Blueberry leaf curling signals an underlying problem, as this distortion is never normal for the plant. Curling is a protective mechanism, either to reduce water loss or to shield new growth from a threat. Correctly diagnosing the cause is the first step toward recovery, as solutions for pests differ greatly from remedies for soil imbalance. Causes generally fall into three categories: environmental stressors, insect pests, and diseases or viruses. This guide explores each category to help you identify the issue and implement an effective solution.
Cultural and Environmental Causes of Curling
The most frequent reasons for leaf curling relate directly to growing conditions. Water stress is a common culprit, causing leaves to curl inward or upward due to either too little or too much moisture. Underwatering causes the plant to curl inward to reduce surface area and slow transpiration. Conversely, overwatering waterlogs the roots, preventing effective water and nutrient uptake and leading to drought-like symptoms above ground.
Blueberries require very acidic soil, ideally with a pH range between 4.5 and 5.5. When the soil pH is too high (alkaline), the plant cannot absorb essential nutrients, leading to nutrient lockout. This often presents as a deficiency, commonly affecting iron and magnesium. Iron deficiency (chlorosis) causes the leaves to turn yellow while the veins remain green, often accompanied by upward leaf curling.
Temperature extremes also trigger leaf curling. Extreme heat and intense direct sun can cause leaves to cup or curl upwards to shield the stomata, the pores responsible for gas exchange. Sudden exposure to cold or frost can damage new growth, causing it to distort and curl during recovery. Checking soil moisture and pH should be the initial diagnostic steps, as these factors are frequently the source of the issue.
Pests That Cause Leaf Distortion
Certain insects physically damage the leaves, causing them to curl or distort from their feeding activity. Aphids are small, soft-bodied insects that feed on the sap of new growth using piercing-sucking mouthparts. This removal of plant fluids causes young leaves to twist, curl, and become distorted. Infestations are often recognizable by the presence of honeydew, a sticky, sugary substance the pests excrete.
Spider mites, common during hot and dry weather, cause leaf curling. These tiny arachnids feed by piercing plant cells, resulting in fine yellow or white stippling on the leaves. Heavy infestations cause leaves to curl upwards and may include fine webbing on the undersides of leaves. Thrips also cause leaves to curl and distort, leaving a silvery or speckled appearance on the foliage. The larvae of the blueberry gall midge feed on developing leaf buds, causing emerging young leaves to be deformed.
Disease and Viral Causes
Leaf curling can signal serious, systemic issues caused by pathogens. Viral diseases, such as Blueberry Shock Virus (BlShV), cause dramatic reactions that vary by cultivar. BlShV is spread through pollen and often causes a “shock” reaction where flowers and new vegetative shoots suddenly die in the spring. Although the plant may produce a second flush of growth later, the new leaves can still show signs of distortion or discoloration.
Viral infections are generally incurable, and infected plants remain a source of disease. Fungal issues like Mummy Berry disease primarily affect the fruit, but initial infection can cause new vegetative shoots to wilt and curl, resembling frost damage. Accurate diagnosis is important because fungicides will not treat viral infections.
A non-biological issue that strongly mimics disease is herbicide drift. This occurs when weed-killing chemicals vaporize and are carried by the wind to the blueberry plants. Even minute amounts of certain herbicides can cause severe leaf distortion, including cupping, curling, and abnormal shape. Since the damage is systemic, effects can appear suddenly and affect multiple plants simultaneously.
Immediate Steps and Long-Term Prevention
Immediate action should focus on correcting environmental factors. If water stress is suspected, check the soil moisture several inches down. If the soil is dry, provide deep, slow watering; if soggy, stop watering temporarily to allow the soil to dry. For suspected nutrient issues, an immediate soil test is necessary to confirm the pH level. If the pH is too high, amend the soil with elemental sulfur or peat moss to gradually increase acidity.
For visible pests like aphids or mites, small populations can be controlled by washing them off the plant with a strong jet of water. Larger infestations can be treated with horticultural oils or insecticidal soaps, ensuring the entire plant, especially the undersides of the leaves, is thoroughly coated. For suspected viral diseases like Blueberry Shock Virus, the primary management step is to remove and destroy the infected plant to prevent further spread.
Long-term prevention centers on maintaining the specific cultural conditions required by blueberry plants. This includes yearly maintenance of soil acidity and ensuring good air circulation through proper pruning, which minimizes conditions favorable for fungal growth. Using certified, virus-free nursery stock prevents the introduction of viral diseases. A yearly application of dormant oil in late winter can help manage overwintering insect pests like scale and mites.