Powdery Mildew

Powdery mildew is caused by the fungus Sphaerotheca pannosa (also classified as Podosphaera pannosa). It is one of the most damaging diseases on roses, and climbing roses are particularly susceptible. The disease thrives under dry but mildly humid conditions with poor air movement.

The characteristic sign is a greyish-white powdery layer visible on the surface of leaves, stems, and shoots. The mycelium penetrates the rose's epidermis and disrupts plant tissue. The disease spreads readily by contact between infected and healthy plants, carried on wind, in the air, and via water splash. Early symptoms appear as small white patches on the leaves; heavily infected foliage becomes distorted, contorted, discolours to yellow, red, or purple, and drops prematurely.

The fungus also colonises peduncles, sepals, and petals, preventing buds from opening properly. The petals thicken and take on a leaf-like, padded texture. In severe infections, affected tissues quickly turn brown and senesce.

Powdery mildew does not penetrate deep into the vascular tissue but forms a mycelial mat on the surface. It draws nutrients directly from the epidermal cells, causing them to dehydrate and contract. This results in the leaf margins curling, twisting, and becoming hard and brittle.

At the first sign of white powdery patches on young leaves or buds, act decisively:

• Prune heavily infected shoots and improve airflow through the canopy. Bag the pruned material tightly and dispose of it as rubbish (do not compost) to prevent spore dispersal.
• Disinfect pruning tools with alcohol between cuts to avoid transferring inoculum.
• Apply micronised sulphur or potassium bicarbonate, thoroughly wetting both leaf surfaces in the early morning (once the dew has dried) or in the late afternoon.
• [After 7 days] Reapply if the white powder persists; rotate active ingredients between Anvil (Hexaconazole) and Score (Difenoconazole) to prevent resistance.
• [After 14 days] Inspect young shoots for residual mildew; if any remains, continue spraying and pruning more aggressively.

Recommended products

1. Anvil 5SC (Syngenta) — Strong systemic and translaminar fungicide. Beyond eradicating the pathogen, it leaves the foliage greener and the canes harder.

   • Dosage: 1–2 ml per 1 L of water.
   • Application: Spray evenly across the canopy. For severe infections, two applications 3–5 days apart, then preventive sprays every 15 days.
   • Where to buy: Shopee — Anvil 5SC genuine

2. Score 250EC — A fast-acting powdery mildew killer with deep penetration into leaf tissue, making it rain-fast.

   • Dosage: 0.5 ml per 1 L of water (highly concentrated — measure carefully).
   • Application: Reserve for severe outbreaks. Apply a maximum of two consecutive sprays, 7 days apart, to avoid resistance.
   • Where to buy: Available on Tiki — search for "Score".

Additional advice: Rotate between Anvil and Score so the fungus does not develop resistance. Once recovery is underway, supplement with a foliar feed rich in potassium to thicken and harden the leaves.

• Maintain good air movement around each plant.
• Avoid an overly dense canopy.
• Apply preventive sulphur sprays on a regular schedule.