How to identify, treat & prevent plant diseases
Protect your garden by spotting and managing diseases
Caring for your garden occasionally involves dealing with diseases that attack your plants. Our guide will give you all the help you need to identify the main culprits, treat the diseases and protect your garden to keep your flowers, fruit and vegetables, shrubs and trees in good health.
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Common plant diseases you may find in your garden include:
Rose black spot
A fungal disease that causes black or dark purple spots to appear on rose leaves. Some rose varieties are more resistant to it than others, but this does not always last as new strains of fungus soon arise.
Once infected, the unsightly spots will stay on the leaves until they drop off or pruned out. As the problem gets worse, the leaves gradually turn yellow and badly affected roses can shed most of their leaves.
Cut off spotted stems when pruning and be sure to burn, not compost, any pruned or fallen leaves. Unfortunately spores can be blown onto the rose from elsewhere, but repeatedly treating (once a fortnight) with an anti-fungal spray like Rose Clear Ultra or Fungus Clear will help control this unwanted disease. Spores can lay in the soil or on fallen leaves, so always add a fresh layer of mulch and feed each spring to encourage stronger, healthier growth.
Powdery mildew
Powdery mildew is a fungal disease that appears on the leaves and stems of a wide range of edible and ornamental plants including courgettes, peas, apple, gooseberry, roses, honeysuckle and azaleas to name a few.
A very common and easy to identify disease, powdery mildew forms a white or grey powdery coating of spots, which gets larger and denser as more spores spread over the surface of the plant.
How to treat and prevent it:
Mulching, balanced feeding and regular watering will help deter this unattractive disease. On ornamental plants, spray the infected plant with Fungus Clear between April and September. Prune out and dispose of infected shoots as soon as you see them, but don’t compost them. Raking up and destroying fallen infected leaves in autumn will help reduce spores spreading the following spring.
Grey mould
Grey mould is a common fungus often found on ornamental plants like primroses and cyclamens and soft fruit like strawberries, grapes, gooseberries, tomatoes and blackberries.
The mould is often seen on plants grown in a greenhouse or where conditions are humid and the plants overcrowded.
How to treat and prevent it:
Improving the ventilation in a greenhouse should help reduce the risk of mould. Additionally, hygiene is also important. Prune off any mouldy leaves, buds or fruit as soon as you see it and burn them, and don’t forget to clean your tools afterwards. Reduce overcrowding of plants and allow for better ventilation and reduce humidity to prevent mould establishing.
Clematis wilt
Clematis wilt is a fungal infection that will wilt buds, leaves and stems, turning the plant black as it dies back.
How to treat and prevent it:
The plant can recover with some hard pruning.
If you see any signs of wilt, first of all check that the plant is not dehydrated. If you suspect it is suffering from clematis wilt and the leaves are turning black (not brown) cut all the impacted stems back to healthy growth and burn anything you remove.
Any heavily affected plants that have been pruned right back, should be replanted about 10cm deeper. This will encourage better root and bud growth.
Help keep your clematis healthy and strong by mulching and feeding every spring. Mulching keeps the roots moist and cool and out of the sun.
Fire blight
Fire blight is a bacterial disease that enters trees through damaged tips and cuts (fruit trees including apples and pears are particularly prone). The disease quickly turns the leaves brown, giving the appearance that it has been scorched by fire, and can kill the tree if not dealt with quickly. Beautiful springtime blossom will die back, and fruits will turn dark brown and wrinkled. You might also notice a jelly-like fluid on the branches.
If one of your trees develops fire blight, treat it quickly to prevent the disease spreading to other trees.
If you find the disease in Northern Ireland, the Channel Islands or the Isle of Man, it is notifiable to DEFRA.
How to treat and prevent it:
Peel back the bark on the infected branches to reveal the brown staining underneath. Cut back further down into healthy wood (this can be up to 30cm in smaller branches and up to 60cm in larger ones) and burn anything that is removed. All secondary, late blossom should be removed before they open.
Don’t forget to clean your lopper or saw after every cut with an outdoor disinfectant as you will be cutting back into healthy wood and you don’t want to spread the disease further.
Brown rot
This fungal disease attacks tree fruit, turning it brown with white spots. Brown rot infects through wounds in the fruit caused by birds and insects.
How to treat and prevent it:
As this disease spreads by contact, pick off any affected fruits and burn them. And don't forget to collect any that drop onto the ground and burn them too. It’s a good idea to prune away any touching branches as they can harvest spores, which will affect the tree again the following year.
If the tree is small, try netting it to reduce bird damage and provide more fruit to harvest.
Rust
So called because it forms rust-like orange patches, called pustules, this species-specific fungal infection effects many different plants including mint, roses and hollyhocks, among others. As well as rust coloured, pustules can also be black, brown, red, yellow or white.
How to treat and prevent it:
Regularly treat by spraying with anti-fungal treatment, like Fungus Clear, throughout the summer months, but don’t use it on edible crops. Fungus Clear fights existing infections and can protect new growth for up to four months.
Small amounts of infected leaves can be removed to slow down the spread, but large amounts should be sprayed and then the leaves be removed at the end of the growing season. As with most garden diseases, don’t compost the leaves. Burn or dispose of them away from the compost bin.
Damping off
This effects seedlings and cuttings, especially in greenhouses – the base of the plant goes black, they collapse and die. Once damping off takes hold, grey mould can also occur. Early signs of damping off can also prevent seedlings from germinating.
How to treat and prevent it:
Damping off is usually caused by poor hygiene, high temperature and lack of ventilation or overwatering. To reduce the risk, sow seeds thinly and increase the ventilation in a greenhouse to reduce humidity.
Dispose of compost where damping off has been an issue and wash pots and seed trays with an outdoor disinfectant before rinsing thoroughly with clean water.
If you use a water butt to water your seedlings, fit a lid to the butt to prevent leaves and debris falling in and introducing fungi to the water. Use a water butt treatment to keep the water free from slime and algae.
Tomato and potato blight (also called late blight)
Tomato and potato blight can be a big problem for potatoes and outdoor grown tomatoes, especially during wet summers. The fungus-like disease causes leaves to shrivel and brown rotting in the tomatoes and potatoes. Mature produce will decay more rapidly if the fungus infects the plant late in the season.
How to treat and prevent it:
Avoid watering the plants from above to help keep the leaves dry. As tomato plants are a thirsty plant, setting up a drip irrigation system provides regular watering without getting the foliage wet.
Maximise airflow between the plants by giving them plenty of space. Tie vines to a trellis or cane support to help keep them off the ground.
Talk to your neighbours and local gardeners. If they find blight, it will spread quickly during wet, windy weather. If possible, bring the plants into the greenhouse to help prevent them from catching blight and ruining your produce. If you do find blight, remove the infected plants to prevent the spread to others.
Pests and diseases can build up in soil and attack specific vegetable families each year. To help control this, divide your annual vegetable plot into sections and rotate what you grow in each section every year for 3 or 4 years. This will also help the soil fertility as different crops have different nutrient requirements. Here are two examples of crop rotation you can adopt for your veg patch.
It is illegal to put any garden chemical down a main drain or waste water drainage system, even when diluted. You risk contaminating waterways and harming wildlife and could face prosecution. Ask your local Council for advice on disposal of unwanted pesticides or empty containers or visit: http://www.hse.gov.uk/pesticides/
In any garden, as well as the many things that delight, there are others that are less welcome. Disease, if left unchecked, can quickly overwhelm, disfigure or kill your much-prized plants, rapidly undoing all that hard work.
Knowing what to do and the best line of defence will help protect your garden.
If you do need to use any chemicals, there are some important things to remember when using insecticides and fungicides:
- Always read and follow the product instructions
- Only use chemicals when they are necessary, prevention is better than the cure
- To help protect pollinator insects, do not spray plants in flower
- Wear rubber gloves when spraying
- Avoid contact with exposed parts of the body, especially eyes. Avoid breathing in spray or dust. Wash with clean water any splashes immediately
- Keep children and pets away when using chemicals
- Do not spray in windy conditions
- Store chemicals in a cool, dry, safe place away from children and pets
Rid your garden of weeds
Once you’re on top of those garden plant diseases, it’s time to identify and remove problem weeds. Find out more with our step-by-step guide.
The healthier and stronger a plant is, the less likely it is to become unwell. There are very few plants that like extreme conditions, and it's usually during one of these periods of drought, or lots of rain that many plants weaken and become susceptible to problems.
Providing your plants with a happy home from the moment you plant it, is the easiest way to help them avoid infection. So keep them as healthy as possible by emulating their natural habitat.
Always read the labels of the plants or seeds you buy to ensure that you're planting them in the right place.
If you’ve previously lost a plant to disease, check that it’s suitable to replant in the same spot. We advise avoiding that spot for that species, and if not possible, change the soil.
Feed and water regularly
If it’s particularly dry, even in winter, ensure your plants have access to water. A thirsty plant will be weakened and more susceptible to infection.
All plants need nutrients to flourish and making sure your plants are fed regularly will help them stay strong.
This is especially important for plants in containers, as the nutrient value in compost will only last for around six weeks – after that they're reliant on you to provide food.
Keep things clean
Clean pruning tools every time you use them with outdoor disinfectant like Jeyes Fluid – this will stop the spread of infection.
And always clear up and burn any plants, leaves, wood or fruit that you believe have been infected – do not add these to the compost heap.