Grape leaves turning brown and yellow can concern any gardener. While some changes are natural, widespread or sudden discoloration often signals an underlying problem. These issues can impact the vine’s health and fruit production. Understanding the reasons behind these changes is the first step in addressing them.
Healthy Grape Leaves and Normal Changes
Healthy grape leaves are typically vibrant green, firm, and smooth, with clearly defined veins. As the growing season progresses into late summer and autumn, some yellowing and browning are normal. This natural process, called senescence, occurs as the plant prepares for dormancy, drawing nutrients from older leaves before they drop.
Common Causes of Browning and Yellowing
Many factors can cause grape leaves to change color, including nutrient imbalances, pest infestations, and environmental stressors. Each cause presents distinct visual cues.
Nutrient Deficiencies
Grapevines need a balanced supply of nutrients. A lack of specific elements causes characteristic discoloration.
Nitrogen deficiency often results in general yellowing of older leaves, sometimes with stunted growth. Potassium insufficiency typically appears as yellowing or browning along older leaf margins, giving a scorched appearance. Magnesium deficiency causes interveinal yellowing on older leaves, where tissue between veins yellows while veins remain green. This can create a “Christmas-tree” pattern, sometimes leading to reddish-brown spots or necrosis. Iron deficiency primarily affects younger leaves, causing interveinal yellowing where veins stay green but tissue between them yellows. In severe cases, the entire young leaf may turn pale yellow or white.
Water Stress
Both insufficient and excessive watering can lead to leaf discoloration. Underwatering causes leaves to wilt and may result in crispy brown edges, as the plant struggles to transport water. Prolonged drought stress can cause basal leaves to yellow and drop. Overwatering can also cause leaves to yellow and droop due to a lack of oxygen around the roots, potentially leading to root rot. Root rot hinders the plant’s ability to absorb water and nutrients, leading to browning and decline.
Pests
Various insect pests feed on grape leaves, causing yellowing or browning.
Spider mites feed by puncturing leaf cells and sucking out the contents, resulting in small yellow-white spots or stippling on the upper surface. Heavy infestations may cause leaves to bronze or yellow, with fine webbing visible underneath. Leafhoppers also cause stippling and yellowing, often along leaf veins, progressing to browning. Phylloxera can form galls on the leaves, typically on the underside, leading to yellowing and premature leaf drop. Mealybugs feed on plant sap, causing yellowing and a sticky residue (honeydew) on leaves, which may then develop sooty mold.
Diseases
Fungal and bacterial diseases commonly cause grape leaf discoloration.
Downy mildew initially appears as yellowish, oily spots on the upper leaf surface, turning reddish-brown and angular, often delimited by veins. A fuzzy white growth can be seen on the underside of infected leaves, particularly in humid conditions. Powdery mildew presents as white, powdery patches on both sides of the leaves, leading to leaf distortion, curling, and gradual yellowing. Black rot causes small, round, reddish-brown spots on leaves, which enlarge and develop dark margins with light brown-to-tan centers. Pierce’s disease leads to leaf scorching, where leaf margins turn yellow or red and then brown, with desiccation spreading inward. Affected leaves may shrivel and drop.
Environmental Factors
External environmental conditions can stress grapevines and cause leaf discoloration. Sunburn results in bleached or scorched patches on leaves exposed to intense sunlight, leading to browning.
Frost damage causes leaves to darken, wilt, and turn brown or black as cell tissues are destroyed. Heat stress may induce wilting, yellowing, and browning, particularly with insufficient water.
Strong winds can lead to windburn, causing physical damage like tearing and browning along leaf edges. Accidental exposure to herbicides can result in uneven yellowing, browning, or distortion of leaves.
Diagnosing the Problem
Identifying the cause of browning and yellowing grape leaves requires careful observation.
- Examine the pattern of discoloration: is it interveinal, along the margins, or affecting the entire leaf?
- Observe which leaves are impacted—new growth versus older leaves—as this can indicate nutrient deficiencies or diseases.
- Look for additional symptoms like galls, webbing, fuzzy growths, distinct spots, or wilting.
- Consider the timing of symptom appearance: did they emerge suddenly after a weather event, or gradually?
- Reflect on recent environmental conditions, including watering frequency, rainfall, and any applied chemicals or fertilizers.
- Inspect the vine: check the undersides of leaves for pests and stems for lesions. Consider examining roots if a water-related issue or root disease is suspected.
- For suspected nutrient issues, a soil test provides valuable information about nutrient availability and pH levels.
Solutions and Preventative Measures
Addressing browning and yellowing grape leaves involves tailored solutions and preventative measures. Proper care is fundamental to grapevine health.
General Care
This includes deep, infrequent watering for strong root development and well-draining soil to prevent waterlogging. Balanced fertilization, guided by soil test results, ensures the vine receives necessary nutrients. Adequate air circulation, often achieved through strategic pruning, reduces humidity around leaves, discouraging fungal diseases. Proper site selection, offering ample sunlight and good drainage, provides a strong foundation for healthy growth.
Specific Interventions
For nutrient deficiencies, apply appropriate fertilizers or soil amendments based on soil test recommendations. Adjust watering schedules for water stress: increase frequency during dry periods for underwatering, or improve drainage and reduce volume for overwatering. Pest issues can be managed using Integrated Pest Management (IPM) strategies, including encouraging beneficial insects, applying horticultural oils or insecticidal soaps, or using targeted insecticides. Disease control involves fungicidal sprays, pruning and disposing of infected plant parts, and selecting disease-resistant grape varieties.
Environmental Protection and Expert Advice
To mitigate environmental damage, consider providing temporary shade during extreme heat, protecting vines from late frosts, and exercising caution with chemical applications to avoid overspray. For persistent issues, consulting a local agricultural extension service or viticulture expert can provide specialized guidance.