Rose black spot's Achilles heel
Every rosarian sooner of later has to take a position on black spot. Personally, I don't do much about it except try to grow resistant roses and break the over-wintering phase of the fungal life cycle. Otherwise I just put up with it, at least in modest amounts. I don't spray: but mainly out of indolence rather than conviction.
Late season black spot on Paul Neyron
Black spot's Achilles heel is its winter stage. To understand this fact, it is important to understand the life cycle.
Black spot is caused by an ascomycete fungus called
Diplocarpon rosae. It reproduces mainly asexually by conidiospores (spores that produce an identical copy of the parent fungus strain). The other type of spore, ascospores, which are the ones that increase the strain diversity of the fungus, are rather rarely produced so I won't consider them further here. The propensity of black spot to reproduce asexually means that a region may be dominated by a particular strain for a long period of time. If a rose has blackspot in your area but doesn't somewhere else, it could be because of climate differences, but it could also mean you have a different strain of black spot.
Rose leaves when they emerge in the spring are free of black spot. To acquire the disease conidiospores have to contact the leaf. In summer, probably they will be splashed onto the leaf from an existing black lesion. However, in spring all the leaves are new and clean so conidiospores have to be splashed up from a decaying leaf on the ground that fell the previous autumn*. As raindrops hit dead leaves on the ground, fine droplets (aerosols) are formed which may pick up spores and waft them up the the leaves.
Many rose growers remark that their roses don't have blackspot early in the season but develop it in late summer. This isn't strictly true. It is just that there are very few lesions in the spring, but as each lesion spreads its conidiospores, one lesion becomes two, becomes four, becomes eight and so on. There comes a point, usually in late summer, when black spot is suddenly noticeable and ugly.
Control of black spot: winter treatments
When the infected leaves fall in autumn the fungus continues to survive on the dead leaves, lasting long enough to reinfect the newly emerging leaves with conidiospores in the spring. It is in winter that black spot needs to be attacked. The length of time the dead leaves can last as a source of infection depends on climate and soil conditions. A mild winter and biologically active soil can break down the dead leaves before spring arrives and before they can infect the new leaves. Wait until all the leaves have fallen and cover them with a rich mulch, or with manure, to encourage decay and a succession of other fungi and bacteria that will rot the leaves to the state beyond which they will be a source of infection. In spring, just before the new rose leaves open, put on a mulch of bark to isolate any remaining viable spores. Western red cedar bark is ideal as this bark has a mild fungicidal effect. Having broken the life cycle you will find it takes much longer for black spot to re-emerge in the spring. Of course it will come back eventually, from an occasional conidiospore drifting in the wind from some other source of infection, but you will have broken the back of it.
In former times, the life cycle could be similarly broken in winter by applying a fungicidal drench once all the leaves had fallen, to kill the conidiospores in the ground. I remember a beautiful bed of Apricot Nectar that had terrible blackspot one year in my family's garden. It was given a drench of Jeyes Fluid in winter and the following year not a single black-spot lesion appeared - it was almost miraculous. (Jeyes Fluid is an old-fashioned British disifectant made from coal tar). It never seemed to harm the soil or the plants, and the coal tar chemicals probably biodegraded quite quickly, but it was lethal for black spot.
Another line of attack is to increase the natural resistance of the plants. In my experience infection rates are much higher if the plants are stressed, whether drought stress or nutrient stress. A well fed and watered plant that is growing vigorously will have maximum natural resistance.
--
*(Another possible source of overwintering spores is on infected stems, but if your plant has such a bad case of black spot that even the stems are infected, the plant should probably be dug up and burned).
