Mildew on roses: identifying & treating downy & powdery mildew on roses

A white or grey coating on roses is often a tell-tale sign of mildew. Read on to find out how to recognise both downy mildew and powdery mildew as well as how to treat mildew on roses.

The rose (Rosa) is one of the longest cultivated ornamental species of plant and is more diverse than almost any other plant. Several thousand varieties with a wide variety of flowers and growth habits adorn gardens all over Britain. Unfortunately, there are two widespread pathogens that can threaten roses from as early as the beginning of May, namely powdery mildew and downy mildew.

Powdery mildew on roses

First and foremost, it is important to know the difference between powdery mildew on roses (Podosphaera pannosa) and downy mildew (Pseudoperonospora sparsa). Even though they are both referred to as mildew, powdery and downy mildew are in fact only distantly related. While downy mildew is a type of water mould (Peronosporomycetes), powdery mildew is classified as a sac fungi (Ascomycota). As their survival and reproduction strategies are different, it is important to be able to identify which fungus it is you are dealing with in order to treat them properly.

Identifying powdery mildew on roses

• A powdery, white dust on roses that partially or completely coats upper and lower sides of leaves. This coating can easily be wiped off with your fingers unlike spots of lime and sulphur or similar chemical sprays.
• While shoots and buds can also have this white coating, it does not occur on the woody stem of the rose.
• When viewed through a magnifying glass, it is possible to identify small arm-like structures. These are the so-called conidia, which are the spores responsible for spreading the fungus.
• What you cannot see, however, is the fungal mycelium – the network – which grows through the stomata into the leaf and absorbs nutrients from the upper cell layers.
• In case of a severe infection, the leaves turn red and curl up or may even drop.
• As well as plant growth being inhibited, stunted growth of the shoot tips and the formation of deformed leaves can also occur.

Treating powdery mildew on roses

Powdery mildew overwinters not in the foliage of the plant, but in the stems or dormant buds of the younger shoots. This is why it is particularly important to prune roses in spring and remove any shoots as soon you spot symptoms. Apart from general preventive measures, such as fertilising your roses and planting in the right location, home remedies can help in the case of less severe infections. You can find a list of effective home remedies for using against powdery mildew below. Sometimes, in the case of a severe infection, you may have to resort to fungicides to treat powdery mildew on roses.

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Downy mildew on roses

Identifying downy mildew on roses

• Brown-red or purple spots of mould visible on upper and lower sides of rose leaves, often between the leaf veins.
• In humid conditions, a thin, grey-white layer of mould forms on the underside of the leaf. With a magnifying glass, you can see the individual spore carriers, each carrying only a single spore with which the fungus spreads.
• Shoots can also become infected, displaying reddish spots and possibly a layer of mould.
• In the case of severe infestation, the plant may drop its leaves.
• Buds can dry out.
• Plant growth is inhibited.

Tip: Downy mildew occurs much less frequently than powdery mildew on roses. As the symptoms are very different, it is not easy to mistake downy mildew and powdery mildew for one another. Downy mildew, for instance, never causes the distinctive white coating typical of powdery mildew.

Treating downy mildew on roses

As downy mildew spores survive during winter in the leaves and shoots of the plant, it is important to remove the foliage once the leaves have fallen in autumn. Annual pruning is also important to help keep mildew at bay. To help prevent downy mildew, prune your roses heavily once new growth appears. Dispose of or burn leaves and shoots to prevent the spread of disease. If preventative measures have not helped, or you simply failed to take such precautions, there are a few home remedies as well as other natural shop bought solutions that are effective against downy mildew on roses.

Home remedies for mildew on roses

Just as with mildew on cucumbers, there are a few effective home remedies for when you find your roses are suffering from a mildew infection:

• Milk spray: Fill a spray bottle with a mixture of milk and water (ratio 1:9) and spray onto leaves daily. The lactic acid bacteria in milk attack the powdery mildew fungus and could potentially suppress it completely. Whether this milk treatment is just as effective against downy mildew has not yet been proven.
• Baking powder: The potassium bicarbonate found in baking powder is strongly alkaline. A 0.5% solution made by mixing 5 grams in 1 litre of water has proven successful in treating both powdery mildew and downy mildew in several trials.
• Chamomile tea: One of our readers uses chamomile tea as a preventive measure against mildew. The essential oils found in chamomile probably inhibit the initial development of the fungus so that the infestation does not occur in the first place.
• Fennel and horsetail sprays: According to scientific studies, sprays made of fennel and common horsetail can also be effective as long as the mildew infestation is not too severe.

Note: These methods have proven successful for some hobby gardeners, others have had no effect. It is worth giving home remedies a try, whether to treat a light infestation of mildew on roses or even as a preventive measure, as it will save you money you would have otherwise spent on shop bought products.

Other treatments

If you do not have time for home remedies or your roses are already badly infected, there are other biological options available to help control mildew on roses. Equisetum Plus, for instance, is made up mainly of common horsetail extract and is effective in treating mildew. There are also several sulphur based products from various manufacturers that are highly effective against fungi, but can also harm beneficial insects if the dosage is too high. The same applies for copper-based fungicides which, whilst suitable for use in organic farming, may build up to harmful concentrations over time.

When used correctly, these remedies can help deal with even severe cases of mildew. However, it is still important to take preventive measures in autumn and the following spring to avoid a renewed infection.

Preventing mildew on roses

To help prevent any kind of fungus attacking your roses in the first place, follow these tips:

1. Choosing the right location for your roses is an effective way of preventing many diseases and pests. Roses require a sunny location and good air circulation. Ensure that your roses have enough space so that the foliage can dry quickly and, if possible, avoid placing them under tree branches or the eaves of buildings.
2. Do not plant roses where roses or any other plants belonging to the rose family have grown before; this includes almost all fruit trees and strawberries.
3. Roses require a well-suited, high quality soil as well as an optimal supply of nutrients – our gentle, plant-based Plantura Rose Food and our Plantura Organic Flower Compost are perfect for this.
4. When watering your roses, avoid getting the leaves wet. Fungal spores need plenty of moisture in order to germinate and penetrate the leaf.
5. It is worthwhile to search for strong, disease resistant varieties. Some resilient new varieties of mildew-resistant roses include, for instance, the ‘Apple Blossom’, ‘Heather Austin’, ‘Palmengarten Frankfurt’ and ‘Summer Wind’ varieties.

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Tip: Downy mildew requires warmth and moisture to germinate on leaves; 20°C and dripping wet leaves are optimal. Whereas powdery mildew develops best in dry and sunny conditions; it requires only slight dampness (like that of a dewy, cool night) and can actually be harmed by too much water.

If you are looking for more tips on how to care for your roses, why not check out our article on pruning roses and fertilising roses.