Red or purple discoloration on strawberry stems (petioles) often signals a potential problem. This color change from green is the plant’s visible response to internal or external environmental changes. The reddening is caused by the production of anthocyanin pigments, which are naturally occurring compounds responsible for red, purple, and blue hues. Determining the cause requires a methodical approach, starting with common explanations before moving to nutrient deficiency or disease.
Is It Normal? Variety and Age Factors
The simplest explanation for red stems is often the plant’s genetic makeup. Strawberry plants, depending on their specific cultivar, are naturally predisposed to produce varying amounts of red pigment in their non-fruit tissues. Certain healthy varieties display reddish or purplish coloration along the petioles and runners due to high, genetically determined anthocyanin levels.
This natural discoloration is not a sign of distress and requires no intervention. An older stem on a mature plant may also deepen in color as it ages, preparing for senescence, a natural process where the plant recycles nutrients. If the plant is otherwise growing vigorously and producing fruit, consult documentation for your specific cultivar to confirm if this color is typical.
The Role of Temperature Stress
One frequent non-pathogenic cause of stem and leaf reddening is environmental stress, particularly cold temperatures. Sudden drops in temperature, especially early in the spring, can trigger this color change. This response is a protective mechanism where the plant produces anthocyanins to shield leaf tissue from excess light energy that cannot be utilized due to the cold.
Cold soil temperatures temporarily inhibit the plant’s metabolism, reducing the roots’ ability to absorb and process phosphorus. Even if phosphorus is abundant, the plant cannot access it, creating a functional deficiency known as nutrient lockout. This temporary inability to utilize phosphorus causes sugars to accumulate, which stimulates the production of red-pigment anthocyanins.
The reddening caused by cold stress is typically uniform and resolves once soil temperatures warm consistently above 60°F. If a late frost is expected, covering the plants with a row cover or blanket can insulate them and prevent discoloration. This is a temporary stress, and the plant should return to normal coloration once the weather stabilizes.
When Nutrients Are Missing
When red discoloration persists despite consistently warm temperatures, a true nutritional deficit should be suspected. Phosphorus (P) deficiency is the most common nutrient imbalance causing purplish-red stems and leaves in strawberries. This nutrient is fundamental for energy transfer, root growth, and flower development.
Symptoms of a true phosphorus deficiency usually appear first on older petioles and leaves, as the plant moves its limited P supply to new growth. Affected stems and leaves may display a dull, dark green color before turning a distinct purplish-red, often accompanied by stunted growth. This persistent discoloration, unlike cold stress, indicates a long-term issue with nutrient availability.
The problem may not be a lack of P in the soil, but rather the soil’s pH, which affects nutrient solubility. If the soil is too acidic (below pH 5.5) or too alkaline (above pH 7.0), phosphorus becomes chemically bound and unavailable for uptake. A soil test can confirm both the P levels and the pH. If a true deficiency is confirmed, amendments like bone meal or a balanced fertilizer high in phosphorus can be applied, or the pH can be adjusted to the ideal range of 5.8 to 6.5.
Signs of Disease and Pathogens
While less common than stress or deficiency, certain diseases can cause stem and petiole discoloration distinguishable from uniform color changes. Pathogen-related reddening is rarely a solid, uniform hue; instead, it typically presents as localized spots, streaks, or lesions. This visual difference is key to identifying a disease issue.
Fungal infections like Leaf Scorch (Diplocarpon earlianum) manifest as irregular, dark purple or brown spots on the stems and leaves. These spots can merge into larger, sunken lesions that give the plant a burnt appearance. Another condition, Verticillium Wilt, can cause bluish or brownish-black blotches on the runners and petioles, often accompanied by the wilting and collapse of older leaves.
When disease is suspected, inspect the plant closely for accompanying symptoms like wilting, lesions, or general decline. Good air circulation and proper water management help prevent many fungal issues. Infected plants should be removed immediately to prevent the disease from spreading throughout the planting area.