Roses vary a lot in their susceptibility to the fungal disease, Black Spot. This can be a killer and a reason why you cannot grow some varieties. I use organic methods to cope with disease most of the time but I make an exception for my (our) roses. That means that I spray the plants most seasons from time to time and if the season is a bad one for black spot, like this one has been (wet and cool), I try to spray every fortnight.
Two things affect how much black spot you get. One is the kind of black spot you have in your neighbourhood; this can affect which varieties show resistance and which are relatively susceptible. The other thing is the microclimate in the garden. 'Agnes' is a rugosa that often does not show a lot of the disease. The 'Agnes' bush I have in the open border is fairly healthy but the one planted against the house is showing more black spot than just about any other.
So by looking around the garden at this time of year, you can see most clearly which roses are more resistant - the ones that are healthy even although they have never or rarely been sprayed. The suscepts are those that are looking very sad even after spraying regularly throughout the season. The list below has the most resistant at the top and least resistant (susceptible) at the bottom. Remember that this is according to infections observed in my garden over several years.
Tuscany Superb
Perle D'Or
Rose de Rescht
Blush Noisette
Rosamudi
gallica Versicolor Rosa Mundi
Ispahan
MutabilisRoseraie De L'HayPhyllis Bide
Brenda ColvinAgnesCupidWilliam LobbLittle White Pet
Old Blush ChinaStanwell PerpetualPenelope
Scharlachglut
Albertine
Mme Alfred Carriere
American Pillar
Windrush
Complicata
I use the systemic fungicide, 'Systhane' (active ingredient myclobutanil). The chemical prevents new infection of healthy tissue but also has a curative effect on any disease already present on leaves and stems.
IIn 2016. I plan to use another systemic substance, the chemical Phosphite (also known as phosphonate, a salt of phosphonic acid). This substance induces the natural defences of the plant. The mechanism is fairly well understood and is known as Systemic Acquired Resistance. This can also be seen when early infection of lower leaves of the plant can induce resistance in upper leaves and vice versa. The resistance appears to be non-specific as the plant is protected from viruses and a range of fungal and downy mildew diseases including rose downy mildew.
Tweet me if you have a comment about your findings compared to mine. @rudebotanical
