Black Spot on Roses
To the rose enthusiast (especially those in the more humid parts of the country) the appearance of black spot can be an endless headache.
However, with a few proactive health strategies for your roses, black spot needn’t darken your mood.
What is black spot?
Black spot is a fungal disease characterised by the appearance of circular spots over the foliage of affected roses. Black spots with yellow-fringed edges, up to 12mm across, will appear on the leaves of affected bushes. These affected leaves will eventually become yellow and fall off. If left untreated, black spot can cause a rose bush to completely defoliate, leaving frustrated gardeners scratching their heads over the demise of their favourite rose bush.
What causes black spot?
Black spot is caused by a fungus, which thrives in warm, humid conditions. It is more common in particular rose varieties, particularly those with yellow genetic parentage.
The first yellow rose was introduced into rose breeding in Paris in the mid-19th century. This Persian rose, R. foetida ‘persiana’ - which was most likely introduced by Andre DuPont, Empress Josephine’s rose-breeding confidante - went on to play a significant role in the development of new rose cultivars, creating never before seen colour variations. It is reported that with the introduction of R. foetida, the Empress’ collections at ‘Malmaison’ increased from 182 varieties, to over 6000 between 1814 to 1850.
Sadly, over time R. foetida proved to be very susceptible to black spot. Today the susceptible gene is the cause of many frustrated rose growers worldwide. So prevalent was the Persian rose in breeding, that many modern rose varieties, especially yellow varieties, are believed to have R. foetida ‘persiana’ as part of their ancient parentage.
Managing black spot on rose bushes
Controlling black spot in roses requires a multi-faceted approach. Rather than treating the symptoms in isolation, a holistic approach at managing the disease will yield better results.
1. Consider the weather conditions
Many fungal diseases proliferate in warm and wet conditions, particularly is the leaf remains wet for extended periods. Reduce humidity by avoiding overhead watering. Watering in the evening should also be avoided, as it allows moisture to remain on the foliage, creating ideal conditions for fungal spores to germinate and cause disease. Make sure there is good air circulation around your rose bushes.
2. Select resistant rose varieties
As most roses are genetically susceptible to black spot it is important to choose resistant varieties that are well suited to your climate.
3. Choose your planting location carefully
Growing conditions can play a big part in a plant’s susceptibility to pest and disease. Most roses require a minimum of five to six hours of direct sunlight each day to bloom properly, so it is best to avoid semi-shaded positions when planting. Roses also prefer to grow without root competition from other plants, such as large trees. For example, if planted under large gum trees roses will compete for water and nutrients making the plant even more prone to infestation.
4. Maintain good plant hygiene
Good sanitation is important to eliminate contamination by fungal disease. Remove and dispose of diseased leaves, including those on the ground, and put them in the rubbish, not the compost. Leaves left lying on the soil have the potential to pass fungal spored on to other roses. Apply a layer of mulch prior to spring so that there can be no splashing of remaining fungal spores from the soil to the lower foliage of the plant.
5. Keep your roses healthy
Just like humans, the healthier your roses, the stronger their resistance to black spot. By improving the general vigour of your rose plants with generous applications of a specific rose fertiliser that includes potash, as well as improving the growing conditions, you can reduce, if not overcome, the incidence of pests and diseases.
Suitable products for treating black spot
Members of the Rose Society of NSW, and other Australian rose societies, have conducted trials of the rose fertiliser, Sudden Impact for Roses, which consistently show an improvement in overall health of the roses trialled, with increased resistance to fungal disease, resulting in a significant reduction in preventative spraying; up to 66%.Rose sprays with tau-fluvalinate and myclobutanil as the active ingredients (like in Yates Rose Gun, and Yates Rose Shield), will be effective in the control of black spot and insects such as thrips and aphids.OCP Eco-Rose is effective in the control of black spot because it contains a specially formulated potassium bicarbonate that alters the pH of the leaf, dehydrating the fungal spores. It can also be combined with OCP Eco-Oil for further benefit. It is good practice to spray roses after pruning in winter with lime sulphur, to disinfect them and clean up fungal spores and insect eggs. If untreated fungal spores can multiply over winter then germinate in spring in the warm, humid conditions.
Remember… prevention is better than a cure
High humidity combined with warm weather encourages black spot, so it is important to start treatment early. Start a program of preventative spraying every two weeks, beginning early in the growth season (October) and continuing through spring, summer and Autumn (April). Alternate each fortnight between Yates Rose Shield and a mixture of OCP Eco-Oil & Eco-Rose. A regular watering with a foliar seaweed tonic like Organix Ecoguard will increase the health and vigour of roses by thickening cell walls, making them inherently stronger. Even with the best hygiene practices, if your roses become stressed for any reason, they will get black spot. Healthy and well-nourished plants grown in the appropriate environmental conditions are always naturally better equipped to fight off pests and diseases.