Black spots on roses

the reasons

Black spot is caused by the fungus Diplocarpon rosae, which can live on fallen leaves and plant debris during the winter.

Propagation conditions

Black spot fungus grows in warm, humid conditions with temperatures between 18°C and 24°C (65°F and 75°F).

The fungus requires long periods of keeping the leaf moist (at least 10-12 hours) for spore germination and infection.

The disease can spread rapidly via water splashes, wind, and infected plant material.

Disease course

The black spot fungus overwinters as dormant threads or fruiting structures (pseudocotyls) on fallen leaves and plant debris.

In the spring, when temperatures and humidity levels are favourable, the fungus produces spores (spore-forming spores) that can start new infections on young, susceptible leaves.

The fungus then produces additional spores (achenes) that can spread the disease throughout the growing season.

Losses

Severe black spot infestations can result in significant leaf stripping, reduced plant vigor, and reduced flower production.

In extreme cases, the disease can weaken the plant to the point that it becomes vulnerable to other pathogens or environmental stresses, leading to the plant's death.

Control strategy

An integrated approach including cultural practices, resistant varieties, and fungicide application is recommended to control black spot.

Preventive measures

• Planting resistant or tolerant rose varieties.
• Ensure good air flow.
• Avoid sprinkler irrigation.
• Remove and destroy infected plant remains.

Organic/chemical control

For organic control, apply baking soda, neem oil, or sulfur-based fungicides.

Chemical fungicides containing active ingredients such as chlorothalonil, propiconazole or trifloxystrobin can also be used according to label instructions, alternating between different classes of fungicides to prevent the development of resistance.