Why do tomato plant leaves turn yellow?
Written by
Kiana Okafor
Reviewed by
Prof. Martin Thorne, Ph.D.Yellowing tomato leaves indicate different plant health conditions that may require attention. Generalized yellowing or patterns of yellowing are typically observed due to deficiencies in nutrients. Diseases cause yellowing in well-defined patterns along with other symptoms. Environmental stress (heat or insufficient/too much watering) plays a large role as well. Each week, I take a photo of the leaf discolorations to track the progression of the condition. Accurate diagnosis saves both time and plants and avoids unnecessary treatments.
Nutrient Deficiencies
- Nitrogen shortage: Uniform yellowing of older leaves
- Potassium deficiency: Yellow edges with brown tips
- Apply balanced fertilizer every 3-4 weeks
- Test soil annually to identify deficiencies
- Use compost tea for gentle nutrient correction
Disease Indicators
- Fusarium wilt: Yellow leaves with brown stems
- Viral infections: Mottled yellow patterns
- Remove infected plants immediately
- Sterilize tools between plants
- Apply copper fungicides preventively
Environmental factors cause short-term yellowing. Heat stress, with temperatures above 90°F, causes a leaf to curl and turn yellow. Overwatering leads to root rot, which is evident in yellowing foliage. Underwatering causes drought stress with the same symptoms. I use shade cloth during heat waves. Use moisture meters to optimize watering. Mulching helps with soil temperature and moisture.
Scrutinize leaves closely for proper identification. Inspect both surfaces for signs of insects, such as spider mites. Trim stems to evaluate vascular tissue color: test pH and nutrient levels of the soil. I follow protocols and mail samples to the extension services. Compare observed symptoms with trustworthy online resources, document weather trends prior to the onset of symptoms.
Put prevention practices in place. Ensure proper spacing of plants for optimal air movement, rotate crops annually to break the disease cycle, and water from the soil level using drip irrigation. I use compost tea every two weeks. Use resistant cultivars like Celebrity tomatoes. Healthy plants are better able to resist yellowing than stressed plants.
Read the full article: 10 Common Tomato Diseases and Prevention Tips
