Rose Diseases and Their Control

To control a fungal disease, you need a fungicide. From "least toxic" to "most toxic," these can include:

Baking soda/horticultural oil — Tests performed by Cornell and Auburn universities in the U.S. confirm this as my favorite powdery mildew control. Mixed at a rate of 1 rounded tablespoon (20 ml) baking soda (sodium bicarbonate) and 1 tablespoon (15 ml) horticultural oil per gallon (4 l) of water, this material is sprayed on foliage and stems weekly when the disease first appears. Use caution to keep the spray confined to the plant, thus reducing the potential for sodium bicarbonate buildup in the soil.

Fish emulsion/seaweed (kelp) — Many rosarians have achieved excellent results by applying these mixtures normally used in foliar fertilization applications. Mixed and applied per label directions, it is possible these ingredients encapsulate the fungi and prevent the spread of spores to neighboring, non-infected areas. Another theory suggests the kelp which remains on the leaves may alter the growing conditions conducive to the fungal spore's haustoria (fungal "roots").

Whole neem oil — This ancient botanical insecticide and miticide from India also serves as a fungicide. Applied per label directions when conditions are favorable for disease is much more effective than when trying to control the disease after it appears. If I must use a pesticide, this is my favorite because it is virtually harmless to most beneficial insects and microorganisms in the garden.

Antitranspirants — Originally developed by farmers in Israel to reduce water transpiration in crops, these products encapsulate the fungal spores with environment-friendly, low toxicity waxy polymers, thus preventing their spread to nearby leaves and stems. Apply per label directions.

Sulfur and sulfur compounds — Both wettable powders and premixed solutions are currently available to the rosarian. Lime-sulfur is good as a preventative. Use caution when applying these during warm days, however. Phytotoxicity can occur when temperatures exceed 90°F (32°C).

Ammonium chlorides — Usually marketed as Consan 20, Triple Action 20 or TA-20, these products are commonly used as an algaecide in hospitals and schools. Recently, they have been rated for certain fungal diseases in roses. They biodegrade to harmless compounds in the environment. Use caution when applying, however. Ammonium compounds are very caustic in the eyes.

Fenarimol (Rubigan, Bloc, EL-222) — A chlorophenyl topical fungicide which acts primarily as a foliar and stem protectant against fungal germination. Oral lethal toxicity (LD50) in laboratory rats is 2500 mg/kg, or about half as toxic as aspirin.

Copper and