Common Plant Diseases to Look Out for— UK Summer Edition

Reading Time: 5 minutes

As the UK summer rolls in, gardeners across the country are preparing their flower beds, vegetable patches, and containers for a season of vibrant growth. With the warmth and humidity of summer, however, comes an increased risk of plant diseases that can threaten your carefully nurtured plants.

To ensure your plants stay healthy, we’ve put together this detailed guide on common plant diseases to watch for this summer, packed with practical tips to keep your garden disease-free.

Let’s dive into the key culprits and how to tackle them!

Powdery Mildew

Powdery mildew is one of the most widespread plant diseases in the UK, particularly during warm, dry summer spells. This fungal disease, caused by a variety of fungi such as Erysiphe species, appears as white or greyish powdery spots on the leaves and stems of plants like roses, courgettes, peas, and apple trees. While it thrives in warm conditions, it can also spread in humid environments, making it a frequent summer nuisance. It is always more prevalent on plants under stress, especially those growing in dry soil or compost.

Powdery mildew diseases are specific to one genus of plant, so clematis powdery mildew can’t spread to roses for example.

Symptoms:

• White or greyish powdery coating on leaves, stems, or buds.
• Yellowing or curling leaves that may then turn brown.
• Stunted growth or distorted flowers.

Prevention and Treatment:

• Improve Air Circulation: Space plants adequately and prune overcrowded areas to reduce humidity around foliage.
• Water Wisely: Water at the base of plants rather than overhead to keep leaves dry.
• Boost Plant Health: Use our nutrient-rich, peat-free compost to promote strong, disease-resistant plants. Healthy plants are less susceptible to infections.
• Fungicides: For severe cases, consider organic fungicides like sulphur-based sprays, but always follow label instructions.
• Remove Infected Parts: Prune and dispose of affected plant parts promptly to prevent spread.

Pro Tip: Regularly apply a diluted milk spray (one part milk to nine parts water) as an effective remedy to suppress powdery mildew. It’s eco-friendly and safe for your garden!

Rose Black Spot

If you’re growing roses, black spot is a disease you’ll likely encounter. Caused by the fungus Diplocarpon rosae, black spot thrives in warm, humid conditions, making it a common issue during UK summers, especially after periods of rain. This disease primarily affects roses, causing unsightly damage and weakening plants over time.

Symptoms:

• Black spots with yellow halos on upper leaf surfaces.
• Premature leaf drop, reducing photosynthesis, and plant growth.
• Weakened plants with fewer blooms.

Prevention and Treatment:

• Choose Resistant Varieties: Opt for roses labelled as black spot-resistant, such as ‘Flower Carpet’ or ‘Knock Out’ varieties.
• Clean the Area: Remove debris, especially fallen leaves, from around plants, as the fungus overwinters in plant material.
• Conscious Watering: Water early in the day to allow foliage to dry before evening and avoid overhead watering.
• Mulching: Apply a layer of organic mulch (paired with our peat-free compost) to prevent soil-borne spores from splashing onto leaves during rain.
• Fungicides: Use eco-friendly fungicides like potassium bicarbonate-based (these are not approved/available in the UK) products for control, but prioritise cultural practices first.

Pro Tip: Regular feeding with a balanced, organic fertiliser strengthens roses, helping them resist black spots. Our all-purpose compost provides essential nutrients for robust growth.

Late Blight

Tomato/potato blight, caused by the water mould Phytophthora infestans, is a devastating disease for outdoor tomato crops and potatoes in the UK. It spreads rapidly in warm, wet summer weather, often striking in July or August. This disease can wipe out entire crops if not managed promptly.

Symptoms:

• Dark, water-soaked spots on leaves, often with a greyish-white mould on the underside.
• Brown or black lesions on stems and fruits.
• Rapid wilting and collapse of the plant.

Prevention and Treatment:

• Grow Resistant Varieties: Choose blight-resistant tomato varieties like ‘Crimson Crush’ or ‘Ferline’.
• Improve Ventilation: Space plants far enough apart, stake well and removed sideshoots from cordon varieties to enhance air circulation.
• Protect Plants: In wet weather, cover plants with cloches, polytunnels or similar to keep foliage dry.
• Crop Rotation: Rotate crops annually to prevent the buildup of blight spores in the soil. Our peat-free compost supports healthy soil ecosystems, reducing disease risk.
• Remove Infected Plants: At the very first sign of blight, remove and destroy affected leaves (do not compost them), which may help delay the spread of the disease. Severely affected plants should similarly be dug up and destroyed.
• Copper-Based Treatments: In severe cases, use copper-based fungicides approved for organic gardening, but apply sparingly.

Pro Tip: Monitor weather forecasts for “blight warnings” (warm, humid conditions), or visit blightspy.huttonltd.com, and act quickly to protect your crops.

Rust

Rust, caused by various species of fungi like Puccinia and Uromyces, are common summer diseases affecting plants such as hollyhocks, leeks, garlic, and beans. It grows in warm, damp conditions and spreads via wind-borne spores, making it a frequent issue during humid UK summers.

Rust diseases are specific to one genus of plant, so hollyhock rust can’t spread to leeks for example.

Symptoms:

• Small, rust-coloured or orange pustules on leaves and stems.
• Yellowing or wilting leaves.
• Reduced photosynthesis leads to weaker plants.

Prevention and Treatment:

• Plant Spacing: Ensure proper spacing to improve airflow and reduce humidity.
• Sanitation: Remove and destroy infected plant debris to limit spore spread.
• Water Carefully: Water at the base of plants and avoid wetting foliage.
• Healthy Soil: Use our peat-free compost to maintain nutrient-rich, well-draining soil, which supports vigorous plants that are less prone to rust.
• Organic Controls: Apply neem oil (not approved for use in the UK) or sulphur-based sprays as a preventive measure during high-risk periods.
• Crop Rotation: Rotate susceptible crops to disrupt the disease cycle.

Pro Tip: Inspect plants regularly, especially the undersides of leaves, where rust pustules often form first.

Botrytis (Grey Mould)

Botrytis, or grey mould, caused by the fungus Botrytis cinerea, is a widespread problem in damp, cool summer conditions. It mainly affects strawberries, grapes, lettuce, and ornamentals like peonies. Poor air circulation and prolonged leaf wetness exacerbate this disease and is often worse under cover, such as in greenhouses and polytunnels.

Symptoms:

• Fuzzy grey or brown mould on leaves, stems, flowers, or fruits.
• Soft, rotting tissue, especially on fruits.
• Wilting or collapse of affected plant parts.

Prevention and Treatment:

• Improve Airflow: Prune dense foliage and space plants to reduce humidity.
• Water Carefully: Water at the base of plants and avoid wetting foliage.
• Remove Dead Material: Clear away dead flowers, leaves, and fruits, as botrytis often starts on decaying tissue.
• Healthy Roots: Coco and Coir® peat-free compost ensures optimal drainage and nutrient availability, reducing plant stress and susceptibility.
• Fungicides: Use organic treatments like potassium bicarbonate(these are not approved/available in the UK) if the disease persists.

Pro Tip: For soft fruit, like strawberries, elevate plants with straw or matting to keep the fruit off damp soil and reduce botrytis risk.

General Tips for Disease-Free Gardening

To keep your garden thriving this summer, adopt these sustainable practices:

• Monitor Regularly: Check plants every few days for early signs of disease, especially during warm, humid weather.
• Encourage Biodiversity: Attract beneficial insects like ladybirds and lacewings, which control pests that can spread diseases.
• Maintain Soil Health: Rotate crops, add organic matter, and avoid over-feeding with high nitrogen fertilisers, which can weaken plants. High potash ones, on the other hand, strengthen growth.
• Apply Seaweed Tonic: Liquid seaweed tonics have been shown to strengthen plants, enabling them to fight off problems.
• Choose Resistant Varieties: Select plants bred for disease resistance to reduce the risk of infection.

Are there any other plant concerns that are stressing you out? You might find a solution in this blog post.

P.S. Don’t panic if your plants are getting sick. They have natural power to heal— with the right care and time. Happy growing!