Mildew on roses: expert tips for prevention & treatment

Mildew on roses: expert tips for prevention & treatment

A white or grey coating on roses can indicate mildew. We reveal how to detect downy and powdery mildew on roses and how to combat it effectively.

Roses (Rosa) are one of the ornamental plants with a long tradition of cultivation. Because of breeding, roses get increasingly diverse with each year. Several thousand varieties with incredibly varied flower and growth forms adorn gardens all over the globe. However, there is a widespread pathogen that can threaten rose plants as early as at the beginning of May. In fact, there are two diseases that might be pestering your roses: powdery mildew and downy mildew.

Powdery mildew on roses

First of all, it is important to know how powdery mildew on roses (Podosphaera pannosa) and downy mildew on roses (Pseudoperonospora sparsa) differ. Not only are their names different, in fact, the two fungi are only distantly related. While downy mildew belongs to the water moulds (Oomycote), powdery mildew is a member of the sac fungi (Ascomycota). Their survival and reproduction strategies are different, too. It is essential to distinguish between the two mildew pathogens for successful control.

Detecting powdery mildew in roses

  • Mealy-white coating on the roses, which partially or completely covers the leaf on the upper and lower side. This coating can be easily wiped off with fingers to distinguish it from lime stains or other type of plant disease.
  • Shoots and buds can also carry the white coating. However, the coating does not occur on the woody stem of the rose.
  • With a magnifying glass, small arm-like structures can be identified, the so-called conidia. These are the spores that are responsible for spreading the fungus.
  • What one does not see: The fungal mycelium – the network – grows through the stomata into the leaf and absorbs nutrients from the upper cell layers.
  • If the leaves are heavily infested, they turn red and curl up or might even fall off the plant.
  • Crippled growth of the shoot tips and the formation of deformed leaves can also occur, and plant growth is also inhibited.

Combating powdery mildew on roses

Because powdery mildew does not overwinter in the foliage, but only under the buds of the younger shoots, rose pruning in spring is an essential step for prevention and control of powdery mildew. Apart from general preventive measures such as good rose fertilisation and the right location, household remedies can help treat a smaller infestation. You can find a list of household remedies you can use against powdery mildew below. When it comes to heavy powdery mildew infestations, in many cases only quality plant protection products can help. It is still crucial to choose products that are safe for the environment, though. We have compiled natural preparations for you below, too.

Downy mildew on roses

Detecting downy mildew in roses

  • Brownish-red or violet spots are visible on the upper and lower sides, often between the leaf veins of the plant.
  • Under humid conditions, a thin, grey-white mould layer is formed on the underside of the leaves. With a magnifying glass, the individual spore carriers can be identified, each carrying only one single spore. The fungus spreads with these spores.
  • Shoots can also become infected. In such a case, they display reddish spots and also possibly a layer of mould.
  • In the case of heavy infestation, the plant sheds its leaves.
  • Buds can dry up.
  • Plant growth is inhibited.

Tip: Downy mildew occurs much less frequently in roses than powdery mildew. It is also not usually as easy to mistake it for powdery mildew, as the symptoms are very different. Downy mildew never causes the unmistakable white coating on the rose petals.

Treating downy mildew on roses

Because the spores of downy mildew overwinter in the leaves and shoots of the plant, it is very important to remove the dead foliage after the leaves have fallen in autumn. In addition, roses should be pruned regularly. To prevent downy mildew, prune your roses generously at the first budding. Leaves and shoots should be disposed of or burnt to hinder the spread of disease. If these prevention measures did not help, you can find a small selection of household remedies and natural products for targeted control of the fungus. We will discuss both options in the following.

Mildew on roses: natural treatment with household remedies

Just as with powdery mildew on cucumbers, there are a few effective household remedies for powdery mildew on roses:

  • Treatment with milk: if an infestation occurs, a mixture of milk and water (ratio 1:9) is sprayed on the leaves every day. The mildew fungus is attacked by the lactic acid bacteria and hopefully completely suppressed. Whether the milk mixture is equally effective against powdery mildew is still debated by gardeners and scientists alike.
  • Treatment with baking powder: the potassium (hydrogen) carbonate contained in baking powder is strongly alkaline and makes life difficult for both powdery and downy mildew. The effect of a 0.5% solution with water (5 grams in one litre of water) has already been confirmed in several trials.
  • One of our readers uses chamomile tea as a preventive measure. The essential oils contained in camomile probably also inhibit the initial development of the fungus, so that the infestation does not occur in the first place.

According to scientific studies, fennel and horsetail brews also have an effect as long as the mildew infestation is not too severe.

If your grapes are suffering from a mildew outbreak too, read here to learn how to combat mildew on grapes.

Important: The above-mentioned methods have proven to be successful in some gardens; however, many other gardeners have not observed any effect. Despite that, we still recommend giving household remedies a try (especially with light infestations or as a preventative measure). It will save you money you would spend on store-bought sprays.

Mildew on roses: other ways of treatment

You do not have time to prepare a DIY spray or your roses are already heavily infested? Then you still have the possibility to fight mildew on roses naturally. We recommend trying products that consist mainly of horsetail extract, as these have proven to be highly effective. Sulphur-based products from various manufacturers are also very effective against fungi but can also harm beneficial organisms if the dosage is too high. The same applies to copper-based preparations, which may be used in organic farming, but in the long run accumulate in the soil to alarming concentrations.

If these preparations are used correctly and with caution, even a stronger infestation with mildew will soon pass. Keep in mind, that preventative measures should still be taken in autumn to avoid a renewed heavy outbreak of mildew next season.

Preventing powdery mildew in roses

To prevent your roses from being attacked by any kind of fungus in the first place, you can follow these tips:

  1. Roses absolutely need a sunny location with decent air circulation. A good location will automatically prevent countless pests and diseases. To ensure that the foliage dries quickly, let your rose plants have enough space and, if possible, do not grow them under branches of trees or eaves of buildings.
  2. Do not plant roses in places where other plants from their family have already grown before. Almost all fruit trees and strawberries belong to the rose family.
  3. Your rose should be optimally supplied with nutrients. You can achieve this by using high-quality potting soils and gentle, organic fertilisers.
  4. When watering, make sure you only water the soil and not the leaves. Fungal spores need a minimum of liquid to germinate and to be able to penetrate the leaf.
  5. There are also new varieties that are resistant to mildew fungi. For example, the varieties ‘Apple Blossom’, ‘Heather Dream’, ‘Palmengarten Frankfurt’ and ‘Summer Wind’ have proven to be resistant.

Tip: While downy mildew depends on high temperatures and leaf moisture during germination – 20 °C and humidity are ideal – powdery mildew develops best in dry and sunny conditions. Powdery mildew can actually be damaged by dripping water.

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