz equivalent.
The old remedy of treating powdery mildew with a baking soda spray has been shown to be ineffective.
The most common
Funginex Triforine
Rose Protector/Rosecare Propiconazole
Dithane mancozeb
Fungicides absorbed by the leaves
(These have a partial curative action as they clear the blocked capillaries)
Proplant proparmocarb
Proparmocarb proparmocarb
Chronos imidazole prochloraz zinc complex
When Using Pesticides always follow the instructions.
Resources
http://web.aces.uiuc.edu/vista/pdf_pubs/611.pdf http://apps.rhs.org.uk/advicesearch/profile.aspx?PID=748