Herb Leaves Have White Spots: What To Do?

Last updated on October 23rd, 2023 at 08:34 pm

Herbs are very popular in the kitchen. They give a spicy aroma to a wide variety of dishes. Sometimes, however, it can happen that the herb leaves get white spots, then you need to act quickly.

Different causes

When herb leaves get white spots, it is necessary to act quickly so that the herb plants do not suffer major damage and may even die in the end. Before treatment can begin, the cause of the spots must be determined. Various causes can be responsible for the white coloration of the leaves, such as

• powdery and downy mildew
• Leaf cicadas
• spider mites
• Care errors

Care errors should not be underestimated when growing herbs. They can result in a wide variety of diseases. Below are some considerations of the individual causes, their control and preventive measures.

Powdery mildew (Erysiphaceae)

Powdery mildew, is caused by various tubular fungi. Powdery mildew is also called “fair weather fungus” because it occurs in dry, warm weather. It is more prevalent after hot days and cool nights with increased dew formation. All green parts of the plant can be attacked and may even die. The powdery mildew fungus overwinters on the living plant, on the shoots or in the leaf buds. Powdery mildew can be recognized by the following symptoms

• occurring from March to August
• whitish-grey coating on the upper side of the leaf
• white spots are mealy
• later brownish discoloration
• coating can be wiped away
• infestation of the whole leaf possible
• thus deprived of water and nutrients
• leaf dries up
• stems and flowers are also infested

Note: Powdery mildew mainly attacks herbaceous leaves of peppermint, oregano, borage, lemon balm, sage and rosemary.

Downy mildew (Peronospora).

Unlike powdery mildew, downy mildew is referred to as the “bad weather fungus.” It needs moisture to grow. It occurs preferentially during prolonged wet weather, for example, increasingly in rainy summers, but also in spring and fall. Symptoms of an infestation are

• white spots or coating on the underside of the leaves
• sometimes also slightly grayish in color, later brown
• upper leaf surface shows yellow-brownish spots
• also affects stems and flowers

Fight powdery mildew

As soon as signs of powdery mildew infestation of herbs become visible, it must be combated immediately so that it does not spread to other plants and eventually even kill them. First measures are

• cut off all infested leaves
• Disposal in the residual waste
• not on the compost
• otherwise fungal spores will be spread by wind

In addition, there are also various home remedies that can be used for control. These should be preferred to chemical pesticides. Effective home remedies are

Milk

• use only fresh milk
• Prepare mixture of one part milk and eight parts water
• spray affected plants at intervals of a few days

garlic broth

• crush two garlic cloves
• boil 0,5 l of water
• pour over garlic
• infuse for 1 hour, strain
• spray plants with it

Tip: As an alternative to garlic broth, you can also use onion broth. Make just like garlic broth

Baking powder

• 3 packets of baking powder
• 50 ml oil like rapeseed oil
• 5 l water, lukewarm
• 2 to 3 drops of dishwashing liquid
• mix everything well
• spray plants at intervals of 10 to 14 days
• repeat the application five to eight times

Alternative: baking soda

1 sachet
20 ml oil
2 l water

Note: A sodium bicarbonate solution only helps against powdery mildew

Rain fern broth

• crush 5 to 6 fern fronds
• pour 2 l of boiled water over them
• infuse for 2 hours, strain
• spray herbs daily
• directly on herb leaves

alternative: field horsetail or nettle liquid manure

• 30 g dried or 300 g fresh leaves
• add 10 l of water
• let everything steep for 24 hours
• then boil for 15 minutes, strain
• let cool down
• for use dilute with water in the ratio 1: 5

Use of ladybugs

• 16-spotted ladybug (Halyzia sedecimguttata)
• 5 to 7 mm in size
• orange-brown colored
• white spots are typical
• 22-spotted ladybird (Psyllobora vigintiduopunctata)
• 3 to 4,5 mm large
• yellow with black spots

Tip: Larvae of these beetles are available in specialized shops.

In addition, the use of neem oil from the fruits of the Indian neem tree can also be used. The oil has an antifungal effect. If nothing helps, radical pruning should be done. In very stubborn cases, chemical agents must then be used after all; these usually contain copper or sulfur to combat powdery mildew. Compo Bio Mehltau-frei Thiovit is recommended. The agent should be applied at intervals of eight to twelve days.

Preventive measures

However, mildew infestation does not always have to occur if various preventive measures are observed.

• larger planting distances
• leaves can dry well after rain
• avoid waterlogging
• planting on permeable soils
• when watering, do not moisten herb leaves
• water from below
• preferably water in the morning or evening
• regular removal of weeds
• avoid nitrogen overfertilization
• plant resistant varieties
• at the end of August shorten shoot tips by 5 cm
• powdery mildew fungi like to overwinter there
• strengthen the herbs with horsetail broth
• plant garlic, chives, basil, chervil alternately between individual herbs.

Tip: As a preventive measure against powdery mildew, you can also spray the leaves of herbs with a mixture of 5 liters of water, 5.5 milliliters of soy lecithin and 20 milliliters of fennel oil

Leaf cicadas (Typhlocybinae)

These equiflora are 2.6 to 3.3 mm in size and yellow-green in color. The head is decorated with three black spots. They often appear in warm summers. The following signs indicate the presence of these pests:

• white to silvery spots on the leaves
• later brown discoloration
• leaves then usually die
• in case of heavy infestation death of the complete plants

Control cicadas

Note: Leaf cicadas mainly attack marjoram, basil, sage, lemon balm, thyme, with a preference for perennial crops.

The control of these tiny insects is not very easy. The following measures are helpful

• spraying the leaves with vinegar essence, strongly diluted
• most favorable in the early morning hours
• must be windless
• putting up yellow stickers, yellow boards
• Control of larvae by means of universal insecticides such as Neem Azal T/S
• spray mixture of 1l water, 1,5 ml rimulgan, 5 ml neem oil

Preventive measures

• Putting up yellow boards
• cover plants with insect protection net
• powder herbs with rock flour in spring and autumn
• ensure appropriate spacing when planting new plants
• Attract birds to the garden
• are natural enemies

spider mites (tetranychids)

They are not visible to the naked eye, because the arachnids only have a size up to a maximum of 0.8 mm. They mainly appear more frequently in dry weather and suck the plant sap from the underside of the leaf. A preferred target of the spider mites is oregano. Signs are

• small white spots or dots on the upper side of the leaf
• sucking spots usually lighter in color
• white webs cover the plants
• with time the leaves dry up

Combating spider mites

It is good to know that spider mites do not like moisture. When an infestation is detected, quick action is necessary:

• remove infested plant parts
• Disposal in the residual waste
• Increase the humidity
• mulch plants in the open
• Water thoroughly on warm days
• shower affected plants with a moderate stream of water
• if necessary, wrap plants with foil afterwards
• use a mixture of 0.5 l water and 15 ml liquid soft soap
• spray leaves with it
• leave to act for 15 minutes
• fertilizing with cigarette ash

Preventive measures

• use of nettle and field horsetail to strengthen the plant
• also watering with cold extract from garlic and onion peels possible
• spraying with decoction from seeds (pressed) of the neem tree
• predatory mites of the species Phytoseiulus persimilis help
• use of ladybugs, ichneumon wasps
• Creation of deadwood corners
• Installation of insect hotels

Care mistakes

In addition, there are still various care mistakes that must be observed so that there are no white spots on the leaves of herbs. Effective care measures are:

• look for a sunny location
• for permeable soils
• do not cause waterlogging
• loosen the soil regularly
• remove weeds
• cut back regularly
• fertilize and water regularly

Frequently asked questions