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If rose petals get black spots with a yellow halo, it is usually an infestation with blackspot. The fungus is extremely persistent and affects almost all types of roses. Therefore, you should protect the plants from this rose disease with preventive measures. If the fungus has already infected the rose, you must start combating it immediately. Preferably before the rose petals have turned completely yellow.
Rose disease: blackspot
Yellow leaves with black spots
If the leaves of your roses turn yellow and get black spots, then it is usually the rose disease blackspot. Blackspot (Diplocarpon rosae) is a fungus of the genus Diplocarpon that survives mild winters well. He prefers to overwinter
- in the ground
- on old, affected leaves
- on old, affected shoots
While black spot disease, as the fungal infection is also known, is difficult to control, recognizing it is fairly easy:
- round, black spots on the upper side of the leaf
- often spread out in a star shape
- are unevenly distributed
- form concentric circles
- have a blurred edge
- yellow discoloration around the black spots
- Leaves turn yellow
- fall off early
The fungus, more precisely the fungal spores, are located in the black spots and are responsible for the spread of the fungus on the plant. Its spread is favored by spray water, insects, wind or cultural measures that weaken the plant (e.g. compacted soil).
course of the disease
Black spot disease usually begins on the lower leaves of the rose. So they turn yellow first. This is because the fungal spores are “splashed” onto the leaves by pouring or rainwater hitting the ground. From there they spread over the whole plant, including neighboring rose bushes. Since affected or lost leaves of the rose are missing during metabolism and energy change (assimilation), they become weaker and weaker as the disease progresses. Sick, she can no longer develop new flowers, and new shoots no longer mature. Since only an incomplete wood ripening takes place, the resistance of the rose bushes to frost is also reduced.
tip: In rare cases, immature, annual shoots are also attacked by the fungus. Initially, purple-red spots form, which then turn black as the disease progresses.
Temporal course of the disease
The first symptoms of this rose disease appear as early as April or May. How quickly the infection spreads, i.e. the spots appear and the leaves turn yellow, depends, among other things, on the variety. It is quite possible that some types of roses have already lost all their leaves in midsummer. Temperatures above 15 °C, wet weather and high humidity promote the spread of the infection.
Once the fungus has started the infection, the first symptoms appear after 3 to 16 days, depending on the temperature and humidity. Under optimal conditions for the fungus, propagation occurs after 10 to 18 days.
Affect rose varieties
Blackspot basically attacks all roses. However, they are particularly at risk shrub roses. The fungus also does not stop at roses that are cultivated as houseplants. Since the fungus also affects other indoor and garden plants, such as laurel or oleander, the roses can also be infected via these neighboring plants.
tip: Rose varieties that are resistant to blackspot are, for example, "Angela", "Dortmund" or "Gelbe Dagmar".
Since blackspot is very difficult to control, you should prevent infection so that the plants do not get sick in the first place. Preventive measures include:
- site selection
- pouring behaviour
- nutrient supply
Mushrooms like it warm and humid. Therefore, a sunny and airy location helps the plants. Choose the location so that the roses can dry well overnight, even if it rains.
notice: Humid and shady locations encourage rose disease.
When watering the roses, be careful not to get the rose petals wet. It is best to water roses in such a way that the water does not splash onto the lower leaves. Avoid watering the roses in the late evening hours as the cooler temperatures prevent the plants from drying out overnight.
Avoid nitrogen-rich fertilization. A balanced potash fertilization, on the other hand, increases the resistance of the roses to blackspot. Silicic acid, which can be administered through horsetail tea, strengthens the cell walls of the leaves, greatly reducing infection triggering.
tip: Use these plant strengtheners from the beginning of May. Plants that were not infected with the fungus in the previous year should also receive this boost to increase their resistance.
When caring for it, it is important to remove all leaves that have not fallen off naturally in the fall. You should also carefully remove all leaves lying on the ground. This is how you deprive the fungus of overwintering opportunities. In the spring, you should remove any leaves, whether green or yellow, that have remained on the plant through the winter.
tip: If the rose was already infected, the leaves must not be disposed of on the compost as fungal spores survive there and the roses will be infected with rose disease when the compost is spent.
You should trim and thin out the rose regularly so that the rose can dry well and light can penetrate into the interior of the stem.
If the black spot disease shows up, then the affected rose petals must be removed and disposed of with household waste before they have turned completely yellow. You should disinfect the tool used after each work step so that the fungus does not spread over it. To disinfect, immerse the cutting tool in spirit for about five minutes and then flame it with a lighter. To further combat the fungus, you can use special fungicides from specialist shops or home remedies.
The home remedies described below can also be used as preventive measures against blackspot. It is important that you spray the plants when the sky is overcast and usually repeat the process several times at intervals of one to two weeks.
Horsetails (field horsetails)
Horsetails are administered as a tea, broth or slurry, which provide the leaves with valuable silica to strengthen the cell walls. For broth or tea you need a kilogram of fresh horsetails for ten liters of water. Proceed as follows:field horsetail
Pour boiling water over horsetails and let steep for ten minutes. Then strain.
Soak horsetails in cold water for 24 hours. Then cook for 30 -60 minutes. Then let it stand for 24 hours and then strain it.
For spraying, dilute the tea and broth in a ratio of 1:5 with water.
Comfrey contains a lot of potassium. You can mulch with comfrey leaves or spritz the roses with diluted comfrey broth.
tip: Do not mulch directly under the roses, as this is where the fungus can thrive.
For a garlic broth or a garlic stock you need 75 grams of coarsely chopped garlic and a liter of water. Proceed as follows for the production:
- Boil the garlic in the water
- Leave for 10 to 15 minutes
- Dilute the mixture in a ratio of 1:10 with water
Application: Spray the roses three times in a row at intervals of three days with the garlic broth.
To make nettle manure you will need:
- 1 kg fresh nettle, finely chopped (or 200 g dried)
- Barrel or container made of wood, earthenware, plastic
- 10 liters of water
- lid for the container
- Stick for stirring
Place the nettles in the container with the water and place in a sunny spot. Cover the barrel with the lid. Stir the mixture with the stick every day. The fermentation process is complete after one to two weeks. The liquid manure is ready when no more foam appears and the liquid has turned dark. Apply the manure in a ratio of 1:10 at intervals of one week.nettle manure