Houseplant diseases are almost always the result of the conditions the plant is kept in, not bad luck. Root rot is the result of wet soil; leaf spot is the result of wet leaves and still air; powdery mildew is the result of overcrowding and humidity. A plant kept in the right light, watered correctly, and given air will rarely get sick, while a plant kept in wet soil in a dark corner will get sick no matter what you spray on it. Prevention is therefore mostly about the basics, and it is far more effective than any treatment.
This guide explains the diseases that affect houseplants, how to recognise them, and the prevention that works. It also covers what to do when a disease appears, and when to throw a plant away rather than try to save it. If a symptom does not match anything here, the plant disease checker can help you compare, and your local extension service can confirm a diagnosis from a photo or a sample.
Root rot: the number-one houseplant killer
Root rot is the most common and most deadly houseplant disease, and it is caused by overwatering. The roots sit in wet, airless mix, the fine roots die, and a fungus finishes the job. The signs are yellow leaves, wilting despite wet soil, a musty smell from the pot, and black, mushy roots when you tip the plant out. The monstera deliciosa, the golden pothos, and the fiddle-leaf fig are all vulnerable if they are kept wet. Prevention is simple: a pot with a drainage hole, a free-draining mix, and watering only when the top few centimetres are dry — our houseplants watering guide explains the method, and the watering calculator can help you build a routine. If you catch root rot early, take the plant out, cut away the black, mushy roots with clean scissors, repot in fresh, dry mix, and do not water for a week. If the stem is black and soft at the base, the plant is usually beyond saving, but a healthy cutting from the top may root — a pothos or philodendron cutting roots readily in water.
Leaf spot and bacterial diseases
Leaf spot appears as brown or black spots on the leaves, often with a yellow halo, and it is caused by bacteria or fungi that enter through wet leaves or damaged tissue. It is common on the monstera deliciosa, the calathea medallion, and the anthurium, and it spreads fastest when the leaves stay wet and the air is still. Prevention is to water at the base rather than over the leaves, to avoid wetting the foliage when you water, and to give the plant air — do not crowd plants so that their leaves touch. Remove an affected leaf as soon as you see it, cutting it off at the base, and clean the blades afterwards so the disease does not travel to the next plant. Do not mist a plant that has leaf spot, since wet leaves spread it. If the spots are spreading fast and the plant is losing leaves, isolate it from your other plants and consider whether the conditions — too wet, too dark, too crowded — are the real cause. A plant in the right conditions will often outgrow a mild leaf spot on its own.
Powdery mildew and grey mould indoors
Powdery mildew appears as a white, powdery coating on the leaves, and it is worst on the African violet, the calathea medallion, and the begonia in warm, humid, still air. Grey mould, or botrytis, appears as a fluffy grey mould on dead leaves and flowers, and it spreads in cool, damp conditions. Both are encouraged by overcrowding, poor air movement, and leaves that stay wet. Prevention is airflow and cleanliness: space the plants so air moves between them, remove dead leaves promptly, and do not leave spent flowers to rot on the plant. Open a window on a mild day for fresh air, and do not mist the plants in the evening, when the leaves will stay wet overnight. Remove any mouldy material at once and bin it rather than composting it. A mild infection can be slowed by removing the affected leaves and improving the air, but a plant that is heavily coated will rarely recover fully, and it is better to remove it than to risk the rest of the collection.
Virus diseases: no cure, remove the plant
Virus diseases cause mottled, distorted leaves, stunted growth, and flowers that are streaked or deformed, and they have no cure. They are spread by sap-feeding insects such as aphids and thrips, and by dirty tools. If a plant shows virus symptoms — and the pattern is usually a mosaic of light and dark green on the leaves, with distortion that does not match a nutrient deficiency — remove the plant and bin it, clean your tools, and check the neighbouring plants for the insects that spread it. Do not compost a virus-infected plant, and do not take cuttings from it. Virus diseases are uncommon in houseplants, but they are the one disease where the only action is removal, and delaying it puts the rest of your collection at risk. Our houseplants pest control guide covers the insects that spread viruses.
Hygiene and the habits that prevent disease
The habits that prevent houseplant disease are the same habits that keep houseplants healthy in general. Use clean pots and fresh mix when you repot — never reuse old mix for a new plant. Clean your scissors and secateurs between plants. Remove dead leaves promptly rather than leaving them to rot on the soil. Water at the base and in the morning, so the leaves dry during the day. Give each plant room, so air moves between them. Quarantine a new plant for two weeks before mixing it with the collection. And inspect your plants weekly, looking at the undersides of the leaves and at the soil surface, so a problem is caught while it is still a single leaf rather than a whole plant. A collection that is inspected weekly and kept in the right conditions will have very few diseases, and the ones it does have will be caught early. If you are unsure about a diagnosis, a product, or a local rule, check your local extension service — its advice is tailored to your soil, climate, and the pests and diseases active in your area.
Frequently asked questions
Why are my houseplant roots black and mushy?
Root rot, caused by overwatering. The roots have drowned in wet, airless mix. Cut away the black roots, repot in fresh dry mix, and water only when the top soil is dry.
What causes brown spots on monstera leaves?
Usually leaf spot, a bacterial or fungal disease spread by wet leaves and still air. Remove affected leaves, water at the base, and give the plant air.
How do I stop powdery mildew on my African violet?
Improve airflow, space plants apart, remove dead leaves, and do not mist in the evening. Powdery mildew thrives in warm, humid, still air.
Can a houseplant recover from root rot?
If caught early, yes — cut away the black, mushy roots, repot in fresh dry mix, and do not water for a week. If the stem is black and soft at the base, it is usually beyond saving.
Should I mist my houseplants to prevent disease?
No. Misting wets the leaves and can spread leaf spot and grey mould. Water at the base and raise humidity with a pebble tray or humidifier instead.