Tar spot infections remain invisible to the eye for days before lesions appear, and that latent period breaks the logic of "see and spray" fungicide programs, Michigan Farm News reports.

The finding carries direct cost implications for corn growers. A fungicide applied only after lesions become visible arrives after the pathogen has already established itself inside the leaf tissue. By the time a sprayer rolls, the treatment window for effective suppression has effectively closed.

Why the latent period changes field decisions

Tar spot, caused by the fungus Phyllachora maydis, does not announce its arrival. Spores land on the leaf, germinate, and colonize tissue beneath the epidermis. The characteristic black stromata — the shiny black spots that give the disease its name — emerge only after this internal development has run its course.

That delay is the crux of the problem Michigan Farm News identifies. A grower who waits to see lesions before spraying is reacting to infections that began days earlier. During the latent period, the pathogen spreads within the canopy while scouting reports still read clean.

The practical consequence: calendar-based or risk-based spray timing, not visual confirmation, governs effective tar spot management.

What growers should do instead

The report points agronomists away from reactive spraying and toward anticipatory decisions. Several shifts follow from the latent-period problem:

• Treat disease-risk indicators, not lesion sightings, as the trigger for fungicide applications
• Factor in weather conditions favorable to infection — moisture and humidity during critical corn growth stages — rather than waiting for visible symptoms
• Scout with the understanding that a clean-looking field can already harbor developing infections
• Treat confirmed tar spot in one part of the operation as a warning for neighbouring fields, since invisible infections may be underway elsewhere

The economics matter. Uncontrolled tar spot can strip yield from infected corn, and a mistimed fungicide pass wastes chemical, fuel and labour while delivering little protection. A spray decision made on visual evidence fails on both counts: it is late for the plants already infected and uninformative about the plants that soon will be.

A caution against misplaced confidence

"See and spray" technology has earned its place in weed control, where targets sit in plain view and treatment matches the moment of detection. Tar spot inverts that logic. The target is inside the leaf before it is on the leaf.

Growers who transfer the weed-control model to foliar disease management without accounting for the latent period risk a false sense of security. The report's core message is that absence of lesions is not absence of disease.

What to watch next

Michigan Farm News's analysis signals that tar spot management advice will continue moving toward predictive, weather-driven spray models rather than symptom-triggered applications. Growers in tar spot-affected regions should track regional disease forecasting as the season develops, and consult their agronomists on risk-based fungicide timing before lesions ever become visible in the canopy.