Diseases and Disorders

Considering the range of diseases found in the rest of the garden there are not many that cause serious problems in the greenhouse.

Powdery Mildew

Powdery Mildew is first spotted when the spores show on the surface of the plant as a ghostly white mould. The mildew itself is at work inside the plant and if not treated it can result in the plant dying. Plants affected include Begonia and grape vines. It is vital to start spraying as soon as the problem is noted.


Rust is so called because the spores are produced in pustules that look like little lumps of rust. Most common is probably Pelargonium rust. Strangely enough it is only ever really bad on zonal pelargoniums. It is important to remove and burn leaves which are infected as soon as it is noticed. Try and control the disease by spraying. Culturally it is important to know that the spores overwinter on the leaves. If rust has been a problem the best policy is to prune stock plants hard to remove all foliage at the beginning of autumn. Keep these plants frost free and very much on the dry side and with luck they should be virtually trouble free the following season, providing a good clean up has been done in the greenhouse.

Cineraria rust

Cineraria rust is disfiguring to the surface of the leaf with the pustules erupting through the surface. This is the same rust which attacks groundsel which is in the same family. Weed all groundsel plants away from the greenhouse so that spores cannot infect the Cinerarias.

Chrysanthemums are the other big rust sufferers. Brown pustules appear on the undersides of the leaves. Chrysanthemum white rust is more serious and is a notifiable disease in Britain which means that it should be reported to the Ministry of Agriculture. Infected plants must be destroyed and the source of infection tracked down. This is vital to prevent the spread of it around the country.

Botrytis or grey mould

Botrytis or grey mould is easily distinguished by its fluffy grey appearance. I have never known it to become a serious problem in the greenhouse. It usually occurs when conditions are cold, damp and ill ventilated. Botrytis usually begins by feeding on already dead matter but can spread to live plants. Having identified the problem it is hard to know what to do as sprays will surely make the situation damper and therefore worse. I would advise that any infected material is removed. Spray in the morning so that the greenhouse can be well ventilated throughout the day. If you have a large greenhouse a Turbair machine is excellent because a botryticide can be applied without making the whole atmosphere damp.

Sooty Mould

Sooty Mould is a black growth which feeds on honeydew which is secreted on to leaf surfaces by such pests as aphids, whitefly, scale and mealy bug. I have never found a cure for it other than getting rid of the pest and painstakingly wiping the leaves clean. I have known plants to be so badly disfigured by it that I have had to prune them to promote fresh new growth. Not only is it unsightly but of course the leaf is masked from sunlight which is needed for photosynthesis so that the whole functioning of the plant is affected.

Damping Off

Damping Off is a disease that strikes at the stems of seedlings at the point where they emerge from the soil. What appears to be a healthy batch of seedlings one day can look like a miniature forest of felled trees the next. It is caused by soil-borne fungi. I would say that if you begin to have problems with this disease you can safely assume that it will happen again. Get into the habit of automatically watering in seed compost before sowing with a drench of the appropriate fungicide. Having pricked out the seedlings water them in with a drench of fungicide, and this will have the effect of sterilising the environment of the seedlings and preventing the disease. Should you notice one or two seedlings in a pot becoming affected, immediately prick the others out and drench them with fungicide.

Crown Gall

Crown Gall is a bacterial gall which can affect some greenhouse plants but I have never seen it seriously damage anything but Pelargoniums. In this case the gall manifests itself as a mass of proliferated distorted growth which usually arises around the base of the plants where plant meets soil. Unfortunately, if this gets a hold on a collection of plants it is best to burn all infected plants and start again, having sterilised the growing area as much as possible. It would seem that although cuttings from infected plants seem to do well at first they too will get the disease eventually as it can move within the plant.


It is hard to know how many of the plants we grow may be infected with viruses which may well stunt their growth yet in a way which we cannot see and may never be aware of.

Indeed, many of the attractive variegations we admire are in fact caused by the presence of virus. The only time when we need to worry about viruses is when they cause disfigurement or serious damage to the health of the plant. The only group of plants that are liable to give problems to amateurs are bulbs. If they are affected they can become distorted and have reddish burnt-looking blemishes on the stems and leaves as they grow from the bulb. Red streaks are particularly common. Should you suspect that a problem might be attributable to virus then the best thing to do is check first with an expert and if they are of the same opinion burn the plant so that the problem cannot be spread to any other plants. There is no cure that amateur growers can use for controlling viruses. Keeping plants clean of pests helps as many viruses are transmitted by sap-sucking insects. It is possible for plants to be ‘cleaned up’ or cured using micro-propagation. The tiny cell at the tip of a shoot can be used which is so new that it is most unlikely that the virus has invaded it.

Spray Damage

Some plants are susceptible to damage by the chemicals we spray them with to control pests and diseases. A few days after spraying this will show itself by the leaves turning yellow and scorched or the plant starting to produce distorted shoots and flowers for a while as a result of the growing point having been damaged. This is similar to symptoms brought about by tarsonemid mite. The difference is that a plant is likely to return to normal after spray damage and get worse if being attacked by the mites. Using some chemicals at too concentrated a strength can produce damage so always take care to use the correct dilution rate. If you have a large greenhouse and use a Turbair sprayer which is battery run this can cause scorching of leaves if the battery is not charged up sufficiently or is held too close to the plants. If the machine is not working at full potential the droplet size of the chemicals will be too large and leave characteristic spotty damage.

26. June 2013 by admin
Categories: Tips and Advice | Comments Off on Diseases and Disorders


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