Garden Diseases

Summary: Find out about the various diseases that can affect your garden, how you can spot them and also how you can cure them

Sometimes it seems like we are fighting a battle to keep the garden looking good and healthy – there are a huge range of diseases that can strike down a plant or ruin a crop. Our guide below covers some of the most common diseases and gives advice on prevention and treatment.

Apple Canker

Commonly known as apple canker, this disease can actually affect a wide range of trees including apple, pear, hawthorn, beech, willow and poplar.

Caused by the fungus Nectria galligena, canker shows itself as deformed and diseased areas of bark. The affected area will look withered and sunken, with the bark around it cracked and distorted. You may also notice discoloured leaves and new shoots dying off.

Spores will spread so it’s important to check your trees regularly and cut off any infected branches as soon as you find them. Burn any infected wood that you remove to kill the spores. If you find a canker on a trunk or large branch that you don’t want to lose, you may be able to cut it out.

For chemical treatment, try products containing either Myclobutanil or Mancozeb.

To help prevent canker, avoid pruning in wet weather. After pruning, apply canker paint to the cuts. Disinfect the blades of your secateurs both before and after pruning.

Bacterial Canker

This type of canker affects cherry, plum, almond, peach and nectarine trees in the UK. The canker first becomes visible in autumn, showing as shallow depressions at the base of a branch. This will grow and spread in spring, often oozing amber-like sap.

Cut out any diseased branches or patches you find, making sure you cut back to healthy wood and burn the infected material.

You can spray trees with a copper-based fungicide (such as Bordeaux mixture) in August, September and October. Avoid planting new trees in poorly-drained areas, as this makes them more susceptible.

Black Spot

Black spot is a disease common to roses, and sometimes seen on other plants too. Look out for black, brown or grey spots appearing on leaves. Remove any infected leaves and burn them. There are spray fungicides available to buy which are specially formulated to work on black spot. Prune back hard at the end of the year to reduce chances of infection the following year.


Blight is a devastating disease that can do great damage to potato and tomato crops. Look out for black or brown patches on the leaves. If you act quickly by removing infected leaves and burning them, you may be able to save the crop. If left, blight will spread quickly, soon affecting the fruit on tomato plants. The stems will get brown patches too, and if allowed to advance the disease will kill the plant off completely. Potatoes affected by blight show brown patches that will soon rot the tuber, giving off a bad smell.

Blight is more of a problem in damp, humid conditions, so wet springs and summers can be particularly problematic. Spray plants with Bordeaux mixture to help protect them, and do not allow any stray potatoes left in the ground to grow.

Blossom End Rot

Affecting tomatoes, peppers, squashes and aubergines, this problem is not actually a disease, but a disorder caused by irregular watering. Both under-watering and over-watering can bring on the problem, which is shown by leather, black or brown sunken areas on the bottom of the fruit. Acid soil can make it worse, so try to keep the pH level of your soil level and water plants regularly to avoid developing rot.


Also known as grey mould, this fungus comes in a range of species and can affect many different plants, including fruits and vegetables, dahlias and chrysanthemums. Watch out of brown patches on leaves and fruit, which if left will grow fluffy grey mould.

Botrytis is more likely to affect weak and dying plants, and will only occur when conditions are cool and humid. It can spread quickly, so make sure you completely remove and destroy any affected parts of the plant. Try to ensure plants have enough space around them to allow healthy air circulation, and remove excess foliage to help with this. Chemical treatments containing Myclobutanil or Penconazole can be effective.


This disease affects members of the brassica family – cabbages, broccoli, cauliflower, Brussels sprouts, swede and wallflowers and leads to swollen and distorted roots. It will also stunt growth which in turn will mean reduced yields. It is important to avoid getting the disease in the first place, as once it is in your soil it can survive there for 20 years! One way to help reduced Clubroot is to apply lime to the soil. A rate of 500g per sq m for the first year is advisable with a lighter dressing of 270g per sq m for following years. Also, try growing transplants in 9cm size or slightly larger pots. this will encourage a larger and healthier root system to start with.


Fireblight is a serious disease that affects pears, apples, pyracantha, hawthorn and roses. There is no chemical control available, and it can kill a plant quickly. The disease gets its name from the way infected leaves have a scorched look – they will go brown and eventually drop off. Depending on the time of year, blossoms may die back or fruit will shrivel up and brown. You may see a white, slimy goo around infected areas.

If you think you have a tree affected by fireblight, remove the affected branches, fruits and blossom as soon as possible and burn them. Make sure you disinfect any tools used to cut infected trees. The white slime contains the bacteria that causes fireblight, so if you see this it may already have spread. Avoid replanting any other trees that could be affected in the same area.

Peach Leaf Curl

This distinctive disease affects all trees in the peach family – nectarine, almond and apricots as well as peach trees. As leaves grow in spring they become distorted in shape, and covered in pink and red blisters. If left on the tree, the distortion will become worse and the leaves will eventually fall off. If only a few leaves are affected, it won’t have a great impact on the tree, but if the infection is severe it can weaken the tree greatly and reduce its fruiting potential.

Inspect trees as they come into leaf and pick and dispose of any infected leaves. As new leaves grow in their place they should be okay. If possible, set up a plastic cover for the tree from about January to April – this will protect the tree from the wind and rain which carry the spores that cause the disease.

If you aren’t able to cover the tree, spray with a covering of Bordeaux mixture as soon as leaf buds start to develop. Repeat the spraying 2 weeks later. If the tree is too large to pick off infected leaves, make sure you remove all fallen leaves from the ground and destroy them. Spray the tree again after the leaves have been dropped to stop spores from living on the branches and infecting the tree again next year.

Powdery Mildew

Caused by fungus spreading out its spores, this distinctive disease can affect many different plants in your garden. Usually the fungus causing the disease is limited to one plant type, so don’t worry that if one plant suffers your whole garden will soon be covered! Look out for a white powdery coating on the tops of leaves. Infected leaves may yellow and die off, and the spores can spread over the plant, eventually causing it to die.

Remove any infected leaves and destroy them. Powdery Mildew is much more likely to occur where the soil is dry but the air is humid – excess foliage and crowding of plants will encourage the fungus. You can use chemical treatments that contain Myclobutanil, Penconazole or Flutriafol.


Rust can affect many different plants including roses, chrysanthemums, fuchsias, pelargonium and broad beans. Like powdery mildew, the spores are usually limited to one plant type. Look out for circular rust coloured patches on leaves, accompanied by spots or pustules on the underside of the leaf.

Rust needs a damp, moist environment to survive and spread, so is more likely to be a problem during wet weather. It also needs a certain temperature, but once it starts it can spread quite quickly.

Remove any infected foliage immediately and destroy it – check the stems as well. Help to prevent rust by spacing plants well apart and watering directly onto the compost, avoiding getting leaves wet. You can use chemical treatments that contain Myclobutanil, Penconazole or Flutriafol.


Viruses can affect almost any plant – they are usually carried by aphids, greenfly and whitefly, so be careful to keep these pests under control. Viruses can present a number of different symptoms, but look out for spotted, mottled or mosaic marks on leaves and distorted leaf growth.

If you spot any plants that look like they have a virus, you should remove them before the virus has a chance to spread to other plants. The same virus can have different symptoms or effects on different plants. Regular weeding to remove possible host plants will help to keep viruses out of your garden, as will keeping the garden tidy, removing dead or dying plants quickly.

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