Sandy’s Garden ... The Black Spot

Sandy Simpson
Sandy Simpson

My friends often say to me, “Did you see such-and-such on television last night?”

The black spot was a pirate’s intimation of a sentence of death, sure, certain, inescapable death. And black spot on rose bushes, if not usually a death sentence, will assuredly destroy their health and their beauty. But unlike the pirates of yesteryear, for whom there was no escape from the black spot, rose bushes can be reprieved if their carers will but try to reprieve them.

Technically, black spot is a fungal disease of rose bushes … Diplocarpon Rosae … which begins to appear when the air temperature approaches 18°C (say 65°°F) and there is plenty of rain. The infection starts near the ground and the youngest leaves are the first to be affected, developing black spots surrounded by a yellow rim. Within a few short weeks the leaves have become a yellowish-pink colour and have started to fall off. In severe cases, the bush can lose all its leaves by midsummer. Such defoliation weakens the plant and encourages the spread of the fungus to neighbouring bushes so that the entire rose bed can be devastated in a single season.

Now, if you are thinking, “Hold on, we’re nowhere near midsummer and the air temperature is struggling to reach 8°C some days, never mind 18°C,” bear with me, for this is the time of year to choose a half-decent day and spend half-an-hour in the rose bed, collecting all the fallen rose leaves that have not been blown away by the winter gales and are still lying about, a doubly important task if you think there may have been some signs of black spot among the bushes last year. If any black-spotted leaves are still clinging to any of the bushes, prune them off, add them to your collection and dispose of all these infected leaves by burning them or, if you value your neighbour’s friendship, by binning them. Don’t put these leaves in the compost heap, for the spores of the fungus will rather like that and will multiply quite happily, eager to be about your bushes when the weather warms up.

The second part of the treatment you should give your rose bushes involves visiting your local garden centre to choose one of a variety of sprays to combat the return of black spot; they will all work if you follow the instructions carefully; and that should keep your precious roses black spot free for the summer.

Sandy Simpson, Polmont Horticultural Society