Mentha belongs to the perennial herbs that should not be missing in any garden. The plants are generally very hardy and rarely get sick. In optimal conditions, they spread rapidly in the garden. If the mint suddenly gets yellow leaves, something is wrong and you need to act. Often it is heat and drought that cause trouble for the plant, but various diseases and pests are also possible causes.
There are countless animal pests on the loose in the garden, but they generally give the various Mentha species a wide berth. Their essential oils protect the plant from the insects to a certain extent. Most pests feed on the plant juices, and some also eat the leaves. If left untreated, severe infestations can kill the mint.
Aphids prefer to stay on fresh shoot tips and the undersides of leaves. They are very small and have a spherical, usually greenish body. Aphids excrete a sugary, sticky substance called honeydew. This promotes the growth of sooty mold on the plants, which can be identified by its black color.
- yellow leaves
- stunted shoots
- necrotic spots
- black shoot tips
If the aphid population is limited to only a few leaves or shoots, cut them out. You can spray off robust plants with a sharp stream of water. A soap or neem oil solution has proven effective for control. If the infestation is severe, it is easier to cut off the entire plant near the ground.
Ground flea beetle (Psylliodes)
Ground flea beetles, also called earth fleas, are only around two millimeters long. Their body is yellow-brown to reddish, the elytra usually yellow in color. The adult beetles overwinter in the soil and lay their eggs starting in the spring after feeding on the leaves of the mint. However, the cause of peppermint dieback is usually hidden in the soil, as the tiny larvae feed on the roots beginning in April.
- tiny, round holes in the leaves
- yellow leaves and dieback of the mint when root feeding occurs
Earth flea beetles can be controlled naturally with plant dips made from wormwood or tansy. A solution of cooked onions or garlic also repels the little animals. In addition, it is useful to place a board coated with caterpillar glue next to the mint. Jumping ground flea beetles will stick to the glue.
Tip: Frequent hoeing and keeping the soil moist minimizes the risk of infestation.
Mint leaf beetle (Chrysolina species)
Occasionally, the mint leaf beetle appears on mint starting in April or May. The beetles are easily recognizable by their shiny metallic exterior. Depending on the species, the beetles can be green, blue, or even copper to black in color.
leaf edge feeding (mainly on the shoot tips)
brown, shiny larvae on the underside of leaves from June onwards
Cut back infested shoots early to reduce egg laying. In addition, spray a solution of soap with a little canola oil to contain the population. If in doubt, cut the plant back to the first leaf emergence.
Dwarf cicadas (Eupteryx spec.)
The flying insects mainly attack labiates such as basil, marjoram, oregano, sage, thyme and mint. The cicadas are only two to four millimeters in size and mottled yellowish to gray-green. If you shake the plant, the insects jump up and fly away.
- punctiform lightening on the leaves
- yellowing of the leaves
Control of the cicadas is not necessary. As a rule, the damage is not particularly great and the mentha is still suitable for consumption. Whoever is bothered by the little animals can attract and catch them with yellow boards.
Pathogens such as bacteria, viruses or fungi can enter the plant through the roots or wounds and cause great damage. The most common diseases of mint include:
Among the many species of rust fungi that attack plants in our gardens, there is one that specializes in labiates. The cause of the disease is a fungus called Puccinia menthae, which thrives in humid weather. It spreads very quickly and in a short time also attacks plants in neighboring beds and gardens. This includes not only mentha species, but many other herbs such as marjoram, savory, and oregano.
- light yellow or rusty brown spots on stems and leaves
- pale and bent young shoots
- withering of the entire plant
Early pruning at the first symptoms often still provides relief. Heavily infected mints can usually no longer be saved. To prevent the fungus from spreading further, remove all diseased plants and their roots from the bed if possible.
The risk of disease can be minimized by removing all dead shoots and foliage in the fall and disposing of them in household trash. Adequate spacing between individual plants allows air to circulate better and moisture to dry off.
Tip: Preferably plant resistant varieties such as ‘Multimentha’.
The spores of powdery mildew can be found almost everywhere. While powdery mildew occurs preferably in warm and dry conditions, downy mildew is an absolute bad-weather fungus. It loves humidity. Although they are different types of fungi, the symptoms and their treatment are very similar.
- white, powdery spots on leaves and stems
- later: yellow and brown coloration of leaves
- death of the plant
Remove all infested leaves and shoots as soon as the first symptoms are visible. The fungus can be well controlled with plant broths. These include field horsetail, tansy or even garlic broth. In case of doubt, it is advisable to cut off the entire shoots close to the ground.
Note: The cuttings of diseased plants must be disposed of in household waste.
If mint is exposed to dry winds, frost or even extreme heat, it will develop yellow leaves. Potted plants are particularly sensitive to these factors. If these are possibly in the blazing sun, not only the foliage but also the roots can be damaged by the heat and lack of water. The most common causes of yellow leaves include:
- Waterlogging (root rot)
- Temperature shock due to watering water that is too cold
- Potted plants: too sunny location (especially in the root area).