Introduction

Downy mildew refers to any of several types of oomycete microbes that are obligate parasiteof plants. Downy mildews exclusively belong to Peronosporaceae.

Downy Mildew is not as widespread and much more dependent on ideal temperatures and moisture conditions to cause devastating results on roses.

Description

Downy Mildew symptoms occur on leaves, stems, peduncles, calyxes and petals.

The leaves develop purplish red to dark brown irregular spots or blotches which might be mistaken for spray burns or possibly black spot.

If one is not familiar with the symptoms of Downy Mildew one could easily overlook this until infections have caused leaf drop and deformation of the bud. The biggest problem is the speed with which infection can take place with the resultant devastation of a whole flowering flush.

To visibly identify this disease one needs to pick an infected leaf, fold it at a brown spot and hold it against the light. White “hair’ are visible.

Damage

The ideal stage for a Downy Mildew attack seems to be when the buds are still green and about pea-size. Germination takes place in the upper two, still soft leaves and soon after, this Mildew enters the peduncle, spreading upwards and blocking the normal sap flow to the bud. The buds are unable to develop normally and are eventually totally distorted and unpickable. The bark of the still soft basal shoots become affected by severe Downy Mildew infection. This is evident from a brown-purplish discolouration of the skin. Soon after it bursts open to release more conidia or to serve as a seasonal hiding spot for the Pathogen. This is why sanitation is so important and it is best to cut out affected parts, remove them from the site, and burn them.

Environment

The optimal conditions for the appearance of Downy Mildew are constant high humidity, low night temperatures and moisture on the leaves. The optimal temperatures for spore germination is between 10°C and 18°C no germination take place at temperatures below 5°C and the spores are killed at temperatures above 27°C. Certain cultivars are more resistant to Downy Mildew than others. The causal agent of Downy Mildew is Peronospora sparsa, this fungus might be on your roses under normal conditions – with a very sparse spore production – until such time that conditions become favourable. The spores germinate within 4 hours in water, enter the leaves, and reproduce in three days. Spores survive on dried fallen leaves for as long as one month.

Areas of occurrence

Downy Mildew was virtually unknown on the South African Highveld until the nineties and even in the cool, moist climate of the Western Cape of South Africa it was not recognized as a problem on roses two decades ago. The main areas that are now suffering from Downy Mildew on and off are in Kwazulu Natal of South Africa

Prevention & Treatment

In practice this means one has to lower the humidity by improving ventilation and aeration. Only by correcting the optimum climatic conditions (or a sudden change in weather – sunshine) will an immediate stop be made to the spreading of Downy Mildew.

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. Watering late in the afternoon or evening must be avoided. Any sudden influx of cold air will immediately increase humidity and cause drops of evaporation.

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 such as Coastal regions and on Inland regions if autumn rains are prevalent. If Downy Mildew breaks out and one is unable to spray within 24 hours it will be too late for curative action coupled with the danger of severely affected main stems which are essential for the next flush.

Protecting the leaves by spraying is effective. During ideal “downy mildew” weather condition, spraying on a weekly 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 very many fungicides registered under various trade names.

The most common:

• Dithane WG                 mancozeb
• Mikal M                     mancozeb + fosetyl aluminium
• Ridomil Gold                 mancozeb + metalyaxyl

Fungicides containing copper:

• CoppercountN                  copper ammonium
• Copperoxychloride                copperoxychloride

Fungicides absorbed by the leaves

(These have a partial curative action as they clear the blocked capillaries)

• Proplant                   proparmocarb
• Proparmocarb               proparmocarb
• Benlate                   Benomyl
• Chronos                  imidazole prochloraz zinc complex

Fungicide to eliminate spores

• Phytex                   phosphorous acid equivalent

Also see

Peronosporaceae (list of downy mildew genera)

References

Taschner, L. Ludwig Taschner’s Roses 2010:37 ISBN 978-1-77007-803-1