This year we have a lot of ‘black spot’ on the second flush of roses. Is this within the actual rose, or is it just showing in the leaves? I cut them off and don’t allow them near plants, but the next lot also has black spot in some cases. Can you help?
Black spot is one of the most prevalent fungal diseases on roses in New Zealand. It must be prevented as once it is in the leaves it cannot be cured and will spread very quickly.
Some rose varieties are almost immune to black spot, while others are exceptionally susceptible. Climatic conditions have a large influence on the appearance and severity of infection, ie. the warmer humid midsummer months are a haven for black spot.
Prevention should begin at the end of the growing season. Carefully remove all old leaves and small twigs; this will help prevent the fungus overwintering in your garden. Applications of copper oxychloride and horticultural oil during the dormant winter months will help ensure new spring growth is clean.
During the summer flowering months, fortnightly applications of Shield will hopefully help control outbreaks of black spot. If a particular rose is continually covered with black spot it should be removed and replaced with a more robust variety.