Prevent and Treat Blackspot
How to Prevent and Treat Blackspot
Blackspot is the most prevalent disease that affects rose bushes and is dreaded by gardeners around the world. However, it doesn’t need to take over your garden. Follow these preventive measures to keep on top of your plant health.
What does Blackspot look like?
Blackspot first appears as just that - black spots - measuring 2-12mm in diameter on leaves. These spots will enlarge over time and turn the foliage yellow. If left untreated, the plant will drop the yellow leaves and potentially defoliate the entire plant. Blackspot can also be seen on immature wood of first-year canes as raised, purple-red blotches which become black and blistered.
How is Blackspot spread?
This disease spreads by rain or overhead watering and can affect other nearby plants. Blackspot cannot survive in the soil, and cannot live longer than a month on infected tools. But it can remain active year-round on plants in mild climates, or overwinter in fallen leaves. This is why it is crucial to always clean up any debris around your roses prior to winter setting in.
How can a gardener prevent Blackspot?
There are several ways to prevent Blackspot, and all begin with good planning.
- Choose a more disease-resistant rose.
While no rose is immune to Blackspot, there are several varieties that show a strong resistance to it and other fungal abnormalities. Some of our favorite varieties that exhibit good to great disease resistance include: Apricot Abundance, Electron, By Appointment, Welsh Gold, Morning Has Broken, Carefree Beauty, Highfield, William Baffin, Amber Abundance, Lawrence of Arabia, Sharifa Asma, Velvet Abundance, Soaring Flight, Black Pearl, Belle Epoque, and Berolina.
- Find the right spot to plant.
Plant roses in an area that gets 6-8 hours of sunlight daily. Good air circulation is also critical. Space your plants out and prune out some of the inner branches to allow more air movement into the center of the plant. Also, be sure to plant your roses in a spot that has good drainage. You can add well-composted, organic matter into the soil to improve the quality of your soil and improve drainage.
- Water your roses correctly and keep the foliage dry.
Too much water and watering at the wrong time of day will encourage the outbreak of Blackspot. The best rule of thumb is to water when the soil is dry to the touch at about 2-3" below the surface. If it is during the cooler spring months, a deep thorough soaking once a week is sufficient. If it is raining, check the soil to determine if watering is necessary. During the hotter summer months, it will be necessary to water more frequently and deeply.
If using an overhead sprinkler, it is best to water mid to late morning, to allow the roses a chance to dry off during the day. The best and preferred method to keep foliage dry is to water the soil only. We recommend using a drip system or soaker hose. Also, avoid standing water around your roses and keep the area around your roses debris free. Blackspot spores will fall to the ground and stay in the leaf matter/mulch at the base of your roses. Avoid splashing water as the spores can reattach to the undersides of the leaves when they are carried by splashes of water.
- Preventative care and a watchful eye keep Blackspot at bay.
Recognizing the disease quickly allows you to “nip it in the bud” Look for circular black spots that are serrated in appearance on the surface of the leaves. Always check the lower leaves, as they will become infected first. Upper leaves will be yellow and fall off easily. Roses with Blackspot start to grow less vigorously and blooming will be reduced or stop altogether.
Check your roses often so you can treat Blackspot immediately upon first notice of infection. Using a fungicide once every 7-14 days during the growing season will help prevent an infection.
- If you do see signs of Blackspot, treat immediately.
If your rose has been affected, remove all infected leaves from the rose and the ground. Do not compost these leaves. Keep the ground surrounding your roses free of leaf debris and weeds. Then, apply the right type of chemical controls at the right frequency and duration during the most critical times. There are a variety of chemicals and organic sprays that can be used to treat Blackspot and with fairly good success rates If using chemicals, it is important to alternate chemicals throughout the growing season to avoid chemical resistance by the fungus.
- Prune in spring.
Not all varieties of roses respond the same way to spring pruning. The once blooming types of roses will need to be pruned hard just after flowering in the spring and summer to encourage flower bud set for the next spring. This is also a good time to clean them up and take out any diseased wood. Make your cuts well below the Blackspot-damaged area of the plant to ensure you are removing anything that may be on or in the canes. In the spring, be sure to cut back the canes that are infected with Blackspot. It should be fairly easy to see the black spots on the canes and easy to remove them.
It is critical to clean up the entire area of all debris when pruning is complete and all pruning tools are thoroughly sterilized. We recommend a 10% bleach solution to keep tools clean and sterile.
While Blackspot is common among roses, it is preventable and treatable. Keep a watchful eye on your garden and be on top of things at the first sign of infection to ensure a continuous and beautiful growing season.