Early blight on tomatoes

Early blight is the most common foliage disease of tomatoes in the eastern US, and in most summers it is also inevitable. Nearly every tomato planting shows some early blight by late July or August, even with good management. The practical goal is not elimination -- it is slowing the rate of.

Early blight is the most common foliage disease of tomatoes in the eastern US, and in most summers it is also inevitable. Nearly every tomato planting shows some early blight by late July or August, even with good management. The practical goal is not elimination — it is slowing the rate of defoliation enough that plants continue producing fruit through the season.

I don't grow tomatoes at my Long Island property, so this guide draws on University Extension research.

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The pathogen

Early blight is caused by Alternaria solani. Per Cornell Cooperative Extension, the fungus overwinters in infected plant debris in soil and on seeds. Conidia (spores) are produced on infected tissue and splash-dispersed from soil and lower leaves onto upper foliage. The disease is favored by warm temperatures (75–85°F, 24–29°C) with intermittent wet periods, which describes most of the tomato-growing summer in the mid-Atlantic.

Note: Alternaria solani also causes early blight on potatoes (see Early blight on potatoes), but tomato and potato strains are closely related and management is similar.