Blackspot and Powdery Mildew on Roses

Photo © Bulleen Art & Garden

Blackspot on Roses

I grew up in South Australia where the hot dry summers were perfect for rose growing. Now I live in Melbourne and have had to become a lot more proactive in preventing and treating both blackspot and powdery mildew, as the wetter conditions leave the bushes a lot more vulnerable to attack. In a bad season, even the more disease resistant varieties can be affected. Not that this deters Melbournians from rose growing, this is after all the garden state (or was, and should be again) and people here clearly love their 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.

Powdery Mildew on Roses

This starts first on new shoots as crinkled leaves and then a soft white powdery fungus (Podosphaera pannosa) appears, if left alone it will rapidly spread across the plant and infect other roses. The problem is worsened by poor airflow, poor light, soil drying out and poor nutrition. So keep under planting to a minimum, plant in the sun and feed and water roses well. Try to water really well in the morning, so the foliage dries out and to reduce night time humidity. Feed with a balanced rose fertiliser (highly nitrogenous fertilisers produce too much soft growth which is prone to fungal attack) and add in sulphate of potash for added cell wall strength. Unlike many other fungal diseases powdery mildew doesn’t necessarily need wet conditions and can thrive over a hot dry summer.

Powdery mildew over winters on the stems and in some dormant buds. This is a good time to spray with a fungicide to get a jump start on reducing the problem. Then keep an eye on new growth in spring and spray as soon as the mildew is seen. Use a good commercial fungicide or make up your own.