Cucumber

Erysiphe spp.

A fungal disease producing a white powdery coating on leaf surfaces. Thrives in warm dry conditions with high humidity, and is particularly common in late summer. Rarely kills plants but significantly reduces vigour and harvest quality.

Peronospora spp.

A fungal-like disease that causes yellowing on upper leaf surfaces with grey or purple fuzzy growth underneath. Favours cool, damp, humid conditions and spreads rapidly in still, wet weather. Unlike powdery mildew, it requires wet or very humid conditions to spread.

Powdery or downy mildew affecting cucumber foliage, weakening plants and reducing yield. Both types are common in cucumbers, particularly in late summer or in humid protected conditions.

Cucumber mosaic virus (CMV)

One of the most widespread plant viruses, spread primarily by aphids. Affects a broad range of crops including cucurbits, legumes, and herbs. There is no cure once a plant is infected; management focuses on controlling aphid vectors and removing infected material.

Plants are drooping or losing vigour. Most commonly caused by water stress but can also indicate root damage, vascular disease, or heat scorch. Identifying the cause quickly is important as some causes are serious.

Seeds are emerging unevenly or not at all. Usually caused by soil temperature being too low, seeds sown too deep, poor seed-to-soil contact, or old seed stock.

Pythium spp. / Phytophthora spp.

Young seedlings collapse at or just below soil level, caused by fungal pathogens that thrive in overwatered, poorly ventilated growing conditions. Can spread rapidly through a seed tray.

Leaves are turning yellow, which can indicate a range of problems including nutrient deficiency, overwatering, disease, or natural senescence. Identifying whether the pattern is upper or lower leaves, whole leaves or between veins, is key to diagnosis.

Plants are significantly smaller or slower to develop than expected. May be caused by poor soil, nutrient deficiency, root restriction, compaction, pest damage to roots, or disease.

Roots are decaying, usually caused by excess moisture, poor drainage, or soil-borne pathogens. Affected plants cannot take up water or nutrients effectively and typically decline rapidly.

Small sap-sucking insects that colonise soft new growth, leaf undersides, and growing tips. Most species reproduce rapidly and colonies can establish quickly in spring and early summer. Also vectors of several plant viruses.

Trialeurodes vaporariorum

Tiny white-winged insects that cluster on leaf undersides and fly up in clouds when disturbed. Both adults and immature stages suck plant sap, weakening growth and depositing honeydew that encourages sooty mould.

Tetranychus urticae

Tiny mites (barely visible to the naked eye) that feed on the underside of leaves, causing pale stippling and fine webbing. Thrive in hot, dry conditions and reproduce extremely rapidly - populations can explode in a polytunnel or glasshouse during a heat wave.

Frankliniella occidentalis

Minute slender insects that rasp the surface of leaves and flowers, leaving characteristic silvery streaking. Can also spread tomato spotted wilt virus in some situations. More common in warm, dry conditions.

Soft-bodied molluscs that feed on plant material at soil level, most active at night and in wet conditions. A persistent pressure throughout the growing season, especially damaging to seedlings and young transplants.

Shell-bearing molluscs that feed on foliage and young plants, leaving similar damage to slugs. They are more active in mild, damp conditions and tend to shelter in visible daytime hiding spots such as walls, pots, and debris.

Chewing pest damage on cucumber leaves and young growth leaving holes or ragged edges. May be caused by beetles, caterpillars, or other chewing insects.

General chewing damage from an unidentified pest, leaving holes or missing leaf sections. May be caused by caterpillars, beetles, or other insects. Inspect at dusk to identify the culprit.

General sap-feeding pest activity causing weakened or distorted growth. May be caused by aphids, whitefly, scale insects, or other sucking insects. Inspect leaf undersides and growing tips to identify the pest.