Tomato plants turn yellow primarily due to a few common reasons: nutrient deficiencies (especially nitrogen), fungal diseases like early blight, improper watering (too much or too little), and sometimes lack of sunlight.
Nutrient Deficiencies
- Nitrogen deficiency is the most common cause; older leaves turn yellow as nitrogen is mobilized to younger leaves.
- Iron deficiency causes yellowing mostly on young leaves with dark green veins.
- Magnesium deficiency can lead to yellow speckling on older leaves.
- Tomatoes are heavy feeders and need adequate fertilization to avoid yellowing from nutrient lack.
Diseases
- Early blight, a fungal disease, typically starts with yellowing and brown spots on lower leaves, spreading upwards.
- It thrives in wet, humid conditions and usually originates from soil-level spores splashing up.
- Removing affected leaves helps control the spread when fungicides are not used.
Watering Issues
- Overwatering can cause root rot leading to yellow leaves.
- Underwatering also stresses plants, causing leaf yellowing and wilting.
- Proper watering of about 1 inch per week or adjusted in hot weather is recommended.
Other Causes
- Lack of sufficient sunlight can cause lower leaves to yellow if shaded by upper leaves.
- Excess salts/minerals in soil from water can cause nutrient lockout and yellowing.
In summary, yellowing tomato leaves indicate stress often linked to nutrient shortages, disease, watering problems, or sunlight inadequacy. Identifying the pattern and accompanying symptoms can help narrow down the cause and guide corrective actions such as fertilizing, adjusting watering, removing diseased leaves, or increasing light exposure.
