It was a ‘good news/bad news’ morning out on the balcony today. When I strolled out to survey the estate as I do each day, I was both thrilled and horrified at almost the same moment.
After an unseasonably cool and damp spring (only seven days of sun in the entire month of May here on Canada’s west coast, for example), I’ve had lots of buds on my row of four Drifts, but not one of them in any hurry to actually start opening up. We know that June is peak rose season, yet here it is – almost the end of June! – and still no explosion of red or coral or apricot or pink blooms from my Drift roses along the balcony railing.
The good news was that today I saw the first unmistakably lovely blush of colour on my Coral Drift rosebuds – so far the first of its sister plants to act like a rosebush should by late June.
But as I was admiring those tiny previews of the full coral rose blossoms yet to come, I saw something else that is the bane of rose growers everywhere: the dreaded Black Spot. Rose guru Brad Jalbert from Select Roses, a farm-style home-based rose nursery in Langley, BC (just east of Vancouver), claims that”many favourite roses of the past are just too disease-prone to be sold today. Iceberg roses, for instance, once hugely popular floribunda rose bushes, are now so prone to Black Spot, they should NOT even be sold here.”
According to the U.K.’s Royal Horticultural Society, Black Spot is indeed “the most serious disease of roses. It is caused by a fungus, ‘Diplocarpon rosae’, which infects the leaves and greatly reduces plant vigour. Expect to see leaf markings from spring, which will persist as long as the leaves remain on the plant. The fungus is genetically very diverse and new strains arise rapidly. Unfortunately, this means that the resistance bred into new cultivars usually fails to last because new strains of the fungus arise to overcome it.“
That last sentence was particularly troubling – since one of the key reasons that I’d chosen the Drift® family of landscaping roses was their much-longed-for disease resistance!
But as the RHS adds: “The fungus produces spores in the Black Spot lesions on the upper leaf or stem surface and these spread in water to initiate new infections. Wet conditions are required for the disease to build up. The fungus spends the winter on fallen leaves and also in dormant infections on young stems and buds, producing spores in the spring to infect young foliage.”
We’ve had very wet conditions for months here. And did I mention only seven days of sunshine in the entire month of May? Sounds like ideal environmental conditions if you’re trying to encourage Black Spot. . .
So what should I be doing now that the first brazen signs of Black Spot are inarguably present on my Coral Drift? The RHS has more bad news:
“Badly affected plants can shed almost all their leaves and their vigour is greatly reduced. The symptoms are so severe that, anecdotally, the disease has been blamed for a decline in the popularity of roses in U.K. gardens in recent decades.”
That’s some kind of powerful fungus! Single-handedly responsible for destroying the reputation of garden roses throughout an entire nation!
Is there anything I can do about Black Spot?
The RHS does have something to say about this:
“Collect and destroy fallen leaves in the autumn, or bury under a layer of mulch. Prune out all stem lesions in spring before leaves appear. These actions will help delay the onset of the disease, but are of limited value because spores are bound to blow in on wind-blown rain from elsewhere. Avoiding pests, diseases and weeds by good practice in cultivation methods, cultivar selection, garden hygiene and encouraging or introducing natural enemies should be the first line of control. If chemical controls are used, they should be used only in a minimal and highly targeted manner.”
What kind of “chemical controls” are they talking about?
They tell us that chemical fungicide treatments are “labelled for the control of rose Black Spot.” There are others labelled as a combination of both fungicide and insecticide (the latter attack the pesky aphids that love to chomp on roses). But the RHS warns that if insects are NOT a problem for your roses, avoid combination products if what you need is to treat only your Black Spot.
I’ve spoken to longtime rose growers, however, who see Black Spot as simply an unavoidable reality that can be managed (sort of!) by picking off the affected leaves.
Garden columnist Dan Gill is one of these optimistic types. He adds:
“If you have landscape roses, such as Knock Out roses, Drift roses and others, there is no need to spray. These roses will get Black Spot when conditions are especially favourable (rainy weather), but they get over it on their own and will be fine without treatment. Be sure to rake up the spotted yellow leaves as they fall and dispose of them. If allowed to remain on the ground under the rose bushes, the Black Spot fungus in the leaves will continue to grow and produce spores, increasing the chances of future problems.“
Staying tidy seems to be a good and environmentally friendly option so far. Same goes for the nasty rose fungus called powdery mildew, apparently also thriving quite happily out on my balcony. . . Here’s what it looks like on the leaves and rosebuds of my Flower Carpet Scarlet (below):
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