Can a black spot spray restore your roses to health? The trial 'Silver Jubilee' roses were chosen for the trial. They are a modern variety, more resistant to black spot than older ones, and more typical of the roses you might commonly buy today. They were treated with six chemical fungicides and one organic fungicide from April to October 2012. A new product from Bayer Garden, released too late to be included in the main trial, was also tested. To encourage infection the 'Silver Jubilee' bushes were surrounded with 'Frensham' roses, which are prone to black spot. All the roses were already established in the ground at an independent test site in Cambridgeshire and known to carry the black spot fungus. They were fed with a balanced fertiliser in spring and sprayed to control aphids. In June, black spot appeared on the 'Silver Jubilee' roses. Treatment began at the first sign. Each treatment was used on nine rose bushes and sprayed according to the instructions. Nine rose bushes were left untreated. Experts assessed the health of the roses over six months from May to October 2012, looking at plant vigour, how quickly the disease spread and defoliation. Results None of the products completely stopped black spot in its tracks, but it was found that any chemical-based product was better than nothing. While there were no significant differences between them - they all slowed the progress of the disease to a similar degree and kept the bushes looking healthier than the untreated ones - two of the treatments kept more leaves on the roses. The new product from Bayer Garden that didn't form part of the main trial also showed very promising results. The organic product tested didn't make any difference to the roses, and black spot took hold of them in the same way as the untreated plants. These are the three recommended products: 1 Bayer Garden Systhane Fungus Fighter Concentrate £5.99 for 300ml, makes 15 litres This general-purpose systemic fungicide controls a range of fungal diseases, including black spot. It's a concentrate that needs to be diluted before spraying. While it didn't stop the disease, it slowed its advance and kept the plants looking healthier. The active ingredient is myclobutanil. 2 Scotts Fungus Clear Ultra £6.99 for 225ml, makes 15 litres This is another systemic fungicide that controls rose diseases, such as rust and powdery mildew, as well as black spot. Like Systhane, it kept the leaves on the roses for longer and stopped black spot spreading as quickly as other products. The active ingredient is triticonazole. 3 Bayer Garden Multirose 2 Ready-to-use £5.75 for 1 litre This new spray was launched too late for the main trial, but it also showed good results. It is a different formulation to the concentrate, and has the same active ingredient (myclobutanil) as Systhane. It also contains the pesticide cypermethrin. If you are worried about the possible effect on bees, use one of the other recommended products. How to spray Spray early, as the leaf buds burst if black spot was present the previous year. In new cases, spray as soon as you see black spot. Continue treatment all summer. The spray forms a barrier on the leaves that kills the fungus. To prevent infection, you have to spray regularly to keep the barrier in place. Prevention Buy a rose that has shown resistance to black spot, eg Rosa rugosa varieties are usually healthy. Strong, healthy plants are less susceptible to disease. Plant roses in a sunny spot with good soil drainage. Apply a thick mulch of well-rotted organic matter, ensure good air circulation and protect from strong winds. Prune in spring to remove infected stems. Take out shoots in the centre of the bush to encourage air circulation. Pick off and destroy infected leaves in spring to slow disease progression. Water the soil, not the leaves. Collect and destroy fallen leaves to prevent black spot overwintering. Sign up to Which? for a one-month trial for £1 and get access to all product reviews, test scores and Best Buy or Don't Buy ratings.