Black spot is a common fungal disease that affects roses, caused by the fungus Diplocarpon rosae. The disease is characterized by the development of circular, black spots on the leaves of roses, which can quickly spread to other parts of the plant. As the disease progresses, the leaves will turn yellow and drop off, weakening the plant and making it vulnerable to other infections.
Black spot thrives in cool, moist weather conditions and can be particularly prevalent in areas with high humidity or frequent rainfall. It is a major problem for rose growers and can severely impact the health and aesthetics of rose bushes.
Blackspot Management in Rose
The Disease Cycle of Black spot
- Overwintering: The disease overwintered in diseased canes and infested fallen leaves, which are a source of infection for the next growing season.
- Spring emergence: In the spring, the fungal spores germinate and are disseminated by splashing water. The wind, insects, or other means may also carry the spores.
- Infection: Fungal spores on the leaf surface must be continuously wet for at least seven hours for infection. Once the infection is established, the fungus will develop fruiting bodies called acervuli in black lesions, which produce spores that can spread the disease to new tissue.
- Spore production: As the disease progresses, the acervuli continue to produce spores that can splash to new tissue, spreading the disease further. The wind, insects, or other means may also carry the spores.
- Overlapping cycles: As the season progresses, multiple cycles of infection and spore production can occur, leading to increased disease severity and potential for further spread.
- Winter dormancy: At the end of the growing season, the fungus may again overwinter in diseased canes and fallen leaves, completing the life cycle and preparing for the next growing season.
Symptoms of Black Spot in Rose field
- Black spots: Small, circular black spots, usually ranging in size from one-tenth to one-half inch in diameter, appear first on the upper leaf surfaces. These spots can merge to form larger, irregularly shaped lesions.
- Yellowing: Areas adjacent to the black spots turn yellow, and the leaves may eventually drop prematurely from the plant. This usually starts at the bottom of the plant and progresses upward.
- Cane damage: In some cases, raised purple-red blotches may develop on the immature wood of first-year canes. These spots may later become blackened and blistered, leading to wilting and dieback of the cane.
- Reduced vigor: As the disease progresses, the plant may become weak and stunted, with reduced vigor and a decreased ability to produce flowers.
Cultural control of Black spot
- Avoid wetting the foliage, especially during dark cloudy days: This helps to reduce the time that the foliage remains wet, which can promote the growth of the fungus.
- Rake and discard all fallen leaves in the fall: This helps to minimize the source of the disease and prevent it from overwintering.
- Use resistant varieties for low-maintenance plantings: This is a preventative measure, as resistant varieties are less susceptible to Blackspot.
- Remove infected leaves during dry weather: This helps to retard the rate of disease spread by removing the source of the disease.
Biological control of Black spot
Biological control is one approach to managing the disease, which involves using natural enemies to suppress the pathogen. Here are some key points on the biological control of Blackspot, as suggested by the study mentioned:
Use of biological control agents: The study found that the incidence of Blackspot was significantly low in treatments that used biological control agents alone or in combination with fungicide treatments. Specifically, Trichoderma harzianum and Clonostachys rosea (C. globosum) effectively reduced disease ratings.
Chemical control of Black spot