Powdery Mildew on Roses
Rose powdery mildew is a disease of roses caused by the fungus Sphaerotheca pannosa var. rosae. It forms what looks like a white powder over the surface area of the rose leaves, and may also spread to the stems and new rose buds.
The attacked leaves and blooms get disfigured and may not regain their normal shapes even after the powdery mildew has been killed while bud growth is stunted and may fail to open in severe cases of infection.
Warm, dry days followed by cool, humid nights are perfect conditions for an outbreak of powdery mildew.
Powdery mildew not only causes the foliage to curl and distort making it unsightly but the fungus also lowers photosynthetic efficiency that results in reduced plant growth and vigour.
Disease Biology
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 weather conditions.
High humidity is favourable for spore production and infection, and plants growing in areas with poor air flow, which allows the accumulation of humid or ‘stagnant’ air, are likely to be attacked. Plants grown in these situations are also more likely to experience dry soil, which is another factor that increases susceptibility.
Rose mildew overwinters as fungal growth (mycelium) on the stems, or within some of the dormant buds. When these buds resume growth, the shoots soon become completely covered with mildew. The fungus then spreads from these infected shoots onto the rest of the plant.
Signs & Symptoms
Infected plants show the following signs and symptoms;
- A white, powdery fungal growth on the leaves and shoots. Upper, lower or both leaf surfaces can be affected.
- There may be discolouration, i.e., yellow, reddish or purple of the infected 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.
- 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.
Disease Control
Chemical control methods
The use of fungicides is the best and more effective management strategy for controlling the disease. The following fungicides are recommended for use in prevention and eradication of powdery mildew disease of roses;
- ABSOLUTE 375SC 10ml/20l
- TRUSTMATE 250EC 50ml/20l
- SPARROW 500EC 15ml/20l
- SACRIFIDO 125EC 10ml/20l
- JUPITER 125SC 15ml/20l
- CHARIOT 500SC 20ml/20l
- EXEMPO CURVE 250SC 15ml/20l
- RANSOM 600WP 15g/20l
- EXPLORER 3 SL 10ml/20l
- MEGAPRODE LOCK 525WP 30g/20l
- MILESTONE 250SC 10ml/20l
- TOMAHAWK 250EC 10ml/20l
- MONUMENT 400EC 40ml/20l
- LONGSTAR 400EC 4ml/20l
Non-chemical control
- Water the plants regularly during dry spells, and mulch the soil to prevent moisture loss
- Ensure a regular plant nutrition to encourage strong growth, but avoid using too much nitrogen as it produces ‘soft’ growth which is prone to attack.
- Prune badly affected shoots and dispose of as soon as the symptoms are seen.
- Plant resistant cultivars.
- Ensure proper spacing to encourage good air circulation.
- Plant roses in full sunlight.
- Use drip irrigation to avoid wetting the foliage
- Minimize field movements from infected areas to non-infected zones
Note
- Whenever spraying, it is advisable to mix the fungicide with INTEGRA 3ml/20l, which improves the efficacy of the fungicide by acting as a sticker, spreader and penetrant.
- Timely control of the disease helps reduce/prevent losses attributed to its infection.
- Alternating different fungicides throughout a plant’s season prevents the fungus from developing resistance over any of the fungicides.
- Spraying the plants with BIODISTINCTION XTRA helps in preventing and suppressing the activity of the fungus.
