Preventing Rose Mildew/Mould with Sulphur or Soda

Helen Chestnut, Gardening in the Province, 2006-05

Traditional treatment against powdery mildew (white mould coating on rose leaves) and black spot is sulphur, available as a dust or spray, preventing germination of the disease spores.

Read instructions: applying sulphur on hot days can burn plants.

Recently homemade mixtures of baking soda (sodium bicarbonate, NaHCO3) have become popular for preventing powdery mildew and black spot. (Note: this is available in bulk as a pool chemical for total alkalinity from pool suppliers.)

The Home & Garden Pest Management Guide for BC recommends a mixture no stronger than one tablespoon in five Liters of water. Higher concentrations can burn and/or stunt plants.

Add one teaspoon insecticidal soap or light vegetable oil to help the spray spread over and adhere to the foliage.

Latest addition is adding one or two tablespoons of a liquid fertilizer such as Raingrow, which contains herbal elements that in themselves may help improve the health of the plants as it feeds.

Shake well before and during application to the foliage.

Alternating a soda spray with sulphur application probably increases control over powdery mildew by reducing chances of the disease building immunity.

Careful: frequent use of baking soda, even in the small quantities called for in the spray, can lead to the accumulation of sodium in the soil, most pronounced in slow draining clay soils.

Even simply washing leaves with generous sprays of water from a hose nozzle on warm sunny days will help reduce powdery mildew.

Skim milk, mixed one part to 9 parts water, has also shown promise as a control for powdery mildew and black spot.