Yellow cucumber leaves can follow age, uneven watering, root stress, nutrient problems, pests, or disease. Inspecting the whole vine is more useful than treating color alone.
Check leaf position
Look at older lower leaves and newer growth separately. Note whether yellowing is even, mottled, edged, or accompanied by spots.
Inspect the base
Check soil moisture, stem firmness, and drainage. Avoid keeping the crown wet or watering shallowly every day.
Look for pests
Turn leaves over for aphids, mites, eggs, or stippling. Remove badly damaged leaves with clean tools and monitor new growth.
Track changes
Mark the plant and recheck after adjusting moisture and airflow. Rapid wilt or collapsing stems require prompt local advice.