House plant diseases tend to be fungal rather than bacterial or viral, and the most likely are listed here, in this guide to indoor plant disease identification. If the symptoms don’t match, check the further resources section for where to find more information.
A Healthy Environment
As with preventing pests, infection is much less likely on healthy plants, so look after them well. Good housekeeping is also essential to avoid cultivating an atmosphere in which disease could take hold and also prevent spreading any existing disease. Keep plants tidy, removing dead leaves and other detritus from around the base of them. Make sure tools and equipment, including the pots themselves, are clean, by washing with soap or other detergent and hot water. If the plants’ compost develops mould on its surface, repot it, washing off the roots entirely before replanting in fresh compost.
Fungicidal sprays are available to treat some diseases but are best reserved for only when they are necessary. Select a treatment which is recommended for the condition you have identified and always read the label before choosing a product. Make sure that all the manufacturer’s instructions are followed, including maximum dose, spray and harvest intervals.
Also known as grey mould and, to wine-makers, noble rot. Spores are ever-present in the air, everywhere, and readily infect dead or weak tissue, before spreading to the rest of the plant.
Signs of damage: Fluffy grey moulds developing quickly on leaves and stems, or spots on petals.
Control: Prevent infection with good air circulation and by promptly removing dead or damaged tissue with neat cuts to minimise open wounds. Remove and dispose of infected tissue, trying to contain it in the process, to avoid spreading more clouds of spores.
Both powdery and downy mildews are fungal diseases which cause white mould on leaf surfaces.
Signs of damage: Patches of white mould; powdery on the upper side, downy on the underside with corresponding yellow patches above. Leaves yellow and die; growth is inhibited.
Downy mildews invade plant cells, and only the spore-bearing structures show on the leaf surface as a white or grey fuzz. Early stages of fungal attack can be seen as yellowish blotching on the lower leaf surfaces. These blemishes are followed by a bloom of white or greyish-white spores, and if left untreated, the plants may be severely crippled.
Plants suffering from these diseases are easily recognised, as their leaves, stems, flowers or fruit may be covered with a white-grey powdery coating. The fungus causing the disease may be specific to the host plant; for example, rose mildew will not affect any other type of plant, or it may have a broad host range attacking many varieties of herbaceous plants. The disease is superficial, but if it is not combatted, it may result in a loss of leaves.
Control: powdery mildew often infects plants with dry roots and wet leaves, so ensure proper air circulation and correct watering. Downy mildew is also characteristic of damp environments and usually affects young plants. Remove affected parts as soon as they are seen and improve the atmosphere around the plant.
These tend to be brought in via the plant or occasionally through infected pests or tools.
Signs of damage: The whole plant will become stunted or distorted, with yellow spots or streaks on the leaves.
Control: It’s impossible to treat a virus, so the plant must be disposed of, and all tools and equipment thoroughly cleaned afterwards to avoid the infection spreading to other plants.