Leaf discoloration in grapevines is a common issue, indicating a problem interfering with the leaf’s ability to produce chlorophyll or maintain healthy tissue. This symptom, which can drastically reduce the vine’s productivity and fruit quality, is not a definitive diagnosis. Root causes are varied, ranging from microscopic biological threats like pests and pathogens to non-biological stressors related to soil chemistry and environmental conditions. Proper identification requires careful observation of the discoloration’s pattern, location, and the presence of any associated physical signs on the vine.
Discoloration Caused by Pests
Tiny insects and mites feed on the vascular tissues and chlorophyll-containing cells of the leaves, leading to distinct patterns of yellowing and bronzing. Spider mites, such as the two-spotted spider mite, pierce leaf cells to suck out the contents, leaving behind a fine, speckled pattern known as stippling. This damage initially appears as tiny white or yellow spots on the underside of the leaves, which eventually coalesce into a bronze or yellow-brown hue. The presence of fine silk webbing across the leaves is a diagnostic sign when infestations become severe.
Grape leafhoppers cause similar stippling damage by extracting cell contents, resulting in a white or pale yellow speckled appearance that progresses to yellow or brown, often called “hopper burn.” These small, wedge-shaped insects and their nymphs typically feed on the leaf undersides. This feeding reduces the leaf’s capacity for photosynthesis, often leading to premature leaf drop.
Grape phylloxera, an aphid-like insect, causes two types of damage resulting in systemic yellowing and decline. The most damaging form involves feeding on the roots, forming galls that inhibit nutrient and water uptake, causing the foliage to turn yellow and growth to become stunted. A separate form feeds on the leaves of certain varieties, triggering the formation of distinct, pea-sized, warty galls on the leaf undersides.
Discoloration Caused by Pathogens
Fungal, bacterial, and viral diseases often manifest as specific lesions that provide clear diagnostic clues. Downy mildew, caused by the oomycete Plasmopara viticola, first appears on the upper leaf surface as small, translucent, yellowish-green spots resembling oil drops, often constrained by the major leaf veins. If humidity is high, a unique, dense, white, cottony fungal growth develops on the corresponding lower surface of these lesions. As the infection matures, the oil spots on the upper surface turn brown and necrotic.
Black rot, caused by the fungus Guignardia bidwellii, presents on leaves as small, circular, reddish-brown spots. As these spots enlarge, the center becomes grayish-tan surrounded by a dark, distinct border. A characteristic sign is the development of tiny, black, pimple-like fungal fruiting bodies, called pycnidia, which often form a ring just inside the dark margin of the lesion. Infected leaves may eventually shrivel or drop prematurely.
Grapevine Leafroll Disease (GLRD) is a widespread systemic problem caused by viruses that disrupt sugar transport. This stress typically becomes most apparent in late summer or fall. A hallmark symptom in red-fruited varieties is deep red or purple coloration of the leaf tissue between the veins, while the main veins remain green. White-fruited varieties show a similar effect, but the interveinal tissue turns pale yellow instead of red.
Discoloration Caused by Environmental Stress and Nutrition
Abiotic factors like weather, soil composition, and water management can significantly impact a grapevine’s health, often producing symptoms that mimic biological threats. Water stress, resulting from drought or excessive heat and sunlight, can cause the margins of leaves to become brown and desiccated, a condition referred to as scorching. In cases of severe water deficit, the oldest, basal leaves may become entirely yellow (chlorotic) and drop. Excessive sunlight combined with water stress can also lead to “blackleaf,” where exposed leaves develop an overall dark, purple-brown discoloration.
Nutrient deficiencies lead to specific discoloration patterns depending on the element’s mobility within the plant. Mobile nutrients, such as Potassium (K) and Magnesium (Mg), can be moved from older leaves to support new growth, so deficiency symptoms first appear on the older, lower leaves. Potassium deficiency causes the leaf margins of red varieties to turn red or purple, while white varieties show a marginal yellowing. Magnesium deficiency also affects older leaves, presenting as interveinal chlorosis where the tissue between the veins turns yellow or red, but the main veins remain green, often forming a distinct wedge-shaped pattern.
Immobile nutrients, like Iron (Fe), cannot be easily relocated, meaning their deficiency symptoms appear first on the youngest, newest leaves. Iron deficiency is characterized by severe interveinal chlorosis where the newest leaves appear bright yellow or nearly white, while their veins remain dark green. This striking contrast between the yellow tissue and the green veins is a reliable indicator of iron chlorosis.
A Step-by-Step Diagnostic Guide and Management
Accurately diagnosing the cause of discoloration requires a systematic inspection of the vine, starting with the pattern and location of the damage. First, determine the age of the affected leaves; problems on the newest leaves suggest immobile nutrient deficiencies, while issues on the oldest leaves point toward mobile nutrient problems. Next, examine the pattern of the discoloration on the individual leaf to distinguish between vein-restricted (disease, immobile nutrient), marginal (water stress, mobile nutrient), or stippled (pest) effects.
Definitive Diagnostic Clues
The presence of physical signs is the most definitive clue, requiring inspection of the leaf’s underside with a hand lens. Finding fine webbing or tiny specks of insects confirms a spider mite infestation. The presence of white, downy fungal growth on the underside directly beneath a yellowish oil spot on the top surface confirms downy mildew.
Management Strategies
Management should be tailored to the specific diagnosis. Adjusting irrigation mitigates environmental stress, while pathogen management often focuses on improving air circulation and using fungicides. Pest control relies on targeted insecticides or promoting natural predators. For viral diseases like Grapevine Leafroll, there is no cure, and management involves removing infected vines to prevent spread by insect vectors.