Your roses should now be bursting into bloom, adding heady fragrance and rich colour to your garden – unless the dreaded black spot has done its worst.
If you see dark brown or black blotches appear on the leaves of your roses before turning yellow and falling off the plant prematurely, they are likely to have been infected by this fungus.
The spots usually appear on the leaves from early summer onwards and spread rapidly. Damp weather can make the problem worse and badly affected plants can shed all their leaves, which can weaken the rose.
Black spot (Diplocarpon rosae) overwinters on fallen leaves and also in dormant infections on young stems and buds, producing spores in the spring to infect young foliage.
But there are many things you can do to prevent and treat it, says Neil Miller, head gardener of Hever Castle & Gardens in Kent, which has more than 5,000 roses in bloom in June and July and is staging Hever In Bloom over two weeks in the summer, featuring garden tours, flower arranging workshops and other events.
Good garden hygiene
“Roses tend to get black spot once the rain comes in the summer – it encourages the spread of this fungal disease. It can spread from year to year. The yellowed leaves fall to the ground and the spores stay in the earth and can transfer to new rose growth the following year.
“Practising good garden hygiene is the best way forward – collecting and incinerating fallen leaves in the summer to prevent the spread, and cutting out any diseased stems.
“Keep clearing any fallen leaves, ensure you cut out any dead wood, keep dead-heading the blooms and mulch every year.”
Remove diseased leaves
“You can pick off the spotted leaves (wearing gloves) to remove the source of reinfection. And remember, each fallen leaf must be removed from the ground – ensure you collect every one. It only takes a leaf or two, over winter, to keep the problem going into the following year.”
Let air circulate
Miller says: “When you plant your new rose bushes make sure they’re planted in an area where air is not restricted – you need a free-flow of air around your bush.”
Think organic
Miller advises rose growers to try to incorporate organic measures like apple cider vinegar spray, or even a milk spray and lemon juice.
“Fungus tends not to enjoy being sprayed with apple cider vinegar, so that’s good option – it’s also safe for the all-important bee population.”
Feed them
“Another key to promoting healthy roses is to feed them with a granular slow-release food at the start of the season and after the first flush in July. Take your feed and draw a little circle with it around the base of the plant, make sure the feed doesn’t scorch the leaves,” he says.
Use a mulch
If you mulch around your roses it will help stop the spores from splashing up from the ground.
Choose disease-resistant varieties
“Rose selection is very important. So when you’re leafing through your bare root catalogue this autumn, look out for rose species that are either specifically bred to be disease-resistant like the peach shrub rose ‘Dame Judi Dench’ (David Austin) or select the older gallica or rugosa varieties which tend not to suffer from black spot,” Miller advises.
“In the Rose Garden itself we have removed plants over the years which have been particularly susceptible to disease and have selected more modern varieties that repeat bloom throughout the summer, rewarding visitors with an incredible display.
“Among our current favourites are ‘Absolutely Fabulous’ – a wonderful yellow rose that looks healthy year on year, ‘Lucky’, and ‘Audrey Wilcox’.
“We removed ‘Rhapsody in Blue’ two summers ago and planted the more resistant, and floriferous ‘Timeless Purple’ in its place. ‘Timeless Cream’ has also performed very well for us over the past couple of years.”
However, the RHS warns that the fungus is genetically very diverse and new strains arise rapidly, which means that the resistance bred into new cultivars usually fails to last because new strains of the fungus arise to overcome it.
Hever In Bloom runs from June 23-July 6.
