blackspot on Sonia Rykiel
One of the Big Three rose diseases. The other two are powdery
mildew and rust; neither is at all common in my garden, in fact
I have yet to see rust. It's mainly a disease of hot dry climates
and that we ain't. Powdery mildew does show up if summer nights
are damp but not a big problem. Blackspot, now there's a problem.
It can totally defoliate roses which will kill them eventually.
And unfortunately the genes for repeat bloom and fragrance are
linked with susceptibility to blackspot.
Blackspot is a disease of warm humid conditions. It prefers temperatures
in the 60s, 70s and 80s F and requires temperatures at least above
65. It also requires a damp leaf surface for a minimum of 7 hours.
That's why we're told to never overhead water our roses, to keep
those leaves dry. Blackspot spores spread by splashing water drops,
not by wind or insects. Blackspot lesions are typically black,
feathery margined, and with yellow halos. Badly infected leaves
turn entirely yellow between the black spots and fall off. Plants
can be entirely defoliated, starting from the bottom usually. If
your rose has black spots with angular margins, and the leaves fall off rapidly
starting from the top, you have downy mildew rather than blackspot. Downy
mildew can totally defoliate a plant in a couple of weeks where blackspot might
take all summer. DM is much more likely to kill your roses too because it
defoliates them so fast.
In dry climates like California there's no excuse for having blackspot.
I only saw it in overhead watered roses or in very wet springs.
Here, blackspot is in season basically whenever the roses have
leaves. Blackspot also infects canes and overwinters in leaf litter
or canes, one reason for pruning hard in spring and stripping
and burning leaves in fall. Many fungicides are sold for blackspot
and they vary in cost, spray interval and environmental hazards.
In general fungicides pose much less hazard to nontarget organisms
than insecticides or herbicides do. Remedy is an industrial strength
version of baking soda. Very safe, but only somewhat effective
against bs. Doesn't do a thing for anthracnose. Cornell Formula
is a good homemade fungicide. The recipe is 2T horticultural oil,
1T baking soda and another T of kelp fertilizer or something similar
in a gallon of water. Works ok in dry weather but not the strongest
thing for blackspot in periods of high disease pressure. Sulfur
seems quite effective. I hear copper can be too. Both are minor
nutrients so not necessarily toxic, however a little goes a very
long ways nutritionwise, and copper can easily burn plants. Sulfur
will too in hot weather but we don't get hot enough here to worry
about that. It was a major limitation in CA though, no sulfur
sprays in summer. I've also used BioNeem and that seems to work
too. However neem is toxic to bees and bees love roses so avoid
spraying the flowers. And both oil and neem will kill many of
your beneficial insects along with the aphids and mites you're
trying to kill. So don't use it too often or you won't have any
predatory insects controlling those aphids and mites. If I were
going to use a synthetic fungicide, I'd consider using Banner
Maxx or one of the new strobilurins. Banner Maxx I know to be
very effective, however it's expensive; though much less so than
the strobilurins which currently are running $200/pint. Funginex
is effective, readily available and cheap but I just don't feel
safe using it.
Of my two main rose beds, one is on a drip system and the plants
kept mulched and weeded; the other is overhead watered and the
bed is 'underplanted' with weeds I hope will be shaded or crowded
out some day. The first bed has the least blackspot but the most
mites. The second bed has the most blackspot but the least mites.
One likes what the other doesn't.
blackspot on Sonia Rykiel
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