Seeing the leaves of a strawberry plant turn brown is often an alarming sign for any gardener. This discoloration, known as necrosis, indicates that the leaf tissue is dying and the plant is struggling to perform photosynthesis. While the symptom is consistent, the underlying causes are numerous, ranging from simple environmental stressors to invasive biological pathogens. Understanding the specific pattern of browning is the first step in diagnosing and correcting the problem to protect the plant’s future harvest.
Browning Caused by Watering and Nutrient Issues
Abiotic stressors, which are non-living factors, frequently cause leaf browning and are often the easiest problems to correct. Inconsistent moisture levels are a prime suspect for foliage damage. Severe underwatering leads to drought stress, causing leaf edges and tips to turn dry, crispy brown, and curl upward as the plant conserves moisture.
Conversely, overwatering creates soggy soil conditions that starve the roots of oxygen, leading to root rot. Damaged roots cannot absorb water or nutrients effectively, causing symptoms similar to drought, such as soft, yellowing leaves with dark brown or black margins. Maintaining consistently moist, well-draining soil is necessary to avoid both types of moisture stress.
Nutrient imbalances can also manifest as leaf necrosis, particularly when fertilizers are applied incorrectly. Fertilizer burn, a type of salt toxicity, typically appears as brown, scorched margins on the older, lower leaves. This damage results from an accumulation of soluble salts that draw water out of the leaf tissue.
A deficiency in potassium, which is involved in water regulation, also causes a marginal leaf burn. The leaf edges turn yellow and then brown or dark purple as the plant pulls this mobile nutrient from older foliage to support new growth. Additionally, soil pH levels that are too high or too low can prevent the uptake of necessary nutrients, even if they are present.
Identifying Fungal and Bacterial Diseases
When browning patterns are highly specific and localized, a fungal or bacterial infection is likely the cause.
Leaf Scorch
One common fungal issue is Leaf Scorch, caused by the pathogen Diplocarpon earliana. It begins as small, irregular, purplish spots scattered across the upper leaf surface. As the infection progresses, these spots coalesce into larger necrotic areas, and the tissue between the lesions often takes on a reddish or bright purplish color. In severe cases, the entire leaflet dries up, curls at the margins, and turns completely brown, giving the foliage a scorched appearance. The fungus overwinters in infected leaves, making sanitation important.
Leaf Spot
Another distinct fungal issue is Leaf Spot, caused by Mycosphaerella fragariae, which is characterized by target-like lesions. The spots start as deep purple, circular blemishes that enlarge up to five millimeters in diameter. A defining feature is the center of the spot turning pale gray or white, while a distinct reddish-purple or rusty-brown border remains.
Anthracnose
Anthracnose, caused by various species of the fungus Colletotrichum, is a serious disease that impacts leaves, runners, and fruit. While known for causing dark, sunken lesions on petioles and runners that can girdle the stem, it also appears on leaves. On foliage, it typically presents as small, round spots with black or gray centers, often leading to general wilting and browning of the entire leaf.
Damage from Pests and Mites
Insects and mites can cause feeding damage that results in browning, often mimicking the symptoms of drought or nutrient issues.
Two-Spotted Spider Mites
The two-spotted spider mite (Tetranychus urticae) is a prevalent pest that feeds on the underside of the foliage. Their feeding removes chlorophyll, causing a fine, pale speckling or stippling on the upper leaf surface. Heavy infestations cause leaves to turn bronzed, reddish, or brown before they dry up and drop. A noticeable sign of a severe problem is the presence of fine, silken webbing found on the undersides of the leaves and between the leaflets.
Cyclamen Mites
A more insidious threat comes from the microscopic cyclamen mite, which targets the new, developing growth in the plant crown. These mites are virtually invisible, often leading growers to misdiagnose the damage as a cultural problem. Their feeding causes young leaves to become stunted, distorted, and severely crinkled or blistered. Severe damage causes the newest foliage to take on a stiff, brownish-green or bronzed appearance, significantly impacting the plant’s ability to produce healthy new leaves and flowers.
Treatment and Long-Term Prevention
Addressing leaf browning requires a targeted approach based on an accurate diagnosis of the cause.
Cultural Management
For cultural issues, establishing proper watering practices is the first step, ensuring the soil remains consistently moist but never waterlogged. Applying a balanced, slow-release fertilizer can correct nutrient deficiencies without risking root damage caused by over-application.
Disease Control
To manage fungal diseases like Leaf Scorch and Leaf Spot, immediate sanitation is necessary by pruning and removing all infected foliage. Improving air circulation by ensuring proper spacing and removing weeds helps the leaves dry more quickly, preventing spore germination. Preventive treatments can include applying a copper-based fungicide, which is effective against many fungal pathogens.
Pest Control
Pest management starts with non-chemical methods, such as dislodging spider mites with a strong jet of water. Introducing natural predators, such as predatory mites or ladybird beetles, can help keep mite populations in check. Horticultural oils or insecticidal soaps can be applied to control populations, ensuring thorough coverage on the undersides of the leaves where they congregate.
Long-Term Prevention
Long-term prevention relies on cultural practices that minimize stress and exposure to pathogens. Planting certified, disease-free nursery stock is a foundational defense against introducing mites or pathogens. Practicing crop rotation and avoiding planting strawberries in the same location year after year reduces the buildup of soil-borne diseases. Selecting resistant strawberry varieties further protects the crop.