Plant diseases

Grey mould (Botrytis): This typical fungal disease causes brown, rotting blotches on the leaves and flowers, sometimes even affecting the stems. In fruits grey mould causes rotting of the fruit with grey areas that taste very mouldy. The disease becomes much worse in warm damp weather. Put the plants in an airy, well-ventilated place where they are not crowded by other plants so that they can dry out quickly after the rain. Trees and shrubs should be thinned, while for plants where the fruit is near the ground such as strawberries, a layer of mulch around the base of the plant is recommended. Avoid fertilisers with a high nitrogen content because this makes the plant tissue soft and as a result more vulnerable to grey mould. Dead parts of plants affected by botrytis on which the fungus can over-winter are sources of infection and should be removed immediately . If you want to use a chemical product during the flowering period, make sure that is not harmful to bees.

Rust: In spring spot-like fungal spores, yellow to rust-brown depending on the pathogen, develop on the underside of the leaves. These spores turn a darkish colour in autumn. Meanwhile, yellow spots develop on the upper side of the leaf. The leaves dry out and become stunted. Make sure that the air is not too humid and the leaves are not constantly wet. The use of a fertiliser with a high nitrogen content also encourages the development of rust. The parts of the plant that are covered with spores must be removed and destroyed immediately because they harbour the rust fungus. The latter survive by passing from one plant to another. All infected plants should be removed as they are sources of infection. If your plants are repeatedly infected, find out from your garden centre which plants could be the culprits. Horsetail and parsley fern extracts will have a toning effect on the plant and will also help prevent mildew attacks.

Seedling blight, root rot and stem rot: These can be caused by several different kinds of fungus, but the symptoms are always the same. The fungus easily penetrates unprotected young tissue or it enters the plant through wounds, so both seedlings and established plants are vulnerable. Roots and stems turn to a brown colour, the roots rot, the plant collapses and finally withers. The appearance of Pythium root rot and Phytophthora blight is the result of a location that is too wet , resulting for example from compacted earth on which water frequently stands. It is therefore worth making sure that all soil is water-permeable, and in the greenhouse the humidity should be not too high. Affected plants must be removed and destroyed. Sensitive seedlings can be treated by watering with a standard captan or other similar preparation.

Wilt: This infection invades in the usual way through wounds and it first withers the leaves on the shoots, which become dried up and remain hanging. In some cases whole shoots perish. The plants grow only weakly and eventually collapse. If an affected shoot is cut open, the tissues are discoloured brown. The roots are not affected. The only option is to remove the attacked plants and destroy them. Asters in particular are prone to Verticillium wilt but there are many varieties that are resistant or immune.

Particular attention should be paid to hygiene in all operations with plants, especially clean vessels and sterile compost. Sclerotinia disease affects many varieties of plants; the fungus has black resting bodies that can over-winter in the soil for many years, and then infect diseased tissue, appearing in stems or roots as a white fluffy mass. It is therefore important to replace the soil completely after an attack. Wilt also frequently affects beans, cucumbers and tomatoes, but there are varieties that are resistant to it.

Powdery mildew: The typical characteristic of this widely dispersed fungus is the appearance of a white powdery coating that mainly covers the leaves, calyxes and young, soft shoots, and in the case of fruit trees, the flowers and fruit. The leaves go brown, curl into a rolls and become dried up.

The fungus multiplies especially quickly in muggy weather, particularly on warm sunny days with nightly dew formation. Therefore an airy location is recommended, with plants not too close together. Feeding with a balanced fertiliser, not too rich in nitrogen, is also helpful. Treating with a decoction of stinging nettles is also effective in preventing the onset of the mildew, as is horsetail tea. Choose the best resistant species that are widely available. With plants that are several years old, all affected parts of the plant must be cut back carefully as far as the healthy wood. In the case of a strong attack, a sulphur-based preparation can provide a remedy.

Downy mildew: The symptoms are similar to powdery mildew although it is caused by different viruses. It is characteristically observed in the leaf veins, which depending on the virus may be turned a bright red-violet colour (in the case of roses). The undersides of the leaves show patches of white to brownish spores.

The fungus also attacks fruits and tubers; grapes change colour to a bluish-brown hue and dry out. The spread of the fungus is promoted by cool, moist weather. Therefore plants should be planted far enough apart for them to dry off well. Since the fungus often spends the winter on diseased leaves and other affected tissue, it is best to remove and destroy any plants that have suffered. As a preventative measure, it is important to provide the greenhouse with good ventilation. Ask in a specialist nursery or garden centre about resistant species, of brassicas and lettuces for example.

Brown rot and leaf rot: This illness is caused by the fungus Phytophthora infestans and attacks potato and tomato plants and fruit trees. It appears predominantly with wet weather, becoming more threatening from July onwards, and its symptoms are brownish-black stains on leaves and fruits. The fruit pulp becomes hard and the fruits are inedible. It can be prevented by selecting resistant varieties as well as by applying a decoction of stinging nettle that increases the plant’s resistance. Covering the fruit with aluminium foil not only raises the temperature, encouraging the ripening of the fruits, but it also protect them from infection by free-flying fungus spores. Horsetail tea is helpful in treating plants that have already been attacked. Use fresh compost each season.

Virus diseases: Viruses cause very varied symptoms of damage. These range from interference with growth to mosaic mottling of leaves and speckling of flowers, and to deformation of leaves, flowers, fruits, shoots and roots until to the whole of the plant tissue is attacked. Viruses are frequently linked to a particular plant species. Important countermeasures include controlling carriers of viruses such as aphids, mites and leafhoppers; destroying affected plants and weeds; choosing healthy plants that are certified ad virus free, or even virus-resistant strains; and being meticulous about hygiene when propagating from cuttings, by disinfecting tools and containers.

Bacterial diseases: Like viruses, bacterial diseases can cause a number of different symptoms of damage. These include leaf staining, canker, bacterial fire, slime formation, over stem rot and root rot and wilt.

Notifiable Diseases

The bacterial disease Fireblight is a Notifiable Disease and must be reported to the authorities if encountered. First the flowers, and then the leaves of infected shoots become brownish-black. They look as if they are have been burned as they hang from the branches. With high humidity, slime develops and drips from the affected branches, carrying the bacteria with it, so they may infect other plants. The plants most frequently affected are from the Rosaceae family, such as apple, pear and pyracantha. There are no chemical means of fighting bacterial diseases, especially fireblight. Plants that are suffering from any of them and any suspicious plants nearby should be removed immediately and destroyed.

03. April 2014 by Dave Pinkney
Categories: Garden Management, Pests and Diseases | Tags: , , , , | Comments Off on Plant diseases

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