Yellow spots on cucumber leaves, known as chlorosis, indicate the plant is under stress due to a loss of chlorophyll. This yellowing is a general reaction, not a specific disease. Diagnosing the exact cause requires observing the spot’s size, shape, location, and associated symptoms. The underlying issues fall into four categories: nutrient or watering problems, insect pests, fungal or bacterial pathogens, or systemic viral diseases.
Nutrient Deficiencies and Watering Issues
Abiotic issues, such as environmental factors or mineral supply, are frequent causes of yellow spots. Cucumber plants are heavy feeders, and an imbalance in soil chemistry quickly manifests as distinct foliage symptoms. These problems are often the easiest for a home gardener to correct once the specific deficiency is identified.
A lack of magnesium causes interveinal chlorosis on older, lower leaves. The areas between the major veins turn yellow while the veins remain green, creating a striking contrast. Nitrogen deficiency also affects older leaves first, as the mobile nutrient moves to support newer growth, leading to a general pale-green or yellow fading across the entire leaf blade.
Improper watering practices can also trigger yellowing. Overwatering saturates the soil, depriving roots of oxygen and leading to root rot. This root damage prevents the plant from absorbing nutrients and water, resulting in wilting and general yellowing. Conversely, insufficient water causes dehydration, resulting in yellowing foliage that may develop dry, crispy, or brown edges.
Insect Pests Causing Yellowing
Yellow spots can result from physical feeding damage caused by tiny, sap-sucking insects. These pests use piercing-sucking mouthparts to remove cell contents, including chlorophyll. This action leaves behind small, localized yellow or white spots, a pattern described as stippling. Stippling is usually concentrated on the upper surface of the leaf where the damage occurs.
Spider mites are a common culprit, thriving in hot, dry conditions and congregating on the undersides of leaves. Their feeding causes fine stippling that merges into larger yellow patches. Heavy infestations are often accompanied by fine, silky webbing visible on the leaf’s underside. Aphids and whiteflies are other sap-sucking insects that cause similar yellow stippling and overall yellowing.
A telltale sign of aphids and whiteflies is the sticky residue known as honeydew. When inspecting the plant, check the underside of the leaves, as pests often hide there along with their eggs or nymphs. Identifying the pest by physical evidence, such as mites or webbing, helps distinguish this cause from systemic disease or nutrient problems.
Fungal and Bacterial Pathogens
Pathogens are microscopic organisms that cause distinct, localized spots on the leaves. Fungal and bacterial infections create lesions differentiated by their shape, texture, and whether they are bound by the leaf’s vascular structure. These diseases are often favored by high humidity and moisture on the leaf surface.
Downy Mildew, caused by a water mold, typically produces angular yellow spots on the upper leaf surface. These lesions are constrained by the small veins of the leaf, giving them a characteristic geometric shape. If humidity is high, the underside of the leaf beneath these spots may develop a fuzzy, grayish-purple growth, which is the pathogen’s spore-producing structure.
Bacterial diseases, such as Angular Leaf Spot, produce angular spots that are initially water-soaked and appear oily or translucent. As the lesion dries, the center may become necrotic and fall out, leaving a hole, and the spots may be surrounded by a yellow halo. A key difference from Downy Mildew is that bacterial spots may exude a white, crusty discharge on the leaf surface or fruit, and they lack the fuzzy growth on the leaf underside.
Viral Infections
Viral infections cause the most severe category of yellowing, as they are systemic and incurable once the plant is infected. Viruses like Cucumber Mosaic Virus (CMV) are typically transmitted by insect vectors, primarily aphids, or through contaminated tools and seeds. Viral symptoms are generally not isolated spots but a widespread disruption of the plant’s growth and color.
A common sign of CMV is systemic mottling or a mosaic pattern on the leaves, where patches of dark green, light green, and yellow are intermixed. The leaves may also become distorted, wrinkled, or stunted in size, and overall growth is significantly reduced. This overall yellowing and distortion distinguishes viral infections from the discrete spots caused by fungi or bacteria.
Since no chemical treatment can eliminate a viral infection, management focuses entirely on prevention and removal. This includes controlling insect vectors, immediately removing and destroying infected plants to prevent further spread, and choosing virus-resistant cucumber varieties. Once the characteristic mottling appears, removing the plant is the best action to protect the remaining crop.