Blackspot on Roses
Blackspot is a fungus (Diplocarpon rosae) that appears initially as black spots on leaves, progressing to large black spots fringed with yellow rings. As the disease spreads, the entire leaf will go from green to yellow and then drop to the ground. Eventually the entire rose bush may become defoliated. This frustrating disease is at its worst in warm humid or wet weather.
You need a multi pronged attack for this disease. One of our wholesale growers gave us a tip; he virtually eliminated the need to spray for powdery mildew and black spot by ensuring his roses NEVER dried out in the pots. To be even more specific, he didn’t want the fine root hairs around the edge of the pot to dry out. We adopted this practice and began twice daily watering, three times on really hot days, and have achieved an astounding difference, have not sprayed once for powdery mildew or black spot (did have to spray for a mite outbreak though). Just incredible. So we are now converted to a less harsh watering regime, and ensure roses are far more generously watered than previously, we are still careful to water the soil and not the foliage. Secondly good garden hygiene is vital. The fungal spores will overwinter on the ground if allowed. So a good autumn/winter cleanup is essential, all the leaves need to be removed and disposed of. If you spot the disease over spring or summer, prune off infected material and throw into rubbish – not the compost. Third, good nutrition makes it easier for plants to withstand attack. Poorly fed plants will succumb to diseases first, so feed your roses well every 6 to 8 weeks throughout the growing season. In addition to a good rose fertiliser, a good tip is to add potash, this is a win/win, as it encourages flowering, but also thickens cell walls which makes them less vulnerable to attack by the fungal hyphae. Then there is spraying. You can use one of a range of commercially available fungicides or a home made sign. Remember to spray to ‘point of run off’ i.e. stop spraying when leaves are misted with spray and before it beads and runs off. The final resort for a constantly poorly performing and badly infected rose is to rip it out, it is just a suppurating source of infection – get rid of it and plant a better variety.