When strawberry leaves begin to turn red or purple, the plant is producing pigments called anthocyanins, which are water-soluble compounds that shield the plant’s cells from damage. This discoloration is not a disease itself but a visible symptom that the plant is under stress, often due to environmental factors or a nutritional imbalance. The pigments act as a protective response, similar to a plant’s natural sunscreen, to manage excess light energy when other processes, like photosynthesis, are slowed or disrupted. Identifying the source of this stress is the first step in restoring the plant’s healthy green color.
Temperature and Seasonal Changes
The most frequent, non-alarming reason for a strawberry plant to develop red foliage relates to temperature fluctuations. Exposure to temperatures below 40°F, particularly during a sudden cold snap, triggers anthocyanin production. This reaction helps protect the plant’s delicate tissues from photo-oxidative damage when light intensity is high but cold temperatures slow down the plant’s ability to use that energy for growth.
This cold-induced reddening is a temporary, defensive measure and usually does not cause lasting harm to the plant crown. Mature plants in late autumn naturally reallocate nutrients from older leaves to the crown in preparation for winter dormancy. These leaves turn deep maroon or red before dying back. Sunscald can also cause a reddish tint, as the plant produces protective pigments in response to excessive, direct sunlight coupled with heat stress.
Nutritional Causes of Discoloration
If the discoloration is widespread and appears during the peak growing season, the cause is often a deficiency in one of the plant’s mobile nutrients. A lack of phosphorus (P) is the most common culprit, as this nutrient is essential for energy transfer and root development. A phosphorus-deficient plant displays dark green leaves with a distinct purple or reddish tint, particularly on the edges and undersides of the older leaves.
This discoloration happens because the plant cannot effectively transport sugars out of the leaves, leading to the buildup of carbohydrates. While phosphorus may be present in the soil, its uptake is often limited by a soil pH that is either too acidic (below 5.5) or too alkaline (above 7.0). This improper pH causes the nutrient to become unavailable to the roots.
Magnesium (Mg) deficiency can also cause reddish-purple discoloration. This symptom is usually accompanied by interveinal chlorosis, where the tissue between the leaf veins turns yellow while the veins themselves remain green. Since Magnesium is a central component of the chlorophyll molecule, its lack impairs the plant’s ability to produce the green pigment.
Identifying Pests and Diseases
Red or purple coloring that manifests as spots, stippling, or concentrated patches often points to a biological threat rather than a generalized nutrient or environmental issue. The two-spotted spider mite (Tetranychus urticae) is a common pest that causes damage by piercing leaf cells and sucking out the contents. This feeding initially creates a fine, silvery-white stippling on the upper surface of the leaves, which progresses to bronzing and eventual reddish-brown discoloration.
Mite infestations are confirmed by inspecting the underside of the leaves, where tiny mites and fine, silken webbing may be visible. Fungal diseases, such as leaf scorch (Diplocarpon earlianum), cause distinct symptoms that differ from generalized reddening. Leaf scorch first appears as small, irregular purplish or reddish-brown spots that merge together, causing the tissue between the lesions to turn a bright red or purplish color.
Viral infections, though less common in home gardens, can cause a generalized purpling, stunting, and deformation of the plant. Since there is no cure, affected plants must be promptly removed to prevent the virus from spreading to healthy surrounding plants.
A Step-by-Step Recovery Plan
Recovery begins with a careful diagnosis based on the pattern and timing of the discoloration. Evaluate the environment first: if reddening occurred after a cold night or appears on older leaves in the fall, it is likely a transient, seasonal response requiring no immediate treatment. If the discoloration is a uniform purple on older leaves during the growing season, a nutrient deficiency is the probable cause.
For suspected nutrient issues, performing a soil test is the most accurate way to confirm the nutrient level and soil pH. If the test confirms a low phosphorus level or an improper pH, apply a balanced fertilizer with a high phosphorus ratio. Alternatively, adjust the soil pH to the optimal range of 5.5 to 6.9 to improve nutrient availability.
If the leaves show stippling or webbing, immediately inspect the leaf undersides for spider mites. Treat a confirmed mite infestation with targeted pest control, such as a horticultural oil or insecticidal soap, ensuring thorough coverage of the leaf underside. Finally, regardless of the cause, remove and destroy any severely discolored, scorched, or deformed foliage to improve air circulation and prevent the spread of disease or pests.