The mountain knapweed can't really harm anything that quickly. But two diseases or pests damage the plant and must be combated.
Mountain knapweeds (Centaurea montana) are quite robust plants that can also bring a lot of joy to inexperienced gardeners. Diseases and pests are rare, but unfortunately cannot be ruled out.
A common cause of damage to the knapweed is care mistakes, in the form of choosing the wrong location, incorrect watering behavior or an overabundant supply of nutrients. The following diseases and pests can occur on the mountain knapweed: downy mildew and spider mites.
❶ Downy mildew - loves moisture
In cold, wet weather, the risk of downy mildew infestation is particularly high. The fungus prefers a moist environment. First of all, a greyish coating appears on the underside of the affected leaves. Light spots soon appear on the upper side of the leaf.
What should be done?
All affected leaves and shoots must be removed immediately. If the infestation is detected in time, the plant can be saved. However, if more than half of the mountain knapweed is infected with downy mildew, it is advisable to dispose of the entire plant. It is once again important to check the plants regularly for possible signs of damage and not only to proceed superficially, but also to keep an eye on the underside of the leaves. Pests prefer to settle there and the first signs of damage can also be seen there.
➔ Tip: Plant parts affected by downy mildew can be disposed of in the compost. The fungus cannot spread via dead plant remains and thus cannot survive on the compost heap.
Downy mildew: How can it be prevented?
Long periods of rain encourage downy mildew infestation. In order not to provide additional lasting moisture, the plants should not be watered directly over the leaves. To allow moisture to drain away, don't set the plants too close together. A distance of 30 to 40 centimeters is thegroup planting.
If you want to give your mountain knapweed a general boost, you can brew it a tea from field horsetail leaves. The silicic acid it contains makes the plants more resistant to diseases and pests.
❷ Spider mites - like the cell sap
Spider mites are only a few millimeters in size and can hardly be seen with the naked eye, but they are all the more voracious against the mountain knapweed. The arachnids feed on cell sap and can cause significant damage to plants if not spotted in time.
How can a spider mite infestation be recognized?
If the leaves lose their chlorophyll, they initially get a silvery sheen. In the further course, the leaves turn yellow and finally the leaves dry up and fall off. However, this is not yet a clear indication of an infestation with spider mites, as other pests also cause a similar pattern of damage. If webs can be seen on the plant, spider mites can be assumed with certainty.
➔ Tip: The webs become clearly visible by spraying with a plant sprayer.
What can be done to combat it?
Spider mites can be eliminated quite well without the use of chemicals. The pest infestation usually occurs during prolonged drought. Arachnids, on the other hand, do not like high humidity. It can already have an effect when the plant is sprayed with water. Although this will probably not completely combat the infestation, it can still be decimated in the long term. Spraying should be carried out for several days in a row. In the case of a stronger infestation, a decoction of soapy water or nettle leaves has also proven useful.
➔ Tip: In the case of outdoor plants, natural predators are also an effective remedy against spider mites. For this purpose, spiders or ladybugs can be positioned near the plant.
Pests and diseases at a glance
| Disease or Pest | Damage | Combat |
| Downy mildew | ■ There is a grey-violet coating on the underside of the leaves. ■ The upper sides of the leaves appear lightly spotted. | ■ All affected parts of the plant must be removed. ■ Field horsetail broth can be used as a preventative tonic. |
| spider mites | ■ The leaves get silvery spots at first,later turn yellow and dry up. ■ On closer inspection, fine webs can be seen on the plant. | ■ Spider mites can be minimized by frequently showering the plant. ■ A decoction of nettle leaves or soapy water is also successful. ■ Spider mites have natural predators in spiders and ladybirds. |
Avoid nursing errors as a cause of illness
Discoloration of the leaves, reduced growth or sparse flowering do not have to be attributed to pests or diseases. In most cases, improper care is the reason that the mountain knapweed is generally weakened and ultimately susceptible to pests and diseases.
The plants must therefore be checked regularly so that care errors can be identified and corrected more quickly.
The most common care mistakes:
- too dark stand
- too much water
- too many nutrients
- too little water
You can often see from the leaves if the plant is suffering from an undersupply or oversupply. Then check the soil. If the plant is too moist, transplanting can help. You have to react quickly. If there has been waterlogging for a long time, the roots are likely to have been affected. A slightly musty odor coming from the plant indicates that root rot has probably already started. Then any help will come too late for the plant.
This is how the mountain knapweed stays he althy:
✔ sunny location
✔ loose soil
✔ no waterlogging✔ water regularly
✔ fertilize occasionally