Fungal diseases: Rose black spot is a fungal disease where purple or black spots develop on leaves, which often turn yellow and drop early. It affects the growth of the plant as photosynthesis is reduced. It is spread in the environment by water or wind. Rose black spot can be treated by using fungicides and/or removing and destroying the affected leaves.

The idea

Rose black spot is caused by a fungus that infects rose leaves. Purple or black spots appear on the leaf surface; surrounding tissue often turns yellow, and infected leaves drop early. Because leaves carry out photosynthesis, losing leaf area reduces the plant's photosynthesis, so growth is reduced. The fungus spreads in the environment by water or wind, so spores land on healthy leaves and start new infections. Treatment combines fungicides, which kill or inhibit the fungus, with removing and destroying affected leaves to reduce the source of spores. Destroying, rather than composting, the leaves matters because surviving spores could reinfect the plant.

What earns the marks

• Rose black spot is a fungal disease of rose plants, so the pathogen is a fungus rather than a bacterium or virus.
• Purple or black spots develop on leaves, and the surrounding leaf tissue often turns yellow.
• Infected leaves often drop early, reducing the total leaf area available for photosynthesis.
• Reduced photosynthesis lowers the plant's growth because less glucose is made for respiration and new tissue.
• The disease spreads in the environment by water or wind, which carry fungal spores to healthy leaves.
• Treatment can use fungicides and/or removal and destruction of affected leaves to reduce reinfection.

Common mistakes and how to put them right

• Calling rose black spot a viral or bacterial disease.

  Put it right by naming the pathogen as a fungus and linking the spots to fungal growth.

• Saying the plant dies immediately from the spots.

  Put it right by explaining that leaf loss reduces photosynthesis, which reduces growth.

• Treating only with fungicide while leaving fallen infected leaves on the ground.

  Put it right: Also removing and destroying affected leaves to reduce spores.

Exam tips

• Link each visible symptom to its effect: spots and yellowing reduce photosynthesis, and leaf drop reduces leaf area further.
• When asked how it spreads, name both water and wind as environmental agents carrying spores.
• For control, give both parts of the treatment: fungicides and removing and destroying affected leaves.