How to control diseases

Prevention is better than cure with diseases in the garden so keep your plants growing as strongly as possible – allowing them to fight off infections naturally. A weak plant is much more likely to fall prey than a good, sturdy one. Also be vigilant! Try to spot diseases in their early stages and act at once. Diseases are much easier to deal with before they get a hold. Caught early they can generally be dispatched with very little harm to the plant.

Grey mould/botrytis

Grey or white fuzzy mould is a very common problem because the fungus can be spread by rain, water splashes or on the air – it can also thrive on live or dead plant material. It is usually associated with high humidity and enters a plant through a wound or an area of damage. Before the mould actually appears the leaves and stems often become discoloured.

Prevention. Increase ventilation indoors and improve air circulation elsewhere by increasing plant spacing or pruning out overcrowding branches. Make sure you do not inadvertently spread the disease – always clean cutting tools thoroughly after using them on an infected plant. Keep plants growing well and avoid physical damage when handling seedlings and young plants as well as when hoeing, training and pruning established ones.

Control. This is a very difficult disease to control because it is so widespread. Cut back any infected plants and make sure all plant debris is cleared away and not left to rot in damp places. Avoid over-wet mulches in shady areas. When taking cuttings, make sure that the rooting medium is well drained.

Rust

Orange-brown pustules develop on the undersides of leaves. These can often go unnoticed and the disease has taken quite a hold by the time the pustules appear on the upper leaf surface. Rust is a nasty fungus, the spores of which need a moist environment to germinate, so most bad infections are in damper places.

Prevention. Mulch with well-rotted farmyard manure to help condition the soil and feed the plant. Reduce humidity levels in the greenhouse and avoid damping down the foliage. Grow rust-resistant varieties.

Control. Pick off and dispose of (not by composting) all infected leaves and cut off infected stems so that the spores are not spread. Make sure any fallen leaves are cleared up so that the spores cannot over-winter on the leaves, ready to re-infect the plant in the spring. In severe cases the whole plant might have to be got rid of.

Scab

This is a fungus which attacks apples and pears, covering the fruit with nasty blackish-brown patches. Similar greyish-green patches are also seen on the leaves, along with a slight blistering. The fungus tends to over-winter on fallen leaves and scabby stems.

Prevention. Grow scab-resistant varieties, such 'Discovery', 'Ellison's Orange' and 'Sunset' apples.

Control. Make sure that all dead leaves are cleared away and disposed of (not composted) so that the spores are not spread. Prune out any scabby looking shoots and also prune so that the center of the plant is open allowing lots of air movement.

Coral spot

Bright pinkish-orange fungal spots on mainly dead wood. However, it can spread on to live wood.

Prevention. Keep plants growing strongly.

Control. The only thing to do is to cut out the affected areas back into healthy wood. Make sure you clean the blades between each cut, so you do not spread the infection.

Powdery mildew

A white powdery mould appears on the leaves, stems and buds. The young growth tends to be particularly affected. The cause is a fungus that is encouraged by the plant being dry at the roots with contrastingly damp stagnant air around the top of the plant. It looks unsightly and can cause leaves to drop early, however, if the plant is well established it will recover. Care should be taken with younger plants as these may be drastically weakened.

Prevention. Mulch well in spring and autumn to help the soil retain moisture. If possible, prune plants so they have an open shape and air can move through the branches. Grow disease-resistant varieties.

Control. Remove all dead leaves in autumn to prevent the spores from over wintering.

Canker

Patches on the bark become discoloured, sinking inwards in rings – usually near a wound, leaf scar, side branch or bud. The fruits may be affected causing them to hang rotten on the tree. Canker is caused by a fungus which is spread on the wind during spring. It enters the tree through a wound. The symptoms are different through the seasons. In summer, white fungal pustules appear; in autumn/winter, red fruiting bodies appear.

Prevention. Some varieties are more prone to canker than others, so if you grow 'Cox's Orange Pippin', 'Ellison's Orange', 'James Grieve', 'Worsester Pearmain' and 'Spartan' watch out for attacks. Otherwise, grow resistant varieties, such as 'Bramley's Seedling', 'Laxton's Superb', 'Newton Wonder' and 'Lane's Prince Albert'. Avoid planting trees in poorly drained, clay soil which makes them much more susceptible.

Control. Prune out any affected areas. Very badly affected trees should be cut down.

Honey fungus

This is a pernicious soil-borne fungus which is very difficult to treat. The symptoms are wilting and die back, or leaves may fail to emerge in spring. A white fungal sheet can be seen growing immediately underneath the bark which smells of mushrooms. Brownish-black 'bootlaces' – the rhizomorphs – can often be seen in the 20cm (8in) surface layer of soil. These are often mistaken for old roots. Clusters of honey-coloured toadstools may appear at the base of infected plants in late summer/autumn.

Prevention. Try to prevent attack by enriching your soil regularly and not allowing plants to go short of water. Honey fungus appears in most gardens at some time or another; it is something we just have to live with. There are some plants which show a good resistance to this disease, including abutilon, actinidia, bamboo, beech, carpenteria, celastrus, cercis, catalpa, choiysa, cotinus, elaeagnus, mallow, oak, photinia, pieris, pittosporum, romneya, tamarix and yew. Others are very susceptible including: birch, cedar, cotoneaster, currants, forsythia, hydrangea, lilac, malus, peonies, prunus, roses, willows, wisteria and most of the common hedging conifers.

Control. Badly affected or dead plants need to be removed. Before replanting you will need to remove all the infected parts of the plant, with as much of the root system as possible. If you choose to try to eradicate it, you will then need to remove the soil to a depth of at least 1m (3ft). Even this maybe unsuccessful at completely eradicating the problem.