Dealing with common plant diseases

There are endless plant diseases and many are specific to certain plants only. If the symptoms you have noticed aren’t listed here, you may wish to search a more comprehensive database of plant diseases – such as that on the Royal Horticultural Society website.

Good gardening practice

Disease has a clever way of winding its way into gardens, despite all your hard work and good intentions. However, it is a good idea to follow some basic rules to make it difficult for disease to strike:

• Do: Maintain good garden hygiene - keep your greenhouse and tools clean and well looked after. If you’re reusing pots from a previous season, ensure they are well cleaned first.
• Do: Be alert - take action at the earliest signs of attack, rather than allowing the problem to spiral.
• Don’t: Let your plants become stressed - take good care of their position, watering, feed and nutrients and ventilation.

Common plant diseases

Powdery mildew (pictured above)

Problem: White powder on leaves, stems and buds

Diagnosis: ‘Powdery mildew’ is a common problem on many plants; a result of fungal disease caused by the plant being dry at the roots with damp air around the top of the plant.

Control: If the plant is well established it’s unlikely to cause too much harm, but remove dead leaves in autumn to prevent the spores from over wintering. Use a mulch in spring and autumn to help trap moisture and prevent roots from drying out.

Read more about powdery mildew.

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Pictured: orange pustules on the underside of a leaf are signs of rust

Rust

Problem: Orange pustules on the underside of leaves

Diagnosis: This is known as ‘rust’ and is a fungus that generally develops in damper places. The signs of rust can take some time to show, so the disease can go unnoticed for some time and cause plants to become weak. If you’re a vegetable grower you may notice ‘leek rust’, a common fungal disease of leeks, which also affects onions, garlic and chives.

Control: Look for rust-resistant varieties if you can. If it’s too late then remove all affected leaves and consider a fungicide if plants are badly affected.

Related: controlling slugs and snails

Pictured: Grey mould on a cabbage

Grey mould

Problem: Fuzzy grey mould

Diagnosis: ‘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.

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

Related: how to get rid of vine weevil

Pictured: Coral spot on a woody stem

Coral spot

Problem: Pink pustules on branches of woody shrubs and trees

Diagnosis: A fungus called ‘coral spot’ which lives mainly on dead wood and can be spread from rain and watering.

Control: It’s important to act quickly or coral spot can be fatal. Heavily prune the shrub or tree, cutting out all affected areas and burning the offcuts to avoid the disease spreading.

Pictured: Fungal leaf spot

Fungal leaf spot

Problem: Grey-brown spots on leaves

Diagnosis: This is likely to be ‘fungal leaf spot’, 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. 

Control: The spots won’t harm your plant too much, but it’s good practice to cut out affected areas. 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.

Related: how to get rid of aphids

Visit our Pests and Diseases section for more garden problem solving, including what to do about honey fungus and how to keep cabbage whites off your plot.

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