Lower tomato leaves that yellow after dark spots appear have a fungal leaf disease (early blight or Septoria leaf spot); leaves that yellow without spots, especially in containers, can be short of nitrogen.
Promising · ignored, it still works, just worse · the planner suggests it
How it works
Early blight and Septoria leaf spot are fungal diseases that begin on the lower foliage in wet weather as deep brown or blackish spots, followed by yellowing or browning of the leaves - early blight with target-like rings, Septoria with whitish centres and dark borders. Separately, tomato leaves can yellow from a lack of nitrogen, which is common in containers because frequent watering and low soil fertility leave little of it.
What to do about it
Where the yellowing began with spots, treat it as leaf disease: remove the infected leaves, mulch to stop soil splashing onto the lower leaves, space or stake for air, and rotate and choose resistant varieties next year. Where the leaves yellow without spots, nitrogen may be short - get a soil test before feeding; do not over-apply.
How much it matters: No fertilizer amount or rate is given - invariant 3: the engine says "get a soil test" and never invents NPK. The source itself names no amount and warns against over-applying. The rule advises and never blocks.
The evidence
Evidence: confirmed against 1 source in the research-based literature.
the source
UW-Madison Extension (Wisconsin Horticulture), 'Troubleshooting Tomato Problems', hort.extension.wisc.edu/troubleshooting-tomato-problems/ (fetched in-container 2026-09-25, read 2026-09-25): 'Early blight and septoria leafspot are fungal leaf diseases infecting tomato plants, especially during wet weather.'; 'Initial symptoms typically appear on the lower foliage of plants, starting as deep brown or blackish spots, followed by yellowing or browning of leaves.'; 'Early blight spots will develop concentric rings and a target-like appearance, often with a yellow halo. Septoria spots typically have whitish centers and dark borders.'; 'Tomato leaves may also show yellowing from lack of nitrogen. This is especially common in containers, due to frequent watering and low soil fertility.'; 'Adding nitrogen fertilizer will help alleviate this problem but be careful not to over-apply.'; 'Removing infected leaves, or even entire plants, might help reduce spreading of disease.'; 'Mulching will help reduce soil splashing onto lower foliage, which often includes fungal spores.'; 'Improve air circulation around tomato plants by avoiding closely spaced plantings, using cages, or staking.'; 'In future years, practice crop rotation and choose early blight- and septoria-resistant tomato cultivars.'
Where this rule stops
Scoped by applies_to.species to tomato, so it surfaces on the tomato page and answers the "lower leaves turning yellow" sign in the Something's-wrong picker (symptoms.ts). It gates nothing. It is the sorting rule: spots first points to the leaf-disease case (early blight, another rule here, among them), no spots to the nutrient case. The soil-test step is this corpus's own (invariant 3), not the source's, which says only to add nitrogen with care.
Plants this fires on
Change history
Every change to this rule is one commit. This is what the project's version history records — the date and the reason — not a summary of it.
- corpus(another rule here): new rule, Promising, verified - yellowing lower tomato leaves - spots or nitrogen
Cite this page
Milpa Gardens. "Lower tomato leaves that yellow after dark spots appear have a fungal leaf disease (early blight or Septoria leaf spot); leaves that yellow without spots, especially in containers, can be short of nitrogen." Evidence grade: Promising. https://milpa.garden/rules/lower-tomato-leaves-that-yellow-after-dark/
A garden-planning corpus: every claim states its evidence grade and links its sources on the page. Cite it with the grade, and add the date you read it.