Herbs come in a wide variety. They are very popular as a spice, decoration of dishes and in the relief of diseases. No wonder the trend is to grow herbs in your own garden. A place is available everywhere. Sage, thyme, peppermint and Co, after all, require little space and are quite frugal in their care. They even thrive in a tub on the balcony. Sometimes, however, it can happen that white spots appear on the herbs. It is then necessary to act quickly to prevent a spread.
Causes of white spots on the leaves
If the leaflets of herbs turn white, then the origin must be investigated as soon as possible to take appropriate measures to prevent their spread. In the worst case, the plants may die. Causes can be:
- Powdery mildew
- Spider mites
- Leaf cicadas
- Care errors
Powdery mildew is caused by a fungus and is also known as the “fair weather fungus” because the fungal spores multiply and spread especially quickly in dry, warm weather and nighttime dew. The fungi love temperatures around 20 °C and a humidity of 70 percent. The powdery mildew fungus belongs to the tubular fungi. It penetrates the outer cell layer of leaves, stems and flowers. The host plant is thereby deprived of nutrients and water. Mainly peppermint, parsley, lemon balm, oregano, rosemary, borage and sage are infected with powdery mildew.
- Dry plants most frequently affected
- Occurring mainly in the months of May to August
- Velvety white-grayish coating on the upper side of the leaf
- Initially only small white, mealy spots visible on leaf
- Increasingly, the entire leaf is affected
- Coating can be wiped off with fingers
- In the advanced stage, the coating turns a dirty brown color
- Leaves eventually dry out
- In case of severe infestation, the plant dies
Powdery mildew must not be confused with downy mildew. The latter is also known as “bad weather fungus” because it loves moisture. It is recognizable
- infestation of both sides of the leaf
- white coating on the underside of the leaf
- on the upper side of the leaf lightening or yellow spots are visible
When the first signs of spot appear, action is called for. Prompt treatment of affected plants can prevent the spread of the disease. Above all it is important
Remove infested leaves
Disposal in household waste or burn, not on the compost
Before immediately resorting to the chemical club, biological measures should first be used. Here are some home remedies that are helpful in the fight against powdery mildew.
- Treatment with milk, mix fresh milk with water in a ratio of 1:8, spray the plant every few days.
- Spraying with garlic tea, to do this, pour 2 thick cloves with ½ l of boiling water and let it cool down
- Spraying with broth made of 3 packets of baking soda, 50 ml of rapeseed oil, 5 l of lukewarm water, a few drops of dishwashing liquid every 10 to 14 days, 5 to 8 applications are sufficient
- use of nettle, tansy or horsetail broth
- Use of ladybugs and leaf wasps
- In very acute cases, radical pruning is necessary.
- If nothing helps, then powdery mildew can be treated with chemical agents. Among others, Compo Bio mildew-free Thiovit Jet can be used. Treatments are carried out at intervals of 8 to 12 days.
Tip: broth of field horsetail contains silica and strengthens the surface of the leaves. To prepare, soak 300 g of fresh or 30 g of dried field horsetail in 10 l of water for 24 hours. Then heated and boiled for 15 minutes. When cooled, it is diluted with water in a ratio of 1:5.
- Choose sufficiently large planting distances, so that leaves can dry after rain
- No waterlogging, but permeable soil is necessary
- Water only when soil surface is dry
- Do not wet leaves, always water from below, morning and evening
- Remove weeds regularly, may be carriers
- Cut shoot tips in fall, provide good hiding places for powdery mildew fungus
- Do not overfertilize especially be careful with nitrogen
- Choose resistant varieties
- Plant strengthening with field horsetail broth
- Spray with 20 ml fennel oil, 7.5 ml soy lecithin and 5 l water
Tip: Always plant garlic, chives or chervil between each herb. These will help contain powdery mildew.
The small suckers are hardly visible to the naked eye. This is because they are only up to 0.8 mm in size. Spider mites are located on the underside of leaves, bite them and suck out the plant juices. They spread mainly in dry, warm weather. Oregano is very often infested. Spider mites can be recognized by the following symptoms:
Small, white speckles on the top of the leaves.
Spider threads between the plant parts
Spider webs can cover entire plants
In case of heavy infestation leaves dry up
Fighting spider mites
The small suckers are usually easy to combat if the causes of the infestation are eliminated. This is because spider mites do not like moisture. Now there are several ways to put an end to the pests:
- Increase humidity, for this purpose ventilate the greenhouse well.
- Ensure even humidity in the open by mulching.
- Water thoroughly on hot days
- Rinse infested plants with a strong jet of water, especially the undersides of the leaves.
- Spraying with a decoction of nettles, field horsetail or wormwood
- Fertilization with cigarette ash
Spray with 500 ml of water and 15 ml of dissolved potash soap, moisten infested leaves for 15 minutes.
Preventive watering with an onion decoction as a cold water extract from onion and garlic peels at intervals
Prophylactic application of a decoction from pressed seeds of the Indian neem tree
When using chemical pesticides, plants can quickly become resistant. It is then always necessary to treat several times with acaricides.
The leaf cicadas belong to the equiflorous insects. There are 45,000 species worldwide. In warm regions they make buzzing sounds, but in Europe the little animals are known as pests. The cicada reaches a size of 2.6 to 3.3 mm and is yellow-green in color. The head has three black spots and two on the top of the forehead.
Leaf cicadas mainly attack sage, thyme, basil, lemon balm, marjoram, but also other herbs, especially perennial crops.
White to silvery mottling over entire leaf, transmission of fungal spores possible
Later brown discoloration of leaves and eventual death
Cicadas visible in sunshine and warm weather
In case of heavy infestation, death of the plant
Control of leaf cicadas is quite difficult. The following measures can be helpful:
Spraying the underside of leaves with highly diluted vinegar essence in the early morning hours when there is no wind
Setting up yellow boards or yellow stickers
Spraying with a solution of 5 ml Nie oil, 1.5 ml Rimulgan and 1 l water
For treatment of larvae use of universal insecticides like Neem Azal T/S
It does not always have to come immediately to an infestation with cicadas, for this then a preventive action is necessary. Even with simple means, herbs can be protected from the like-winged insects:
- Putting up yellow boards
- Covering the plants with insect protection nets
- Powdering the herbs with rock flour in spring and late autumn
- Keeping distance to infested plants when planting new ones
Sometimes care errors in herbs can also result in white discoloration of the leaves. Therefore, it is important to give the plants proper care from the beginning. It starts with the choice of location. Further it must be considered:
- Sunny location, but no direct sunlight, otherwise leaves can get sunburn.
- Soil must be permeable, no waterlogging
- Loosen the soil between plants so that air can reach the roots and water and nutrients can be better absorbed.
- Water directly into the root area, in the morning and in the evening.
- Water only when the surface of the soil has dried out.
- Do not wet leaves
- Regularly remove weeds
- Regularly break off shoot tips and possibly flower sprouts
- For fertilization use small amounts of horn meal, liquid organic complete fertilizers or primary rock meal (minerals and trace elements)
- Regular top dressing of highly nutritious plants such as basil, lovage, chives and parsley.