Prolonged hot, dry weather this summer has put gardens under strain, and roses are no exception. Heat stress makes roses more vulnerable to pests and diseases, but with early intervention, they can continue producing blooms well into autumn. With further hot spells forecast, gardeners are advised to inspect leaves and flowers carefully for the first signs of trouble.
Roses are prone to two main fungal diseases – black spot and mildew – which become more likely when the plant is dry at the roots, according to Alan Titchmarsh. In a YouTube video, he advised: "Make sure your roses are growing in as rich as soil as you can manufacture, with the help of garden compost and manure, and that they don't go short of moisture."
Black Spot: Symptoms and Prevention
Black spot is an airborne fungus that causes premature leaf drop, starving the plant and reducing its ability to produce blooms. It is characterized by dark round patches on leaves, ranging from dark purple to charcoal black, measuring anywhere from 2mm to 12mm across. The green leaf tissue surrounding each spot turns bright yellow as the infection disrupts chlorophyll. As spots expand and merge, the whole leaf becomes yellow except for the dark blotches, before drying out and dropping from the bush, reports the Express.
Prevention focuses on watering the soil, not the foliage. Watering in the morning allows leaves to dry before nightfall, as black spot spores need several hours of continuous surface moisture to germinate. Adding a thick layer of mulch helps prevent rain or watering from splashing soil-borne spores onto lower leaves.
Pruning can also help: cut out inward-crossing branches to create an open vase shape, allowing wind to pass through the canopy and dry wet leaves quickly. Remove all leaves from the bottom 30cm of the bush to reduce the likelihood of infection from ground splashback.
Black Spot Treatment
Remove spotted leaves as soon as they appear – don't leave them on the ground and never compost affected foliage. In late autumn, rake up every fallen leaf around the base of the rose bush to lower the risk of spores overwintering. Apply a suitable fungicide to both sides of the leaves or use a dilute solution of bicarbonate of soda mixed with a little washing up liquid.
Mildew: Symptoms and Prevention
Powdery mildew looks like a dusting of flour or talcum powder across young leaves, stems and flower buds. It causes foliage to twist, curl or pucker, and infected buds often fail to open properly or bloom misshapen. Unlike black spot, powdery mildew spores do not need wet foliage to germinate; in fact, water on leaves can actually hinder spore germination. Instead, the infection takes root when the plant is drought-stressed and the surrounding air is humid and stagnant.
Prevention involves keeping soil consistently moist and mulching heavily around the base to lock in moisture. Keep the centre of the bush open to allow air currents to carry spores away and reduce local humidity. Avoid high-nitrogen fertiliser, as it encourages rapid, sappy, tender growth that's particularly vulnerable to mildew.
Mildew Treatment
Snip off badly affected leaves and stems – but, again, do not compost them. Spray affected plants with a solution of one part milk to nine parts water once a week, preferably on sunny days. This also works wonders on other mildew-prone plants such as courgettes.
By taking these steps, gardeners can keep their roses healthy and blooming despite the challenging weather conditions.
