Water the soil, not the leaves. A film of water on a leaf is what most fungal spores need in order to germinate.
Well established · suboptimal if ignored → advise
How it works
A spore landing on a leaf needs liquid water on that surface to germinate and enter it. Water delivered to the soil never puts a film there; water thrown over the plant does, and the longer it lasts the more of the spores already present can use it.
What to do about it
Put the water at the base - a soaker hose, a drip line, or a wand held low - so the foliage stays dry. If overhead is all you have, water when the leaves will dry quickly rather than staying wet into the night.
How much it matters: Not quantified by any source read. The mechanism is stated categorically - spores require the water film - but no source read gives an infection rate, a wetness-hours threshold, or a difference in disease incidence between watering methods. Deliberately left unquantified.
The evidence
Evidence: confirmed against 3 sources in the research-based literature.
the 3 sources
Univ. of Wisconsin-Madison Div. of Extension, Wisconsin Horticulture, 'Water, Water, Everywhere, but Never on the Leaves', hort.extension.wisc.edu/2021/04/01/water-water-everywhere-but-never-on-the-leaves (fetched in-container 2026-08-02, READ AND CONFIRMED BY THE MAINTAINER 2026-08-02): the mechanism, in the source's own words - overhead watering creates 'a "leaf wetness period," a time when a thin layer of water coats the leaf', and 'This layer of water is exactly what most plant diseases-causing fungi require for their spores to germinate and infect.' The remedy: 'a soaker or drip hose that directs water into the soil rather than onto leaves.' NOTE FOR ANYONE TEMPTED TO ADD AN HOUR TO THIS RULE: this page names no time of day at all. [read]
Iowa State Univ. Extension, Yard & Garden, 'Managing Diseases in the Vegetable Garden', yardandgarden.extension.iastate.edu/how-to/managing-diseases-vegetable-garden (fetched in-container 2026-08-02, READ AND CONFIRMED BY THE MAINTAINER 2026-08-02): 'Soaker hoses, drip irrigation, or watering at the base of the plant all help keep the foliage dry.' And why it matters: 'Most disease-causing microorganisms, like fungi, require water to proliferate. Dry foliage helps slow or prevent their growth.' This page likewise names no watering hour. [read]
Penn State Extension, 'Garden Myths: Watering', extension.psu.edu/garden-myths-watering (fetched in-container 2026-08-02, READ AND CONFIRMED BY THE MAINTAINER 2026-08-02): 'If possible, avoid watering at the foliage of the plant which can lead to disease.' THIS IS THE ONLY SOURCE READ THAT NAMES AN HOUR - 'The best time for watering plants is in the early morning to avoid evaporation and reduce the potential for disease' - and one source's hour, contradicted elsewhere in the same literature by a mid-afternoon recommendation for gardeners with only a sprinkler, is why the clock is in this rule's scope_note rather than in its claim. [read]
Where this rule stops
This rule is about where the water goes, not what time you go out. Of the sources read, one names early morning and another suggests mid-afternoon when overhead watering is unavoidable; both are reasoning about how fast the leaves dry, not about the clock. So the rule says what they agree on and leaves the hour alone. It also says nothing about how MUCH or how OFTEN to water - that is a weather question, and the answer to it is the refusal one rule up.