Diagnosing Tomato Diseases

Is it Early Blight, Late Blight, or Septoria?

Tomato blight
Aleksandra Semyonova / EyeEm / Getty Images

The information in the table below will help you determine which tomato disease you are dealing with. The symptoms of early blight, late blight, and leaf spot can be very similar, but if you know what to look for, you shouldn't have much trouble figuring out what you are dealing with. Included in the table are known organic treatment methods for each type of disease.

DiseaseEarly BlightLate BlightTomato Leaf Spot
Responsible FungusAlternaria solani Phytophthora infestansSeptoria lycopersici
Description of Foliar DamageOne or two spots per leaf, approximately ¼ to ½ inch in diameter. Spots have tan centers with concentric rings in them and yellow halos around the edges.Spots start out pale green, usually near the edges of tips of foliage, and turn brown to purplish-black. In humid conditions, a fuzzy mold appears on the undersides of leaves.Numerous brown spots appear on the leaves, approximately 1/16 to 1/8 inch in diameter. The spots lack a yellow halo, and, upon close inspection, have black specks in the center.
Description of Fruit DamageDark, sunken spots appear on the stem end of fruits.Brown, leathery spots appear on green fruit on the top and sides of the fruit. In humid conditions, white mold also forms.Fruit are not affected, though sun scald can be a problem due to foliage loss.
Description of Stem DamageDark, sunken cankers at or above the soil line.Black and brown spots appear and spread. Entire vines can be killed very quickly in periods of high humidity.No stem damage.
Optimal ConditionsHigh humidity, and temperatures above 75 degrees F.High humidity, temperatures between 60 and 80 degrees F.High humidity, temperatures between 60 and 80 degrees F.
Organic TreatmentRemove lower leaves after first fruit sets, remove affected leaves as they appear, plant tomatoes in a different area next year.Pull and destroy the plant, select resistant varieties next year, and plant tomatoes in a different area of the garden.Remove infected foliage as it appears, clean tools before moving to another plant. Plant tomatoes in a different area of the garden next year.

Photos of Tomato Diseases

Early Blight

Late Blight

Tomato Leaf Spot