Fungal leaf spot diseases

Leaf spot diseases, as their name suggests, are diseases that are produced by fungal attacks and that affect plant leaves! Numerous different plants are affected, and the shape, size and colour varies from plant species to plant species. They can be round or an irregular shape. And they may be either brown, black, yellow, purple – a range of different colours.

Each species of leaf spot disease only attacks one species or closely-related species of plants and generally don’t spread across to other species. This means each one doesn’t spread as far as many people may think.

While most leaf spot diseases are disfiguring, and detract from the overall appearance of the plant, they rarely do much other damage, although severe attacks can weaken plants – especially young ones.

Commonly affected plants include the following.

Ornamental plants

Acer (maple)
Arbutus
Escallonia
Hellebore
Hydrangea
Passiflora (passion flower)
Primula
Prunus laurocerasus (laurel)
Rhododendron
Viola (pansy)

Edible plants

Celery
Currant
Gooseberry
Quince (Cydonia oblonga)
Tomato

Numerous other things – mainly environmental or growing conditions – can cause spotting on the leaf, and are confused with fungal leaf spot diseases. These include damage by water droplets in high light levels, nutrient deficiencies, overall plant stress, virus diseases.

Rose black spot is a type of leaf spot disease, but one that differs to most others.

Some leaf spots, including shothole disease of Prunus species, are caused by bacterial diseases.

• Trees – including fruit trees
• Shrubs
• Herbaceous perennials
• Annuals
• Vegetables
• Houseplants

Several different, often closely-related, fungi cause fungal leaf spot diseases. Each one attacks a different species, or closely-related species, of plants. Airborne spores spread them from leaf to leaf and from plant to plant.

Fungal leaf spot diseases are always worse on plants that are under stress, such as growing in the wrong position or the wrong conditions, water stress and nutrient deficiencies.

They start to become active in spring and the spores are easily carried by the wind, quickly spreading from leaf to leaf and from plant to plant.

They either overwinter on fallen leaves, releasing spores to infect plants the following spring, or as dormant infections on the plant.

Try to keep plants growing as strongly as possible to help them fend off disease attacks. However, when large outbreaks occur, it may be difficult to halt their spread.

While affected leaves will always have the discoloured spots or marks – you can’t get rid of these – it is important to prevent them spreading any further.

Carefully pick off and dispose of affected leaves when the disease first strikes to reduce or slow down its spread. This is highly unlikely to actually totally eradicate the disease. Try not to disturb the leaf too much, or you may spread the spores and so help spread the disease further.

Prune out badly infected growth and dispose of it by either burning it or put it in the council’s green waste bin. Don’t compost it.

Similarly, carefully pick up or rake up affected fallen leaves – especially in autumn – to try and reduce carry-over through the winter.

Congested plant growth may help the disease to build up further. Pruning, where appropriate, will help to open up the plant, improve airflow through the plant, so may help prevent or reduce outbreaks.

Similarly, closely spaced plants may help to spread the disease, so always adhere to recommended spacing intervals – especially with annual bedding plants and vegetables.

You can help protect new growth and prevent the disease – and to a slightly lesser extent, control existing diseases – by spraying ornamental plants with a good garden fungicide, such as Bayer Garden Fungus Fighter or FungusClear Ultra.

Unfortunately, there are currently no fungicides approved for home gardeners to use to treat diseases on edible crops.

Sulphur was one of the active ingredients in suitable fungicides, but this lost its approval some years ago. Sulphur is also a key nutrient needed for healthy plant growth and, if there is a sulphur deficiency, plants may become more susceptible to diseases.

Use plant protection products safely. Always read the label and product information before use. Plants in flower should not be sprayed due to possible danger to pollinating insects. Either spray early in the morning or late in the evening when pollinating insects are less likely to be active.

Leaf spot diseases are worse on plants that are under stress. Make sure they are planted and grown in the correct conditions, that the soil or compost is kept evenly moist – especially during periods of warm, dry weather – and plants are well fed with a balanced or high potash fertiliser.

High nitrogen fertilisers produce soft growth that is more susceptible to disease attacks.

Adding a water-retention gel to the soil or compost will help even out periods of drying out.

Adding a 5-10cm (2-4in) thick mulch of organic matter to the soil around plants will help conserve soil moisture and help prevent it drying out.

• Fertilisers/plant feeds
• Fungicides
• Water-retention granules
• Mulches