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Diagnosing Tomato Diseases - Is it Early Blight, Late Blight, or Septoria?

By July 5, 2009

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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.
Disease Early Blight Late Blight Tomato Leaf Spot
Responsible Fungus Alternaria solani Phytophthora infestans Septoria lycopersici
Description of Foliar Damage One or two spots per leaf, approximately to 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 Damage Dark, 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 Damage Dark, 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 Conditions High 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 Treatment Remove 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


August 21, 2011 at 5:29 pm
(1) Amanda says:

I have a tomato problem that I can’t solve. Nothing I’m researching sounds exactly like what’s going on. The leaves turn brown (no spots or rings or other colorations) and have a dusty texture. (If I don’t wash my hands well enough after touching them, my sink and towels are stained yellow.) Though completely brown, the leaves stay alive for a long time where they are attached to the main stem. I did have a bit of a blossom rot problem early in the season, but otherwise, the fruits are totally unaffected by whatever this problem is. I have 8 different types of tomatoes in my garden. The black cherry tomatoes seem most affected and I actually have yellow pear shaped tomato plants that seem just fine. None of the plants showed any signs of this disease when planted. Recently, some of them have started new growths at their bases. Does anyone know what this problem is and how to solve it?

September 18, 2011 at 5:21 pm
(2) Susan says:

I can’t help you with the disease issue, but as tomato plants mature, they will always stain your hands yellow. I’ll often have to soap up to my elbows a few times to get it all off.

September 22, 2013 at 10:16 am
(3) Gabby says:

I had a tomato plant that was infected with what looked like early blight. I got rid of the plans but I do not know what to do with the pot and the soil it was in. Should I get rid of it? Or can I still plant something?

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