Cucumber leaves turn yellow primarily because of three main reasons: nutrient deficiencies (especially nitrogen), overwatering, and pest or disease problems. Other factors like insufficient or excessive sunlight, temperature stress, and natural leaf aging can also cause yellowing leaves.

Common Causes of Yellow Cucumber Leaves

• Nutrient Deficiency: Lack of nitrogen is the most common cause, as nitrogen is vital for chlorophyll production, which gives leaves their green color. Magnesium deficiency can also cause yellowing, often with green veins remaining (interveinal chlorosis). Using balanced fertilizer with nitrogen, potassium, and phosphorus helps prevent this.

• Overwatering or Underwatering: Overwatering can lead to root rot, causing yellow and limp leaves starting from the top, while underwatering causes dehydration and stress, resulting in yellow, crispy leaves. Cucumber plants need consistent moisture, about 1 inch of water per week, avoiding soggy soil.

• Pests and Diseases: Pest infestations and fungal infections can cause yellow spots or widespread yellowing. Managing pests early and maintaining good air circulation helps.

• Sunlight and Temperature Stress: Cucumbers require 6-8 hours of sunlight daily. Too little light weakens plants, causing yellowing, while excessive sun and heat can burn leaves and lead to yellow, dry edges. Ideal temperatures are between 21-29°C (70-85°F).

• Natural Leaf Aging: Older leaves at the bottom often turn yellow as they age and lose photosynthetic ability, which is normal. Removing these old leaves prevents disease spread.

Recommendations to Fix Yellow Leaves

• Apply a balanced fertilizer with nitrogen during vegetative growth and magnesium during flowering.
• Adjust watering practices to keep soil evenly moist but not waterlogged.
• Ensure cucumber plants get adequate but not excessive sunlight.
• Monitor for pests and diseases and take action promptly.
• Remove old yellowing leaves to maintain plant health.

By addressing these factors, cucumber plants can regain healthy green foliage and improve yield.