Why cucumber leaves turn yellow, and how to fix it

Yellow leaves on cucumber plants come from a short list of causes, and most leave a pattern you can read in a minute. A hungry plant moves nitrogen and magnesium out of its old leaves to feed new growth, so those shortages show at the base first. Iron stays put, so its shortage shows on the newest leaves. The RHS guide to nutrient deficiencies sets out each pattern.

Check the compost first. Push a finger in, and lift a corner of a pot or grow bag to feel its weight. Sodden compost needs drainage, not feed. Dry compost needs water at the base, then a second look in a day or two.

Nitrogen. The RHS describes a shortage that starts on the older leaves and spreads upwards. Nitrogen washes out of compost easily, so cucumbers in pots and grow bags run short first. A general-purpose liquid feed is the RHS remedy for containers. Judge the result by the new leaves: they should come through green.

Magnesium. Here the yellow sits between veins that stay green, sometimes with reddish-brown tints. The RHS gives a reason worth knowing. Plants take up potassium in preference to magnesium, so plenty of high-potash tomato feed can cause the very yellowing it was meant to prevent.

Age. If only the lowest two or three leaves have yellowed and the plant keeps growing strongly, there is nothing to treat. Pick them off.

Where the plant grows decides whether it can be hungry at all. The RHS cucumber guide says cucumbers in the ground shouldn’t need feeding unless the soil is very poor. Plants in containers need a general liquid feed every 10 to 14 days, then a high-potash feed such as tomato feed once they flower. So when a cucumber in a good border yellows, look at water, cold and pests first.

The RHS notes that iron shortage affects the youngest leaves most, and that alkaline or chalky conditions lock iron up even when it is present. For plants in containers it suggests a liquid seaweed feed with sequestered iron, and for a border a chelated iron fertiliser. If the problem returns in the same bed each year, test the soil pH before buying more iron.

Fine pale speckles. Glasshouse red spider mite leaves a fine pale mottling on the upper surface. Underneath, a ×10 hand lens shows tiny mites, white cast skins and egg shells. Despite the name, the mites are yellowish-green and turn reddish-orange only in autumn. They thrive in warm, dry glasshouses from March to October. Damping down the path helps but won’t control them alone. The predatory mite Phytoseiulus persimilis is the most commonly used control, the RHS says, and our red spider mite guide covers it in full.

Angular patches. Downy mildew makes pale green to yellow spots, angular and boxed in by the veins, with a dark purplish-grey fuzz underneath in humid weather. University of Minnesota Extension notes the pattern is clearest on cucumbers, and that the disease needs wet leaves to infect. The RHS advice on downy mildews is to remove affected leaves as soon as you see them, open doors and vents, avoid overhead watering and water early so the leaves dry quickly.

White powder. Powdery mildew starts as pale yellow spots on the oldest leaves before the white patches appear, says the Minnesota diagnostic guide. The RHS links it with plants that dry out now and again and with still air. Keep the compost evenly moist and the air moving; our powdery mildew guide has the full routine.

The RHS describes potassium shortage as yellow or purple-red tints with browning from the edges of mature leaves, and poor flowering and fruiting. Plants in containers may need extra potassium-rich feed. The Minnesota guide links olive-green to brown edges with cold, wet weather below about 10 °C and damp soil below about 17 °C. And if you sprayed a feed in sunshine a day or two before, suspect scorch.

The RHS explains that waterlogging cuts the oxygen supply to the roots. Leaves yellow between the veins, the plant may wilt as if short of water, and damaged roots turn blue-black with a sour smell. Tip a pot out gently to look. For routine care the RHS asks for evenly moist compost, water at the base rather than over the leaves, and watering in the morning.

Greenhouse cucumbers grow best at 18 to 25 °C, the RHS says. From late September an unheated greenhouse often drops below that at night, and older plants yellow from the base as the crop ends. To keep tender plants going into winter, our greenhouse heating guide sizes a heater from your glazing.

The RHS describes cucumber mosaic virus as yellowish patches or green and yellow mottling, leaves that curl down and shrink, and plants stunted by short stems between the leaves. Yellow alone is not a sign of the virus; the distortion and stunting are. Aphids spread it, and so can tools and fingers, so wash both before touching healthy plants. For next year, the RHS lists resistant cultivars, among them ‘Bush Champion’, ‘Crispy Salad’, ‘Petita’ and ‘Country Fair’.