Black spot on roses

There are thousands of fungi capable of producing leaf spot diseases on plants. The pathogen Diplocarpon rosae is responsible for Black spot on roses. (Its asexual infectious phase is termed Marssonina rosae). While leaf spot diseases can be largely ignored, black spot is disfiguring and will weaken your roses. Plant selection and cultural practices can greatly reduce the incidence of black spot. Once it is present, Diplocarpon can be difficult to eradicate.

Black Spot normally appears with the advent of wet warm spring weather. A few drops of water are all it takes to bring spores, overwintering in leaf buds or on canes, to life. Upon germination, fungal mycelia rapidly tunnel into plant tissue producing the characteristic black to purplish sploches on leaves and reddish lesions on canes. After a week, small, black fruiting structures (acervuli), followed by slimy white conidia (spores) will develop in the darkened lesions. A rose leaflet with one small black spot (10mm) can produce over a million spores over the course of a month. As the disease progresses, affected leaflets yellow. This chlorosis is caused by ethylene gas released by the fungi. In susceptible rose cultivars, infected leaves will fall prematurely and the health of the plant will be compromised. Severe defoliation weakens the plant, reduces flowering and makes the plant more subject to winter damage.

If you plan on starting a rose garden, choose resistant cultivars (see below for listings). Most garden catalogs will identify rose cultivars resistant to black spot and other diseases. Only purchase disease free plants.

Spend time preparing the soil, make sure it is rich with microorganisms that will support the rose's health. Position the rose so that it can receive a minimum of six hours of sunlight and lots of air circulation. Interplant with other species to minimize disease transfer. Avoid wetting the foliage when watering and water in the mornings. The drier the plant, the less black spot can take hold.

Unfortunately, because this fungi embeds itself within plant tissue, it is difficult to treat. Frankly, unless the rose is a great beauty and you do not mind the trouble, I suggest replacing disease prone plants with resistant cultivars. That said, you can learn to manage and minimize the disease. If disease appears, prune it out and clean up fallen leaves. Severely infected plants should be pruned back in the fall or early spring to within 1 to 2 inches of the bud union, according to variety and cultivar.

If you know the pathogen is present, begin spraying the plant in winter or at bud break before symptoms appear and continue weekly or biweekly. What can you use? It is always best to begin with the least toxic treatments and remember some treatments will kill beneficial insects and microorganisms, so proceed wisely.

• Oils and fungicidal soaps can help prevent black spot. Use horticultural oil or plant based oils and fungicidal soaps like Safer's (or make your own). Hortico sells an oil based foliar spray, developed at Guelph University, that helps prevent foliar disease and repels insects. (It makes my roses altogether more robust.) 

• Once leaves emerge, anti-transpirants or antidessicants, biodegradable waxy polymers, may reduce infection rates by limiting fungal penetration. Use cautiously as they also reduce photosynthesis and transpiration and can become phytotoxic at high concentrations and elevated temperatures.

• A combination of sodium bicarbonate plus horticultural oil or neem oil has also been shown to be effective in reducing black spot. Mix 1Tbsp (15ml) baking soda, 1Tbsp (15ml) oil, 1/2 tsp (2ml) dishwashing soap with 4liters of water. 

• A combination dormant oil and sulfur spray in the fall before hilling as well as first thing in the spring after you remove the soil covering may be more effective than oil alone. Several companies (Green Earth®, Wilson®, etc.) offer lime sulfate fungicides.

• During the summer, sulfur (dry or wettable) can be used alone. Used with oils, sulfur can cause leaf burn. Use only when temperatures are below 32°C. Safers Garden Fungicide advises spraying at 7 to 10 day intervals.  Read instructions and cautions carefully.

• For more aggressive control, try a fixed copper fungicide (e.g. copper sulfate) or Bordeaux mixture (a copper sulfate-lime mixture). [A standard Bordeaux mixture can be made using 3-1/3 T (50ml) of copper sulfate and 10 T (150ml) of hydrated lime (calcium hydroxide) in 4 litres of water.]  Bordeaux Mixture gives good control, but leaves a whitish deposit. Handle with care, Bordeaux mixture is corrosive. Phytotoxicity and leaf burn can be an issue.

• Several research articles suggest that silicon increases the resistance of plants to pathogenic fungi. It is widely used in Europe. I do not know if soluble silicon is sold here in Canada. However, a Horsetail Tea (Equisetum arvense) would be very high in silicates if anyone wanted to go to the trouble of drying and steeping them. 

• A foliar spray of manure or compost tea can help to prevent foliar diseases. (Fill a bucket with water (let it fill with rain) and add a shovel full of manure or compost. Strain and apply as needed). This is a little hit or miss, depending on the microorganisms in the tea. The right microorganisms will compete for nutrients, induce systemic resistance and in some cases actively destroy fungi.

• Some beneficial microorganisms are commercially available. For instance, Ampelomyces quisqualis, (sold as AQ10) is parasitic fungus that feeds on powdery mildew and Bacillus subtilis (sold as Serenade) a bacteria that reduces colonies presumably by competing for nutrients and or by attacking the pathogen. Bacillus laterosporus sold in the U.S. as Rose Flora does not appear to be available here, but it is in some active yoghurt cultures and sold as a probiotic. You may want to experiment with a yoghurt foliar spray (milk sprays have certainly been found effective for powdery mildew). You may try enriching your soil with mycorrhizal inoculants (beneficial fungi) and beneficial bacteria as they become commercially available, or better still create a healthy compost pile and produce your own beneficial microorganisms.

• A new product in the U.S. is a bacterial protein called Harpin(Ea) - produced naturally by Erwinia amylovora (a causal agent of fire blight). A foliar spray containing Harpin(Ea) appears to trigger a plants natural defense system, inducing systemic resistance to pathogens. In a similar manner, acetylsalicylic acid or aspirin appears to trigger a plants natural defense and induce resistance to certain pathogens. Try crushing an aspirin tablet in a liter of water to create a foliar spray.

• Conventional fungicides, like myclobutanil and tebuconazol, are extremely effective but hazardous and not permitted for home and garden use in Canada.

Let me know what works for you.