Thyme is a popular Mediterranean culinary and spice herb with numerous species in different qualities. But what can be the reason when the thyme leaves get white spots and what can be done?
These cicadas are two to four millimeters small, slender, greenish-yellow animals, usually with a brown-black spotted surface. They sit mainly on the undersides of leaves, are very skittish and have exceptional jumping ability. Dry and mild weather may favor infestation. The greatest emergence of adults is observed in July. The first symptoms of an infestation can be seen from around mid-May.
- Thyme leaves show small dot-like white spots
- leaf green disappears more and more
- at some point completely disappears
- white spots become larger and run into each other
- often whitish skinning remnants on leaf undersides
- leaf edges partly show necroses
- part of the thyme leaves die off
- plants are weakened, but do not die
Note: Cicadas may transmit other pathogens that may eventually cause plant death.
- Yellow boards or yellow stickers can partially intercept pests
- against the larvae neem preparations very effective
- Beneficial insects not endangered by the oil
- refrain from treatment during flowering if possible
- radical pruning after harvest can decimate infestation
- use of potash soap and pyrethrum also helpful
Tip: Pyrethrum is extracted from the flowers of various species of chrysanthemum and is contained in numerous remedies against sucking insects
To prevent cicadas, follow crop rotation and do not grow thyme where cicada infestations have occurred previously. Infested plants should not be overwintered in the greenhouse and vigorous pruning should be done after harvest. Regular removal of weeds is also recommended.
Spider mites are extremely small, measuring less than a millimeter, and are barely visible to the naked eye. They live only a few weeks, but lay up to 100 eggs during this time. Just three days after laying the eggs, the next generation hatches. They feel particularly comfortable in warm locations with low humidity. Infestation can occur in spring and autumn.
- Thyme in the greenhouse particularly affected
- Signs of infestation, fine white webs
- and numerous small white spots
- which are caused by the biting tools of the animals
- as well as the tapping of the plant sap
- evaporation protection of affected thyme leaves is disturbed
- they wither and fall off
- usually the tips of the shoots are also affected and die off
- remove infested plant parts
- every second day with a strong water jet shower off
- or spray thyme leaves with rapeseed oil
- alternatively mix tea tree oil with water
- 15 drops of tea tree oil and 500 milliliters of water
- Shake mixture well
- spray plants several times a day
- neem oil is also effective
- do not use together with predatory mites
- use predatory mites only in the greenhouse
- Repeat spraying several times at appropriate intervals
Since spider mites prefer to attack plants that are already weakened, care must be taken to ensure optimal conditions and proper care. This includes, among other things, a balanced but sparing fertilization to strengthen the plant tissue and make the plants more resistant. Sufficient humidity should be provided in the greenhouse.
If thyme leaves show white spots, powdery or downy mildew may be the cause. Both are caused by different fungi. Differences can also be found in their appearance and on the basis of their development with respect to weather conditions.
Powdery mildew is a so-called “fair weather fungus”, because it develops in warm and dry weather. Changing humidity and dryness favor an infestation. Unlike downy mildew, powdery mildew does not penetrate into deeper tissue layers.
- Thyme leaves initially branch small white spots
- soon become larger and run into each other
- flowers can also be affected
- white spots can be wiped off
- mealy coating, later dirty-brownish
- leaves turn brown and dry up
- flowers wither and die
- severe infestation can lead to the death of entire plant parts
As a so-called “bad weather fungus”, it requires permanently damp and cool weather in order to spread. Consequently, it occurs mainly in spring and fall. Nevertheless, it is rarer than powdery mildew, but no less dangerous, because it penetrates into the plant tissue.
Downy mildew affects both sides of the leaves. However, it spreads mainly on the undersides of the leaves. White spots or white-spotted spore lawns appear there. This is partially purple in color and cannot be wiped away. Yellowish spots are visible on the upper leaf surfaces. Affected thyme leaves die, or the entire plant if not treated.
- Remove and dispose of affected plant parts
- help promises spray solution of milk and water
- mix in the ratio 1:9
- lactic acid bacteria act against the fungus
- Sodium phosphate in the milk strengthens the plants
- Spray the thyme thoroughly with it
- Do not forget the undersides of the leaves
- repeat at intervals of a few days
- natural enemies are ladybug larvae
Tip: A good alternative to milk is a spray solution made from a packet of baking soda, 20 ml of rapeseed oil and two liters of water.
- avoid too dense planting
- water as needed
- preferably in the morning and not over the thyme leaves
- avoid overfertilization with nitrogen
- strengthen thyme with field horsetail broth
- if possible prefer resistant varieties
- treat preventively with copper lime
Frequently asked questions
Can commercial crop protection netting prevent cicada infestations?
Covering with close-meshed crop protection nets can be helpful in the sense that they prevent cicada influx and thus prevent egg laying.
Is control with chemical agents recommended?
Appropriate fungicides against powdery mildew are offered in the trade. However, this is not advisable for thyme and all other herbs that are to be eaten or used in the kitchen. Otherwise, there is a risk of ingesting residues of these agents when thyme is eaten. In addition, soil organisms and, indirectly, beneficial insects may be harmed.
Can powdery mildew be transferred to neighboring plants?
Powdery mildew in particular is highly contagious and can spread to vegetables or ornamental plants. Transmission occurs mainly through the wind. The fungus overwinters on dead plant parts on the ground and can attack the plants again in spring.