Early Blight vs. Late Blight on Tomatoes: Identification and Management

"Blight" is a term used loosely to describe rapid browning and death of plant tissue on tomatoes, but the two diseases commonly called blight are caused by fundamentally different organisms and require different management responses. Using copper fungicide for late blight is inadequate. Using a.

"Blight" is a term used loosely to describe rapid browning and death of plant tissue on tomatoes, but the two diseases commonly called blight are caused by fundamentally different organisms and require different management responses. Using copper fungicide for late blight is inadequate. Using a mefenoxam-based oomycide for early blight is overkill and unnecessarily expensive.

The ability to tell them apart — before spraying — is a practical skill that makes a measurable difference in outcomes.

Pathogen Distinction

This distinction drives all subsequent management decisions.

Per Penn State Extension, early blight is caused by Alternaria solani — a true fungus with chitin-based cell walls. Most conventional and organic fungicides have activity against true fungi.

Per Cornell Cooperative Extension, late blight is caused by Phytophthora infestans — an oomycete (water mold) with cellulose-based cell walls, more closely related to algae than to true fungi. The cell wall difference means fungicides targeting fungal chitin synthesis (most conventional fungicides) have limited effect on P. infestans. Specific oomycide-class products are required for effective late blight control.