Brown leaf spots with target-like rings, starting on the lower leaves of tomato or potato, are early blight - a fungus that overwinters in plant debris and soil and reaches the leaves by soil splash.
Promising · ignored, it still works, just worse · the planner suggests it
How it works
The early blight fungi overwinter in infected plant debris and in the soil. Lower leaves are infected when they touch contaminated soil or when rain splashes soil up onto them; the spores need free water or very high humidity to germinate. Spots enlarge with concentric rings, and fruit can form leathery black spots.
What to do about it
Mulch the soil under the plants and stake them, to keep soil and leaves apart; water at the base rather than over the leaves. Remove spotted leaves early - no more than a third of the plant. Grow resistant varieties (often marked EB), rotate out of tomatoes and their relatives for at least two years, and remove or bury infected plants in the fall.
How much it matters: No yield-loss figure is invented. Resistance lessens the damage rather than preventing it, as the source says; the remedy is a set of cultural steps whose combined effect depends on the season's wetness. The rule advises and never blocks.
The evidence
Evidence: confirmed against 1 source in the research-based literature.
the source
UMN Extension, 'Early blight in tomato and potato', extension.umn.edu/disease-management/early-blight-tomato-and-potato (fetched in-container 2026-09-25, read 2026-09-25): 'Larger spots have target-like concentric rings.'; 'The early blight pathogens both overwinter in infected plant debris and soil in Minnesota.'; 'Lower leaves become infected when they come into contact with contaminated soil, either through direct contact or when raindrops splash soil onto the leaves.'; 'Cover the soil under the plants with mulch, such as fabric, straw, plastic mulch, or dried leaves.'; 'Water at the base of each plant, using drip irrigation, a soaker hose, or careful hand watering.'; 'Do not remove more than one-third of the plant's leaves.'; 'Rotate out of tomatoes and related crops for at least two years.'; 'Spores (reproductive structures) can germinate between 47° and 90° F and need free water or relative humidity of 90% or greater.'; 'Fruit spots are leathery and black, with raised concentric ridges.'; 'Staking will also reduce contact between the leaves and spore-contaminated soil.'; 'Remove leaves with leaf spots and bury or burn them.'; 'There are many resistant tomato cultivars available, often designated with an "EB" in seed catalogs.'; 'In the fall, remove or bury infected plants to reduce the likelihood of the pathogen surviving into the following year.'
Where this rule stops
Scoped by applies_to.species to tomato and potato, the two hosts the source names, so it surfaces on those plant pages and answers the "brown spots with rings" sign in the Something's-wrong picker (symptoms.ts). It gates nothing. The two-year rotation is the source's figure for the home garden and sits alongside, not in place of, the nightshades rotation rule, which carries its own interval and sources.
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 - early blight on tomato and potato
Cite this page
Milpa Gardens. "Brown leaf spots with target-like rings, starting on the lower leaves of tomato or potato, are early blight - a fungus that overwinters in plant debris and soil and reaches the leaves by soil splash." Evidence grade: Promising. https://milpa.garden/rules/brown-leaf-spots-target-like-rings-starting/
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.