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you are here: Disease Guidance
Disease Guidance There are many diseases that can affect the health of our plants. Some are bacteria based, some fungal and others viral. Each type of disease has its own symptoms. Diseases can be superficial or they can be deadly and it can be difficult to know what you are dealing with. The following is a brief guide showing you the type of disease you are likely to come across in your garden, the type of damage it may cause and the action you may take. 
BLACKSPOT Blackspot is caused by a fungal infection (Diplocarpon rosae) that attacks roses. The first sign of the fungus occurs from mid-summer onwards as a dark, rounded or diffuse spotting of the foliage. The leaf turns yellow shortly after the attack and premature defoliation occurs. Bushes can be severely weakened by the fungus. The disease carries over from year to year on infected leaves and on fungal spores on stems. Spores can also remain in the leaf litter on the ground and in the soil, waiting to reinfect the plant. Chemicals need to be applied following a strict spray programme to be effective. Dig up and burn badly affected varieties and grow naturally disease resistant types. It is vital that any fallen leaves are removed and destroyed. Removing infected leaves from the rose is also wise - these should also be destroyed. In winter, prune out any stems that show signs of the disease.  BOTRYTIS (GREY MOULD) Botrytis is probably the most common fungus to be found in gardens and feeds on all manner of dead and dying material. It also spreads into healthy tissue, making the disease a major problem, especially in warm and humid conditions. Fluffy grey patches can be seen on leaves, flowers and fruits, such as strawberries. The patches of mould contain the spores that allow the fungus to move from plant to plant. In dull, damp weather, small, round translucent spots appear on tomatoes. It also attacks perennials, bulbs and bedding plants. A suitable fungicide can be used to treat the condition. All infected parts of the plant should be removed and destroyed. It is also wise to ensure good ventilation in greenhouses to prevent an outbreak as Botrytis loves humidity. If the disease occurs on shrubs and climbers, the diseased stems should be pruned back to 5cm or so below the visible signs of dieback. 
BROWN ROT Brown rot is a fungal disease affecting fruit trees. The whole fruit rapidly turns brown with concentric rings of raised white spots. Affected fruit may remain hanging on the tree in a mummified state. At flowering time, the same fungi can cause blossom wilt. All infected fruit must be removed and destroyed. Diseased fruit must not be placed in the compost heap as this will spread the disease. Prune out and burn infected spurs and blossoms to reduce the amount of fungus available to infect fruit. The disease can enter the tree through wounds therefore it is important to minimise wounding and to treat any that do arise. 
CLUB ROOT Club Root is caused by a fungus and is the most serious disease that affects the brassica family. The disease attacks through the soil, causing the root tissue to swell into large galls in which the fungus multiplies. In severe attacks, the plant dies and the fungal spores remain in the soil in diseased roots. It can be hard to spot the presence of the disease because the attack is going on underground, but stunted growth and purplish foliage can be a clue that something is wrong. Spores can remain in the soil for 20 years, waiting to infect new plantings of brassica crops. Club root is encouraged by acid and wet soil conditions. A crop rotation plan can help to prevent the disease as can liming the soil where brassicas are to be grown. 
CORAL SPOT Coral spot is a fungal disease that affects woody plants. The disease causes branches to die back. Coral spot often indicates that the plant has been weakened by other factors. Among broadleaf trees and shrubs, Acer, Aesculus (horse chestnut), Carpinus (hornbeam), Fagus (beech), Juglans (walnut) and Tilia (plane) are among the more susceptible to attack. It is rarely found on conifers. After death, pinhead-sized coral-red pustules appear on the bark. Pruning in dry weather can limit the occurrence of the disease. If it does occur, the infected wood should be removed and destroyed. There are no specific fungicidal controls for this disease. 
DAMPING OFF This can be as a result of a fungal infection or simply due to wet compost and over humid conditions. It results in young seedlings wilting and toppling over. New compost should always be used when growing seedlings and all containers should be thoroughly cleaned. Seeds should be sown thinly to avoid overcrowding which allows the damping off in. Only water when necessary to prevent the wet conditions that damping off enjoys. 
HONEY FUNGUS Honey Fungus (Armillaria mellea) can attack a wide range of trees and shrubs. Affected plants die back - shoots wilt and turn brown. Under the bark at the base of the affected plant will be a web of white fungal threads. In the soil, black bootlace fungi can be found and this spreads through the soil to infect other plants. Clusters of honey-coloured toadstools appear close to dying trees or shrubs in late summer or autumn. There is no known cure, but honey fungus tends to attack stressed plants, especially woody plants, so keep plants healthy by caring for them. Dig up and burn infected plants, removing as much of the roots as possible. If you replant, choose less susceptible plants and change the soil before planting. 
POTATO AND TOMATO BLIGHT Potato blight and tomato blight are caused by the same fungus, Phytophthora infestans. It attacks mainly during warm, wet weather in late summer and early autumn. In tomatoes, it causes the leaves to turn black and die and the fruit to develop dark brown areas on the skins. A whitish fungal growth develops on the leaves to spread the infection and the fruit then rots. The stems then turn black and the whole plant collapses. However, this disease is not as common on greenhouse tomatoes. In potatoes, a rapidly spreading watery rotting of the leaves occurs, which leads to their collapse. During humid conditions, a fine white fungal growth may be seen on the underside of the leaves. If left unchecked, the disease will spread to the potato tubers, causing them to rot. Planting resistant cultivars can help. Infected material should be removed and destroyed. 
POWDERY MILDEW There are several strains of powdery mildew that each have a range of different host plants. Affected plants become covered with a white, powdery coating of disease spores. In severe cases, the leaves can turn brown and die. General growth and flowering can be reduced. The disease tends to be more serious during spells of dry weather. Plants need to be well fed and watered to limit attacks. Any infected leaves should be removed and destroyed - place them in a plastic bag after removal from the plant to prevent the movement and spread of spores. Systemic fungicides containing carbendazm or sulphur can also be sprayed to eradicate an existing infection and to prevent new ones. 
RUST Rust is a fungal disease and most rusts are specific to a particular species of plant. Hollyhocks, roses, pelargonium and leeks are popular targets. Reddish brown raised spots and powdery pustules appear on the leaves. Good hygiene is the best way to prevent rust - ensure all fallen leaves are removed from the top of the soil before they begin to decompose. Prune the plants to ensure a free circulation of air, especially at the centre. Try to grow resistant varieties. The disease spreads via spores so it is vital that infected leaves are removed and destroyed as soon as signs of rust appear. Spraying with a suitable fungicide, such as one containing mancozeb or penconazale, is also advisable. General Advice Keep plants healthy as healthy plants are less susceptible to attack Monitor plants weekly for signs of infection If plants show signs of distress, eg, wilting, curling etc, take action Clean up any debris that may be harbouring diseases Check the soil or mulch around the plant for clues Buy resistant cultivars of plants where possible Control weeds as these can harbour diseases Rotate crops to reduce the build up of soil-borne diseases
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