Family

Peronosporaceae

Damage rating

Minor or severe

Type of disease

Fungal.

Rose downy mildew - Peronospora sparsa

How to recognise it

This fungal infection can appear similar to rose black spot, with rounded, red/brown patches appearing on the upper surfaces of leaves. However, unlike rose black spot, these patches can develop a grey/white mould growth on their underside in humid conditions. The leaves may wither and drop as a result of the infection.

Stems can also be affected, with long, darkened patches appearing.

Why it’s a problem

Can be unslightly, cause premature leaf fall and generally weaken the plant.

Where you are likely to find it

Mainly affects roses grown in glasshouses (ie for commercial purposes) but may occassionally be found on garden roses.

How to prevent it

Ensure that there is adequate air flow around and between the plants (this fungus thrives in humid conditions) and that good ventilation is provided for glasshouse roses, particularly overnight. Avoid overhead watering which can moisten the leaves making them susceptible to infection.

Maintain good hygiene around vulnerable plants, or areas where there has been an outbreak, particularly by removing and destroying any plant debris at the end of the season to remove any overwintering fungal spores.

Ensure you thoroughly clean any tools you have been using around plants which may have rose downy mildew, including your hands/gloves if you have been handling the plant.

How to get rid of it

Spraying with a fungicide containing mancozeb can be effective in controlling rose downy mildew. Alternativley, copper based fungicides can be used, but mancozeb is generally more beneficial.

Badly affected plants should be destroyed as they are likely to be weakened beyond the point of recovery and will only spread the disease.

Is it good for anything?!

No.