Stefan Buczacki solves your plot problems
Carole Chalmers, by email
Stefan says: This is apparently a simple question, but one with a less than straightforward answer.
Because black spot is a disease of the leaves, one might think, it can’t contaminate secateurs, which are used for cutting stems. However, although some black spot fungal infection does persist on fallen leaves over winter, in Britain and much of Europe it has been discovered that most of the new black spot disease each year originates from tiny spots of fungi that become established on the shoots in the autumn. Sometimes, these are large enough to be seen; in others they’re merely microscopic.
Even so, I rather doubt the significance of pruning as a method of transferring black spot from one plant to another, given the very widespread and general occurrence of the fungus. But
I do prefer to err on the side of caution and a quick disinfection of the blades when moving from diseased to healthy plants will certainly do no harm. You can achieve this with a cloth soaked in meths and use it to wipe over the blades. And you should follow the same precaution after cutting diseased wood from fruit or other trees.
But the question of the overwinter survival of black spot contrasts with that of the two other rose diseases: mildew and rust. Mildew will survive wherever it can; on stems, buds or leaves that remain on plants growing in warmer, more sheltered sites. Climbers growing against warm walls are almost always the first roses to show mildew infection in the spring, simply because they’re the starting points, and the disease subsequently spreads from them to bushes growing nearby.
However, rust is different, surviving through the bad times in the form of tiny black spores on the fallen leaves. So, while clearing up fallen leaves beneath diseased rose plants is unlikely to have much impact on the incidence of black spot and mildew in the following year, the removal of rusted leaves certainly will lessen the likelihood of that problem the next time around.