Hi Yvonne,
Given the current warm weather and recent May-June rains, its possible the yellowing could be a result of overly saturated soil.

That said, its not uncommon for the lower leaves of tomato plants to naturally yellow as plant resources are directed toward new growth.

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In a more typical, less rainy spring, yellowing lower leaves on your tomato plants could be due to several common issues.

Possible Causes

Natural Aging:

• Reason: Lower leaves naturally yellow and drop as the plant matures, redirecting energy to fruit production. This is normal, especially mid-season (June).
• Check: If only the oldest, bottom leaves are affected and the plant looks healthy otherwise (green upper leaves, fruit setting), it’s likely not a problem.
• Action: Trim yellow leaves with clean shears to improve airflow, but don’t overdo it—remove no more than 1/3 of foliage.

Watering Issues:

• Overwatering: Excess moisture from recent rains or overzealous watering can cause root rot, leading to yellowing. Tomatoes prefer consistent moisture but dislike soggy soil.
• Underwatering: Dry spells between rains can stress plants, causing lower leaves to yellow first.
• Check: Soil should be moist 1–2 inches deep but not waterlogged. Test by sticking a finger in—if it’s soggy or bone-dry, adjust.
• Action: Water 1–2 inches per week if rain is insufficient, ensuring good drainage (e.g., raised bed edges). Mulch with straw or black mulch to retain moisture.

Nutrient Deficiency:

• Nitrogen: Lack of nitrogen causes yellowing, starting at lower leaves, as it’s pulled upward for new growth.
• Check: Pale yellow leaves with stunted growth suggest nitrogen deficiency. Soil test kits (available at Garden Factory) can confirm.
• Action: Apply a balanced fertilizer (e.g., 10-10-10) or compost. Avoid over-fertilizing, which can burn roots.

Disease:

• Early Blight: Common in humid climates like Webster, this fungal issue (Alternaria solani) causes yellowing with brown spots and concentric rings on lower leaves.
• Fusarium or Verticillium Wilt: These fungal diseases block water uptake, yellowing leaves and wilting stems. Look for wilting despite moist soil.
• Check: Inspect for spots or wilting. Early blight thrives in wet foliage; wilts show vascular browning if stems are cut.
• Action: Remove affected leaves (bag and discard, don’t compost). Use fungicide (e.g., copper-based) if severe, and space plants 2–3 ft apart for airflow. Rotate crops next year.

Pests:

• Aphids or Whiteflies: These can suck sap, weakening leaves and causing yellowing.
• Check: Look under leaves for tiny bugs or sticky residue.
• Action: Spray with neem oil or insecticidal soap. Encourage ladybugs or lacewings.

Here in Webster

• Weather: Recent rains may have oversaturated soil, especially if drainage is poor. Check your bed’s slope or add organic matter to improve it.
• Soil: Webster’s clay-heavy soil can retain too much water, exacerbating root issues. Mixing in sand or compost could help.

Next Steps

• Prioritize: Start with watering and aging checks—most likely culprits. If persistent, inspect for disease or pests.
• Monitor: Yellowing should stabilize with proper care. If it spreads upward or includes wilting, suspect disease.
• Prevention: Stake plants to lift foliage, reducing soil contact and disease risk.

Hope this helps!
Zoe