How To Treat Common Plant Diseases
Rose bushes and flower seedlings, shrubs and trees, vegetable plots and fruit crops, house plants and even the lawn are all in danger of succumbing to common plant and garden diseases, whether they be fungal, bacterial or viral. Here’s how to spot and treat them.Powdery Mildew
This disease grows mostly on the upper surface of leaves and will occasionally spread to the underside and other parts of the plant. It thrives in dry soil conditions, but where overhead conditions are a little humid. A whole host of plants, including herbaceous and many ornamental, can be at risk from a powdery mildew infection.What to look for: As the name suggests, it is characterised by a white powdery fungus coating on the leaves.
How to treat:
- Remove and destroy all infected parts of the plant and be careful not to transfer spores on to healthy foliage.
- Try not to add too much nitrogen-rich fertiliser as this can promote sappy growth which is more vulnerable to fungi. If opting for chemical solution, choose a fungicide containing penconazole. If looking for a suitable organic treatment, choose a dispersible sulphur spray.
Botrytis
Botrytis (or grey mould) is often found on indoor cyclamen and outdoor flowers, such as lily and chrysanthemum, which are kept in very humid conditions.What to look for: It looks like a grey furry mould and can be found on leaves, petals and the base of stalks.
How to treat:
- Pick off the affected part of the plant and reduce any humidity by increasing space between plants, encouraging air movement.
- Destroy all plants which are severely affected, and avoid over-feeding with a high nitrogen fertiliser as this encourages soft lush growth that is more susceptible to attack.
Clematis Wilt
Clematis wilt mostly strikes early, large-flowering varieties.What to look for: The top of the clematis can wilt, collapse and eventually die. The diseased shoots turn brown and hang loosely from the main stems.
How to treat:
- Cut out all affected shoots and, if the entire plant is affected, cut it down to ground level and feed with a good liquid fertiliser.
- Initial planting of the clematis several centimetres deeper than normal allows for the development of healthy shoots beneath the surface as replacements.
- There's no ready-made chemical treatment, but spraying the soil around the plant with a fungicide containing penconazole can help healing.
Damping-Off Disease
This disease (left) is extremely common in seedlings and is usually caused by over-watering, crowding of seedlings, poor air circulation, dirty containers, infected soil or contaminated water.What to look for: Damping-off disease causes emerging seedlings to rot off at soil level, collapse easily and lose their leaves early.
How to treat:
- Sow your seeds thinly in good compost and clean pots, then dampen in moderation with fresh water and make sure there is the necessary ventilation.
- Avoid sowing seeds too thickly.
- For a chemical treatment, try one of the many copper-based fungicides on the market in the seed trays.
Leaf Spot
Leaf spot is a common plant disease, easily noticeable, and caused by a combination of bacteria and fungi.What to look for: Leaf spots can be of various colours such as grey, brown or black, and can affect all types of plants. Spots can sometimes join together to form larger areas of dead black tissue.
How to treat:
- Immediately remove and destroy all infected foliage, and to reduce the chances of re-infection for the following season, prune shrubs back hard and water sparingly for a few weeks.
- For a non-organic treatment, spray with suitable fungicides (containing myclobutanil) and continue spraying at fortnightly intervals throughout the season.
Rose Diseases
Black Spot
What to look for: Black spots appear on the leaves and can sometimes join together to form larger areas of dead tissue. The spots are in fact dead leaf tissue caused by the fungus which spreads the disease. Some roses also develop smaller black spots on stems.
How to treat:
- On first sign of black spot, remove and destroy all infected leaves and plant parts.
- A Bordeaux Mixture gives excellent organic control. Or alternatively spray with a systemic chemical fungicide such as Rose Clear.
- To reduce the chances of re-infection for the following season, prune shrubs back hard. Try to select resistant varieties of roses when buying.
Rust
This fungal problem mostly develops on the rose leaves, but also on stems, and the rust spores thrive in really moist environments.What to look for: They can develop either as patches or as pustules (like septic spots) and usually they range in colour from orange to brown, depending on the season. The presence of a rust infection may point to an already weakened plant, so check for other diseases or pest problems.
How to treat:
- Once spotted, immediately remove all affected plant parts, and remember to destroy any fallen leaves too.
- Choose a suitable fungicide if looking for a fast chemical treatment – there are many specialist chemical rose formulations available – and spray at fortnightly intervals throughout the season. If you would prefer an organic solution, try one part white vinegar to three parts water and spray.
Rose Viruses
Viral infections on roses are unusual in the garden, but can be particularly troublesome.What to look for: Viral infections on rose plants and bushes can appear as veining which gives the foliage unsightly streaks, marks, or patches. One virus in particular, the Rose Mosaic, will appear as discoloured mosaic patterns in shades of yellow and green.
How to treat:
- Viruses which strike roses are virtually impossible to cure and will seriously weaken the plant, probably causing its death. It is advisable to destroy roses with a virus to avoid spreading the infection to other plants.
Shrub, tree and lawn diseases
Honey Fungus
Honey fungus (above right) affects all woody plants and trees, including ornamentals.What to look for: The disease causes nasty white growths under the bark and honey-coloured threads in the roots, which is how it gets it’s a name. Brown toadstools can also appear at the base of the tree.
How to treat:
- Remove and destroy the affected parts, and dig out as much of the roots as possible because this is where the disease is most easily spread to other plants.
- When replanting in the affected area, make sure the soil is replaced first.
- At present, there are no chemicals available for the treatment of honey fungus for home gardeners.
Silver Leaf
Silver leaf is a vicious tree disease caused when a fungus enters wounds (during a cool and wet season) and slowly kills off parts of the tree by blocking its water-carrying vessels.What to look for: This infection gives the leaves of trees a silvery sheen, and affected branches can split quickly, turn brown and die. Similar silvering symptoms may sometimes develop as a result of cold, drought or other non-disease forms of stress, but beware – this is known as ‘false silver leaf’ and can be told apart from true silver leaf by the lack of a stain in the wood.
How to treat:
- The only treatment for silver leaf disease is to cut down all affected branches completely and burn them. On susceptible trees, consider painting all pruning cuts with protective wound paint.
Red Thread
Red thread disease is caused by a fungus which grows on infected blades of grass. The threads can extend to around an inch past the lawn height.What to look for: As its name suggests, red thread causes red, pinkish threads to appear on the lawn.
How to treat:
- It’s possible to apply a systemic fungicide, but the effect of these is usually fleeting and you will still need to rely on nutrients of a good fertiliser to improve the health of the grass.
- The best plan to thoroughly rake out the affected grass and use a high nitrogen lawn fertiliser.
Coral Spot
Coral spot disease appears on the dead and dying twigs of trees and shrubs, and it can quickly migrate over to living wood by entering in pruning snags.What to look for: Coral spot manifests itself as tiny salmon-pink coloured spots, usually in the winter months. It can move down into living wood rapidly and will usually attack trees such as beeches and magnolias.
How to treat:
- A tricky disease to cure organically, the best action is to remove all dead branches (on and off the tree) and always prune in dry weather.
- Chemical treatment is available if you seal all pruning cuts with a protective dressing like Seal and Heal.
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