Early blight is a common tomato disease caused by the fungus Alternaria solani. It can affect almost all parts of the tomato plants, including the leaves, stems, and fruits. The plants may not die, but they will be weakened and will set fewer tomatoes than normal. Early blight generally attacks older plants, but it can also occur on seedlings. Stressed plants or plants in poor health are especially susceptible. Early blight is also a problem with potatoes.
Early blight is a difficult enough disease to deal with, but don't confuse it with the much more problematic late blight that can quickly kill tomato plants and spread for miles.
What Causes Early Blight
The early blight fungus can come from many sources. It can be in the soil, or it can already be on seeds or seedlings you purchase. It can even over-winter in the diseased debris of your tomato plants and it can persist in the soil or debris for at least one year. Although early blight can occur in any type of weather, it favors damp conditions, like frequent rain or even heavy dews.
Symptoms of Early Blight on Tomatoes
Early blight is sometimes confused with Septoria leaf spot. They both form spots on the leaves, which eventually turn yellow and die off, but Septoria also forms fruiting bodies that look like tiny filaments coming from the spots.
On Older Plants: Dark spots with concentric rings develop on older leaves first. The surrounding leaf area may turn yellow. Affected leaves may die prematurely, exposing the fruits to sun scald.
Dark lesions on the stems start off small and slightly sunken. As they get larger, they elongate and you will start to see concentric markings like the spots on the leaves. Spots that form near ground level can cause some girdling of the stem or collar rot. Plants may survive, but they will not thrive or produce many tomatoes.
On Tomato Fruits: If early blight gets on the fruits, spots will begin at the stem end, forming a dark, leathery, sunken area with concentric rings. Both green and ripe tomatoes can be affected.
On Seedlings: Affected seedlings will have dark spots on their leaves and stems. They may even develop the disease on their cotyledon leaves. Stems often wind up girdled.
Management and Control of Early Blight of Tomatoes
There are some tomato varieties that offer some resistance to early blight, although none are fully immune to it. Even if you buy a resistant variety, early blight is a very common problem with tomato plants and you may not be able to avoid it completely. These measures, however, can help you keep it under control.
- Certified Seed: Buy seeds and seedlings from reputable sources and inspect all plants before putting them in your garden.
- Air Circulation: Provide plenty of space for the plants. Good airflow will help keep the plants dry.
- Fungicides: Keep tabs on your plants, especially during wet weather or if your plants become stressed. If you see signs of blight on a single plant, removing the plant is your best option. Fungicides containing the active ingredients Penthiopyrad or Boscalid will be most effective.
- Garden Sanitation: Since early blight can over-winter on plant debris and in the soil, sanitation is essential. So many tomato diseases can come into your garden this way so it's very important to clean up all plant residue at the end of the season.
- Rotate Crops: If you have an outbreak of early blight, find somewhere else to plant your tomatoes next year, even if it's in containers.
Tomato Varieties With Some Resistance to Early Blight
All of these varieties are somewhat resistant to early blight, but none is completely immune.
- Aunt Ginny's Purple - Heirloom, indeterminate, beefsteak (16 oz.)
- Big Rainbow - Heirloom, indeterminate, bi-color beefsteak (16 oz.)
- Black Plum - Heirloom, indeterminate, plum (2 in.)
- Juliet - Hybrid, indeterminate, cherry (1 ounce)
- Legend - Open-pollinated, determinate, beefsteak (14-16 ounces)
- Manyel - Heirloom, indeterminate, yellow globe (8-10 ounces)
- Matt’s Wild Cherry - Heirloom, indeterminate, cherry (1/2 inch)
- Mountain Supreme - Hybrid, determinate, globe (6-8 oz.)
- Mountain Fresh Plus - Hybrid, determinate, globe (12 ounces)
- Old Brooks - Heirloom, indeterminate, globe (6-8 ounces)
- Tigerella (aka Mr. Stripey) - Heirloom, indeterminate, globe (4-6 oz.)
- Tommy Toe - Heirloom, indeterminate, cherry (1 inch)
University of Minnesota Extension Office. “Early Blight of Tomato.” Umn.edu. N.p., n.d. Web.