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Like all living things, indoor plants also get sick. These 18 diseases are common. There are many causes, ranging from errors in watering and fertilizing to infection with pathogens such as fungi and bacteria.
Diseases caused by improper care
Many indoor plant diseases are caused by simple care mistakes. Incorrect watering and fertilizing or an unsuitable location makes it difficult for both leafy and flowering plants - they get sick just like a person who eats an unbalanced diet over a longer period of time or does not go out in the sun. These diseases of houseplants are noticeable on the outside, often they can be read on the leaves. A healthy leaf (and therefore a healthy plant) has a firm structure and flawless leaf edges and tips. Changes always indicate something wrong that needs to be rectified.
Notice: Once deficiency diseases appear, further infections - for example with fungal, bacterial or viral diseases - are not far away. Houseplants weakened by improper care are more susceptible to additional infection.
leaf wilt
Do the leaves of your houseplants droop and seem to be wilting? Is their coloring unchanged? Then the plant suffers from leaf wilt due to lack of water. Fix the dry condition by watering the plants. The leaves should then straighten up again within a few hours. If this is not the case, other diseases are behind this phenomenon.
Brown leaf spots
Brown spots on the leaves have different causes. You can often tell what is causing the discoloration by looking at where it is placed. Brown leaf edges, for example, are caused by
- too little or too much water
- over-fertilization
- spent earth
- dry air
If, on the other hand, only the tips of the leaves turn brown, you are either keeping the root ball too dry or the humidity is too low. The problems mentioned can be remedied by correct care that meets the requirements of the respective plant. However, once brown leaves do not turn green again, you should cut them off.
Tip: With potted plants, make sure there is drainage in the planter so that excess water can drain off immediately. Do not leave this in the planter, but remove it within half an hour after watering.
chlorosis
Chlorosis is a typical leaf disease on undersupplied indoor plants. It is characterized by the bleached, light to yellowish leaves, with the leaf veins typically remaining bright green. The disease appears insidiously.
Causes are:
- Use of hard (calcareous) irrigation water
- resulting iron deficiency
- Lime impedes the absorption of iron by the roots
- Undersupply also possible due to waterlogging
- causes rotting roots that no longer absorb nutrients
Source: dr Avishai Teicher Pikiwiki Israel, PikiWiki Israel 32204 Saintpaulia Ionantha, Edited by Plantopedia, CC BY 2.5
In the event of waterlogging, you should immediately unpot the affected plant, remove the wet substrate and the rotting roots and repot the plant in fresh soil. Then keep them drier and, above all, do not leave excess irrigation water in the planter or on the saucer.
If the chlorosis results from iron deficiency, then also repot the affected plant in fresh substrate. Additionally fertilize it with iron chelate (from specialist shops) to compensate for the deficiency. In the future, only use soft irrigation water, either well-filtered tap water or rainwater.
Yellow leaves
If the leaves don't fade, but instead turn a bright yellow, you should act quickly. The yellowing indicates significant care mistakes that must be remedied quickly - otherwise the plant will soon shed all its leaves and threaten to die.
Possible causes for the yellowing are:
- Waterlogging / waterlogging
- nitrogen deficiency
- too dark location / lack of light
- too cool or too warm location
As a first measure, check the moisture content of the substrate: does it feel damp or even wet? Does it dry off in between or do you keep it moist all the time? Then you probably watered your plant too much. Repot them in fresh substrate and keep them drier. If there is a lack of nitrogen, moving it to nutrient-rich soil helps, but if there is a lack of light or the temperature is not suitable, a change of location takes place.
bud and flower fall
Diseases of indoor plants caused by incorrect care are not only reflected in the leaves. Many indoor plants - such as some types of cacti or azaleas - often shed buds and flowers if there is a lack of light or the wrong supply of water and nutrients. If you see these signs, find out about the specific needs and requirements of the plant in question and care for it accordingly. Particularly important are:
- location / lighting conditions
- temperature
- Humidity (many tropical plants need high humidity)
- amount of water
- fertilization
- wintering
Some indoor plants do not tolerate a warm hibernation with a lack of light and dry heating air. You have to keep these specimens cool and bright, then the buds will remain.
cork growths
Cork growths show up as brownish, corky and slightly raised leaf spots. They occur more frequently on orchids and pelargoniums when these
- getting too much water with too little light
- are exposed to severe temperature changes
- or the moisture content in the substrate often varies greatly.
The affected leaves fall off after a while. Remove the diseased leaves, make the plant lighter and ensure an even water supply, allowing the substrate to dry off in between.
Silvery leaf spots
If your indoor plants show silvery spots on the leaves, the location is too sunny - the leaves are sunburned. In some plants, the phenomenon also manifests itself in the form of red or brown leaf spots. Remove the stained leaves and place the affected plant in bright light, but not in direct sun. Shading during the midday hours is particularly useful in a location with intense midday sun. Also, avoid watering the leaves of plants that need to be in the sun. The water drops act like a magnifying glass.
root rot
Yucca palm with a soft trunk
Root rot always occurs as a result of excessive watering or even waterlogging. Fungi and bacteria settle on the permanently moist roots, causing the plant to rot from below. An infestation is first noticeable above ground in signs of wilting: leaves and shoots seem to wilt even though they are getting enough water. The opposite is the case: the plant withers because it receives too much water. Now you have to act quickly to prevent the growth from dying:
- Repot the plant.
- Remove any soil stuck to the roots.
- Cut away the black, rotting roots.
- Use a sharp, clean knife for this.
- Dust the interfaces with charcoal to disinfect.
- Place the plant in fresh soil.
Furthermore, the above-ground shoots must also be cut back because the remaining roots can no longer supply all of them.
Tip: Damaged roots are often the reason for the notorious accordion growth in orchids. Cut away the diseased root system and transplant the plant to fresh substrate.
fungal diseases
Diseases of houseplants caused by fungi often occur as a result of care or location errors, since the pathogens more easily attack weakened plants. In such a case, you not only have to treat the actual disease, but also correct supply errors and/or an unsuitable location.
Tip: Strengthen your indoor plants' defenses against fungi by spraying or watering them regularly with freshly made horsetail stock. To do this, let 500 grams of finely chopped field horsetail soak in five liters of water for 24 hours, then simmer for half an hour and finally strain it. Use cool as a spritz.
leaf spot disease
Marssonina leaf spot disease
Leaf spot disease, which is typically characterized by scattered yellow to brown leaf spots, is caused by various fungi. These are sometimes bordered by dark spore deposits and gradually merge as the infection progresses. Once the whole leaf is colonized, it falls off. Leaf spot fungi follow
- waterlogging
- too high humidity
- too cold water
- cold "feet" (e.g. planter standing directly on cold stone floor)
- or stagnant air (lack of ventilation)
on. Diseased parts of the plant must be cut away.
powdery mildew
Powdery mildew on maple leaf. Source: Jerzy Opioła, Powdery Mildew Acer DK43 (1), crop from Plantopedia, CC BY-SA 4.0
Powdery mildew shows up as a white, powdery and wipeable coating on the upper side of the leaves. If the fungal infection progresses, other parts of the plant - such as the undersides of leaves, stems, shoots and even flowers and fruits - are also affected. This fungal disease occurs mainly in dry and warm weather (which is why the gardener also speaks of the "fair weather fungus") and can in principle attack almost all indoor plants. The cause is blown fungal spores, for example in plants that have oversummered on the balcony.
A spray treatment with milk (one part fresh whole milk, nine parts soft water) or horsetail stock helps against powdery mildew. If possible, cut away diseased leaves and other parts of the plant to prevent further spread. Incidentally, roses, which are already susceptible to diseases caused by fungi, are particularly often affected.
Wrong mildew
Downy mildew ("bad weather fungus"), on the other hand, occurs in damp and cool weather / corresponding room temperature. An infestation is indicated by a white-grey to gray-blue coating on the undersides of the leaves, while whitish or red spots appear on the upper sides of the leaves. Affected parts of the plant die off over time and must therefore be cut back. Spray infested houseplants with fresh, undiluted horsetail broth.
Gray mold (Botrytis)
If your houseplants seem to be downright moldy on various parts, then gray mold rot caused by the fungus Botrytis cinerea is at work. A white to grey-brown, dusty coating on leaves, stems or flowers is typical. In addition, other parts of the plant can also be affected, because the gray mold spreads very quickly. The causes of infection are mostly
Source: John Yesberg, SemillonBotrytisCinerea, Adapted from Plantopedia, CC0 1.0
- splash water
- cool and humid room air
- too high humidity
Remove diseased parts of the plant and place the affected plant in a warm, light and airy location. Reduce humidity.
rust diseases
Rust diseases are caused by various fungi of the Pucciniales genus and are easy to recognize by their characteristic damage pattern.
Mallow rust, source: Björn S… , Puccinia malvacearum on Marsh-Mallow - Althaea cf. officinalis (30709427647), crop from Plantopedia, CC BY-SA 2.0
- orange to brownish elevations on undersides of leaves
- these are about the size of a pin
- often light spots on the upper side of the leaves
- these often only appear as the infection progresses
- Leaf spots and spore beds turn black over time
Rust fungi get to indoor plants via the air - for example through an open window. Remove affected leaves and spray the affected plants with horsetail decoction or fern tea (made from bracken or male fern, see horsetail broth for preparation).
sooty mold 
A blackish, dirty-looking and wipeable coating on the upper side of the leaves is due to sooty mold fungi. This disease only occurs in connection with an infestation of plant sap-sucking pests such as aphids, mealybugs and scale insects, since sooty mold fungi feed on the sweet residue of the insects - the honeydew. Infection is treated as follows:
- Cut away heavily blackened leaves
- wipe faintly blackened with a damp cloth
- fight pests
Bacterial and viral diseases
In addition to fungi, bacteria and viruses also cause diseases in indoor plants. These pathogens are more dangerous because they often cause the affected plant to die within a very short time. A remedy or treatment option is not always available. At the beginning of the disease you may be lucky and save the plant with a strong pruning. In many cases, however, it can only be disposed of.
bacterial wet rot
Bacterial wet rot occurs in cyclamen, Dieffenbachia and calla lilies. The disease is initially characterized by a decaying stem base, the decay inevitably spreads as it progresses.
frizz disease
This bacterial disease mainly affects fuchsias and geraniums. You can recognize them by stunted, unnaturally curled leaves. Caution: There is a fungal disease of the same name that occurs on peach trees. However, these are different pathogens.
mosaic disease
Source: DieterO, Cucumber Mosaic Virus, Edited from Plantopedia, CC0 1.0
The mosaic virus prefers to attack flamingo flowers (anthurium), orchids, hydrangeas, gloxinias and the knight star. Typical symptoms are different light and dark green leaf spots that look like a mosaic.
oil stain disease
Oil spot disease is also a leaf spot disease, but it is not caused by fungi but by bacteria. The infection appears particularly on geraniums, begonias and ivy. The glassy, light or dark oil stains on the upper side of the leaves and on the stems are characteristic. Cut back diseased plant material vigorously. If the infestation is severe, however, the plant must be disposed of.