Most common plant diseases

Many green fingered Brits will be starting to plan their 2019 gardens, but experts have warned to be wary of these nine common plant killers.

The team behind online garden centre have revealed the most common and destructive plant diseases to try and prevent in order to keep your flowers alive and healthy this year.

From bacterial spot to black root rot, most of these diseases are relatively easy to control but for some, preventative measures are best taken.

The key is to know what symptoms to look out for and to take action at the first sign.

Chris Bonnett from said: “There’s nothing more disheartening than spending hours planning and planting a new flower bed or vegetable patch, for it then to be ravaged by aphids or grey mould.

'Liquid copper sprays can treat a range of different symptoms so is definitely one to keep to hand, but like most things, prevention is better than cure, so make sure you inspect newly purchased plant material for symptoms before planting them.'

1.  Bacterial spot

Symptoms of bacterial leaf spot may include black edged lesions, brown spots with yellow halos or just light and dark on the foliage. Symptoms of bacterial leaf spot may also appear on the edges of a leaf, where it appears brownish yellow and the tissue dries and breaks off.  

Your best bet is prevention and mechanical control at the first sign of symptoms of bacterial leaf spot. You can also use copper fungicide on crops.

2.  Black root rot

Plants affected by black root rot may show above-ground symptoms such as yellowing leaves, plant stunting, wilting or even plant death. As the name suggests, a diagnostic feature of black root rot is the presence of black lesions on the roots, which are visibly very different from healthy white roots.

Good sanitation is key to prevent this disease, so make sure you inspect newly purchased plant material for symptoms. There are many fungicides that have been shown to be effective against black root rot, but they must be applied preventatively or at first detection of the disease for maximum effect.

3.  Aphids

Aphids are attracted to warm environments and high nitrogen in early growth stages. Look out for small green or yellow bugs on leaves and stems, and wipe soapy water or rubbing alcohol over the plants to get rid of them.

4.  Grey mould

Grey mould is a common disease for many ornamentals and soft fruit such as strawberries, blackberries and raspberries. It often develops when grown under glass and in humid conditions.

If you’re growing under glass, improve ventilation and ensure plants aren’t overcrowded. Remove affected areas as soon as you detect them.

5.  Downy mildew

This occurs in cool, moist weather usually in early spring or late autumn. It affects many plants and appears as yellow to white patches on the upper surfaces of older leaves. As the disease progresses leaves may eventually turn crisp and brown and fall off even though the plant as ample water.

The best way to prevent downy mildew is to avoid the environmental conditions that favour the disease. It’s fairly easy to control on most plants when the foliage and fruit are kept protected by a liquid copper spray.

6.  Fungal leaf spot

These grey-brown spots won’t harm your plant too much, but it’s good practice to cut out affected areas nonetheless. This disease is caused by a range of fungi and can also be a sign your plant is stressed in some way. You may notice that the spots join together to create large, dead patches.

Check that the plant is happy in its position or isn’t getting too much or too little water – other factors like this could be at the root of the problem.

7.  Mealybugs

Mealybugs are common sap-feeding insects that infest a wide range of houseplants and greenhouse plants. They weaken plants and excrete a sticky substance on foliage, which allows the growth of sooty moulds.

Dead leaves and prunings should be removed from the greenhouse as these may have mealybugs or eggs on them. Ladybirds also do a good job of controlling mealybugs.

8.  Powdery mildew

Powdery mildew is a result of fungal disease caused by the plant being dry at the roots with damp air around the top of the plant. It shows as white ‘powder’ on leaves, stems and buds, but if the plant is well established it’s unlikely to cause too much harm. Use a mulch to help trap moisture and prevent roots from drying out.

9.  Rust

The signs of rust can take some time to show, so the disease can go unnoticed for some time and cause plants to grow weak. The underside of leaves will develop orange pustules, and if you’re a vegetable grower you may notice ‘leek rust’, a common fungal disease of leeks which also affects onions, garlic and chives.

Remove all affected leaves and consider a fungicide if plants are badly affected.