by Mark Whitelaw
Like all living organisms, when a rose undergoes stress it can get sick. That is, its normal defense mechanisms break down and it opens itself to disease.
The first rule for controlling rose diseases is prevention. You prevent diseases by ensuring your rose has its Three Basic Needs plenty of sun, plenty of air, and plenty of nutrients which include water, the proper and deep, organically improved, well-drained soil. By providing these needs, you reduce the stress on your roses, their susceptibility to diseases, and ultimately your need for potentially toxic pesticides in the garden.
The next rule for controlling diseases is to select resistant cultivars. Although many hybridizers are beginning to produce less disease-prone roses, others are still more concerned about "show form" than their rose's ability to withstand the onslaught of fungi and viruses. I refer to these latter roses as "chemically addicted" to pesticides.
Various rose texts can offer suggestions for disease resistant cultivars, but much of this determination is a matter of experimentation on your part. Find out which roses do best in your garden. Here's where you local rose society and their consulting rosarians can help. And ask your neighbors or fellow "cyber-rosarians" which roses they prefer for disease resistance.
Despite all of your efforts, however, sometimes nature plays tricks on your garden and your roses just get sick. Identifying which disease they have will go a long way in determining the cure. To help in that regard, the Texas A & M University Agricultural Extension Service's Plant Answers database will definitely help. There, you will find that rose diseases generally fall into three groups bacterial, viral and fungal.
Bacterial diseases (like crown gall) and viral diseases (like mosaic, rosette, and strawberry latent ringspot) are pretty rare for the average rosarian. Transmission of these diseases is usually spread via grafting irregularities or by the lack of clean horticultural practices when pruning. If these diseases have found a way into your garden, most likely they came in with the plant. And although some extraordinary measures can be taken to cure some of them, your best solution is to "shovel prune" the offender and try again.
Fungal diseases, on the other hand, are the most common for rosarians. They exist because environmental conditions favorable to their reproduction have appeared. The more common ones are powdery mildew, blackspot, downy mildew and rust. The less common diseases are botrytis, anthracnose, and canker.
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)
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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 copper compounds Usually dusted on canes and bud eyes as new growth first appears in spring. Reportedly, these compounds inoculate the rose for the entire season. Despite some promising results, I'm not a big fan of copper compounds because of their potentially damaging effects to the soil. Likewise, some copper compounds can be toxic to the person using them with LD50's around 1000 mg/kg.
Triadimefon (Bayleton, Amiral, Bay MEB6447) A chlorophenoxy-dimethyl-trizol-butanone systemic fungicide. If you can pronounce that, you might want to know it works against rust and powdery mildew for about 28 days, but its overuse can cause short bloom stems.
Benomyl (Benlate, Tersan 1991) A methyl-bensimidazole carbamate systemic fungicide effective for rust and powdery mildew. If you like carbamates like Sevin as an insecticide, you might like this material as a fungicide. What you should also know is these products may affect your kidneys after prolonged and repeated exposures. Some are known teratogenics and mutagenics. (Note: Some forms of benomyl are not labeled for roses in the U.S.)
Triforine (Funginex, Saprol) A trichloroethylene formamide systemic fungicide long considered the rosarian's favorite. Unfortunately, despite its low oral lethal toxicity, more detailed studies have shown it will metabolize into chloralformamide, four per cent of which remains in your body. This latter chemical is potentially life-threatening in that it has psychogenic and cardiovascular effects after prolonged use. If you like this stuff, stock up now. It won't be around much longer.