Powdery mildew is one of the most common foliar diseases of roses. The white, powdery fungal growth can be very disfiguring, with repeated heavy infection reducing plant vigour. Cultural techniques play an important role in minimising outbreaks, and fungicides can also be used.
What is rose powdery mildew?
Rose powdery mildew is a disease of roses caused by the fungus Podosphaera pannosa. The conspicuous white growth can affect all aerial parts of the plant, producing microscopic spores that spread the disease. High humidity is favourable for infection, and plants growing in areas where air movement is poor or the soil is dry can be severely affected.
You may see the following symptoms:
- A white, powdery fungal growth on the leaves and shoots. Upper, lower or 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
- Heavily infected flower buds frequently fail to open properly
- Mildew growth on stems (where it is often found surrounding thorns) and flower stalks is usually thicker and more mat-like than that on the leaves
- The mildew growth on all parts may turn browner as it ages
- Water plants regularly during dry spells, and mulch the soil to prevent moisture loss
- Climbers and ramblers grown in situations with good air circulation (e.g. over arches, rope swags or pergolas) are less likely to be affected than those grown in still air (e.g. planted closely against walls, in sheltered corners, etc.). Bush roses grown in sheltered situations are also more likely to be attacked
- Feed regularly to encourage strong growth, but avoid using too much nitrogen – this produces ‘soft’ growth which is prone to attack
- Badly affected shoots are best pruned out and disposed of as soon as the symptoms are seen. During routine spring pruning any shoots showing large patches of mildew around the thorns should also be cut out
- There are considerable differences in susceptibility between rose cultivars. However, any claimed resistance to the disease may not persist for the lifetime of the plant, or be effective in all localities
Like that of most powdery mildews, the majority of the growth of rose powdery mildew is on the surface of the plant. This exposed growth makes it susceptible to a range of chemical control measures.
Some of the fungicides available for the control of rose mildew will have activity against blackspot and rust, which are also common and damaging diseases. These include difenoconazole (Westland Plant Rescue Fungus Control), myclobutanil (various products, including Bayer Garden Systhane Fungus Fighter, Westland Rose Rescue and Doff Systemic Fungus Control), tebuconazole (Bayer Garden Multirose Concentrate 2) and triticonazole (Scotts Fungus Clear Ultra, Roseclear Ultra).
Some formulations also contain insecticides but these are best avoided if no insect pest problem is specifically identified. For example, some formulations of myclobutanil (Westland Rose Rescue) contain cypermethrin, some formulations of tebuconazole (Bayer Garden Multirose Concentrate 2) contain deltamethrin, and some formulations of triticonazole (Scotts Roseclear Ultra and Scotts Roseclear Ultra Gun) contain acetamiprid.
Plant and fish oil blends (Vitax Organic 2 in 1) and the sulphur and fatty acids formulation (Scotts Nature's Answer Natural Fungus and Bug Killer) may also be used but the latter must be kept away from fish.
Fungicides are likely to need several applications during the growing season, particularly in still, humid weather. Sprays in late summer and early autumn may help to reduce the number of infected buds in which the fungus can overwinter.
Fungicides for gardeners (Adobe Acrobat pdf document outlining fungicides available to gardeners)
Rose powdery mildew spreads during the growing season by means of microscopic, air-borne spores produced on the powdery growth. 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.
High humidity is, however, favourable for spore production and infection, and plants growing in areas with poor air flow (allowing the accumulation of humid, ‘stagnant’ air) are likely to be attacked. Thus climbers and ramblers grown against walls and fences are often heavily infected. Plants grown in these situations are also more likely to experience dry soil, which is another factor thought to increase susceptibility.
Rose mildew overwinters as fungal growth (mycelium) on the stems, or within some of the dormant buds. When these buds resume growth in spring the shoots soon become completely covered with mildew. The fungus then spreads from these infected shoots (known as ‘primaries’) onto the rest of the plant.