- correctly identify the disease
- Consequences of fungal infestation
- preventive measures
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Every year from early summer to autumn, roses delight us with their delicate blossoms and beguiling scents. With proper care, they can live for many years. However, their flowering splendor can sometimes be damaged or the plant can die completely due to various diseases. Black-brown spots are very common on the leaves. In this case, quick action is called for, because these are the first signs of the most dreaded disease that can affect roses, blackspot.
What is that exactly?
Many varieties of roses are susceptible to blackspot (Diplocarpon rosae), but only a few are really resistant. Mainly this disease occurs shrub roses on. But what exactly is blackspot?
- difficult-to-control fungal disease
- also known as black spot disease
- The disease is caused by a fungus (Ascomycota)
- Occurrence of the disease mainly in long-lasting cool and humid weather
- Pathogen overwinters on fallen leaves on the ground and in the soil
- Infection of the whole plant in spring, for example via irrigation or rainwater
- rapid spread of fungal spores (conidia) in wet weather, temperatures above 15 °C and high humidity
- Favoring of the disease by waterlogging, very loamy and lean soils as well as dew and rain
Fungal spores continue to form on leaves that are already infected, from spring through to summer.Black spot disease on rose petals
tip: Other plants can also be infested with blackspot, for example oleanders, laurels, houseplants such as orchids, spider plants and cucumber plants, as well as apple trees.
correctly identify the disease
The first symptoms can appear as early as April or May. Mainly the leaves close to the ground are attacked at first. Attention should be paid to changes such as:
- approx. up to 3 cm large black-brown spots or fruit bearing (Acervuli) with
Conidia (fungal spores) on the top and bottom of the leaf
- Spots are irregularly distributed in a star shape or in concentric circles, starting at the petiole and ending at the leaf margins
- initially affects older leaves near the ground
- eventually leaves turn yellow and fall off
- in midsummer, roses are sometimes completely leafless
The fungal spores are contained in these spots or fruit deposits. Splash water, unfavorable cultural measures, insects and wind can spread it further.
In the case of a severe infestation, similar symptoms sometimes appear on other green parts of the plant and flowers, for example on the immature wood of the annual shoots, purple-red irregular spots appear at the beginning, which turn black over time. They also contain fungal spores.
Consequences of fungal infestation
This disease severely affects the energy and metabolism of roses and ultimately weakens the roses because leafless plants can:
- hardly form any new flowers and
- new shoots no wood
- Shoots no longer mature until autumn and
- the plant is only slightly frost hardy, sometimes frost damage
What must be done now?
It is very important that at the first sign of the presence of blackspot, action is taken as quickly as possible to prevent further spread of this disease to other plants. For this it is necessary:
- Regularly check roses for fungal infestation from May onwards
- Cut off diseased leaves and shoots with a sharp knife or secateurs
- Always dispose of the parts of the plant in the household waste or organic waste bin to prevent further spread
- Then clean cutting tools thoroughly with hot water and disinfect with alcohol
- regularly remove and dispose of fallen leaves well into autumn to prevent the fungal spores from overwintering
Since this is a stubborn fungal disease that is difficult to combat, it is not enough just to dispose of infested parts of the plant, but in the case of a stronger infestation it also requires the use of specially approved ones fungicides, so-called fungicides. These should be used at least three times in a row at intervals of 7 to 14 days, only then can they develop their full effect. It is important that the underside of the leaf is also well wetted so that the fungal spores do not germinate again.
A wide variety of remedies are available on the market, some of which have a good effect:
- Rose-fungus free Saprol
- Mushroom-free Ectivo
- Duaxo Roses Mushroom Free
- Fungisan rose and vegetable fungus free
- Compo Duaxa Universal Mushroom Free
Some rose lovers swear by the use of various biological ones for a light infestation and for prevention home remedies, for example
- onion broth
- Spray with baking soda (dissolve 2 tablespoons soda in 1 l of water)
It should be used every 10 days.
Onion broth made quickly
- Quarter 150 g onions with skin, then separate into slices
- Pour 2 liters of boiling water over everything, cover and leave to steep for 30 minutes
- Strain the brew through a sieve, allow to cool and dilute with 5 parts water
- If the infestation is severe, spray the soil and leaves in the morning and evening for 4 days
- after that only once a day
- Treat endangered roses with it every 14 days as a preventive measure
Since an existing infestation is very difficult to combat, susceptible roses should be treated with plant strengtheners as early as May. Such a treatment is also recommended for roses that were already affected by blackspot in the previous year, since the plants will continue to suffer from the infestation the following year and a little strengthening will certainly do them good. Herb broth and liquid manure prepared by yourself achieve good results against blackspot.
tip: These herbal preparations contain high levels of nutrients and trace elements in a rapidly soluble form. They are easy to dose and due to their low concentration, over-fertilization is completely impossible.
An existing infestation with blackspot is usually difficult to combat, so in order to keep the roses healthy, attention should always be paid to prophylaxis. A few things have to be taken into account:
- When buying a new one, look out for resistant varieties with the ADR rating (“Recognized German Rose”), for example
"Angela" or "Dortmund"
- avoid damp and shady locations
- keep the appropriate distance to neighboring plants for good ventilation
- only water the roots, do not wet the leaves, so as not to offer any surface for fungi to attack
- Watering in the morning hours, exception in summer, also possible in the evening
- Water in good time during prolonged dry periods
- Nutrient supply through balanced, potassium-based fertilization to increase resistance
- make heavy pruning in spring
- Cut and thin out regularly during the growing season
- Bark mulch as ground cover to keep spores from penetrating soil
- regular inspection of the plant parts for infestation from spring onwards
- preventively treat plants with plant strengtheners from May onwards when the leaves sprout
- Spraying at intervals of 2 weeks, for example with horsetail manure
tip: A garlic manure applied to the soil around the plant helps control fungal spores in the soil. To do this, 10 large bulbs of garlic are crushed and poured over 2 liters of boiling water, everything must then cool down and finally another 5 liters of water are added and left to ferment.
Make your own horsetail manure
Due to its high silica content, manure has proved effective in the fight against blackspot. The silicate compounds form a protective coating on the leaves, the cell walls of the leaves are strengthened and further germination of the fungal spores is prevented.
You will need:
- 1 kg fresh horsetail or 100 g dried
- 10 liters of water
- 1 hand scoop of stone flour
- 1 larger container (bucket, barrel)
- coarse mesh
- a stick for stirring
- Chop the horsetail with secateurs
- put in a container with 10 liters of water
- Add a hand shovel of stone flour and stir well
- Cover the container with a coarse-meshed cloth to deter mosquitoes and prevent liquid evaporation
- Allow the starter to infuse in a warm, sunny place for two weeks
- stir well regularly until no more bubbles rise, fermentation is now complete
- Strain broth, put plant residues on compost
- Dilute the mixture with water in a ratio of 1:5
- Application by watering can
- when applying with a sprayer, the nozzle can clog, broth must not contain any plant parts
- Application from the time the plants emerge until midsummer at two-week intervals
- to prevent burns, only use in the evening or when the sky is overcast
tip: Horsetail is found in wet places with compacted soil, in damp places on hay meadows, on ditches or water bodies. Alternatively, the trade also offers dried horsetail.