If you find dark brown spots on the lower leaves of tomato or potato plants with concentric rings inside them like an archery target, and a yellow halo around them, you are most likely looking at early blight, caused by the fungus Alternaria solani. It is a disease that starts quietly at the bottom and then climbs leaf by leaf, until you find the plant, midway through the season, bare at the bottom with its fruit exposed to the sun.
But before you spray anything, answer one question with confidence: is it early blight or late blight? The two names are close, but the difference between them is the difference between a disease you manage over the whole season and one that can wipe out a field in two weeks, and each has completely different fungicides.
The disease: early blight (Alternaria) shows brown spots with concentric rings on the lower leaves first.
The difference: it is marked by target-like rings and infects the older leaves first, unlike the fast, water-soaked late blight.
Conditions: it is active with warm weather and alternating moisture and dryness, and becomes severe on stressed, poorly fed plants.
Common mistake: leaving infected lower leaves on the plant, so the disease climbs to the leaves above.
The rule: remove infected leaves, feed the plant in a balanced way, rotate crops, and spray preventively before the disease intensifies, not after.
Early blight: a fungus that lives on residue and waits for a tired plant
The cause is a fungus of the genus Alternaria. On potato the main species is Alternaria solani, and on tomato a close species called Alternaria linariae (also known as A. tomatophila) shares the role. It infects tomato and potato, and sometimes eggplant and pepper, and some nightshade-family weeds such as black nightshade.
How does it survive between seasons? On infected plant residue and in the soil for several years, on volunteer plants and nightshade weeds, and it may arrive with seeds or seedlings. From these sources its spores travel by wind, splashing water and overhead irrigation to the lower leaves first.
It is essentially a disease of older tissue: a young, vigorous plant is rarely severely affected, while the disease intensifies at the start of heavy fruit load and plant stress.
Symptoms on each part of the plant
The best-known sign of early blight is the spot with concentric rings, but it is not limited to leaves. It may also appear on the stem, fruit and tubers, and its look differs slightly on each part:
- Leaves: it almost always starts on the older, lower leaves, with small dark dots that grow to a centimeter or more, with clear concentric rings inside, and often a yellow halo around them. The spots may be bounded by the veins and take a slightly angular shape, and as they multiply the whole leaf yellows and dries, and often stays hanging on the plant.
- Stem: slightly sunken dark spots on stems and petioles, and in small seedlings collar rot may occur: a dark lesion at the base of the stem near the soil surface that weakens or kills the seedling.
- Fruit: in tomato the infection usually enters from the calyx area, producing a dark, leathery, sunken spot at the stem end, which may carry rings, and small infected fruit may drop.
- Tubers: in potato, dark sunken round or irregular spots on the skin, with slightly raised edges and dry, corky brown tissue beneath. Infection usually happens at harvest through wounds.
Examples by crop
- Tomato: spots climbing from the bottom up with leaf drop, which exposes the fruit to sunscald, with collar rot in seedlings and stem-end spots on fruit.
- Potato: ring spots on large leaves near the end of the season, and the biggest loss is in weak yield and spotted tubers in storage. Late-maturing varieties are generally more tolerant.
- Eggplant: ring spots on leaves and fruit in severe infections.
Early blight or late blight?
Late blight is caused by Phytophthora infestans, which is not a true fungus but an oomycete, closer to the cause of downy mildew, which is why most early blight fungicides do not help against it: (downy mildew)
- Weather: early blight likes warmth with dew. Late blight likes cool, highly humid, cloudy weather.
- Shape of the spot: early blight gives dry dark spots bounded by rings with a yellow halo. Late blight gives large, irregular, pale green or dark spots that look wet and are not bounded by the veins.
- Underside of the leaf: in late blight white fuzzy growth appears at the edges of the spots on the lower surface on a humid morning. You will not find it in early blight.
- Where it starts: early blight from the lower, older leaves. Late blight may start anywhere, even at the top.
- Speed: early blight progresses gradually over weeks. Late blight can kill plants within roughly two weeks if conditions allow, and sometimes a smell of decay comes off the field.
- Fruit: in late blight, large, firm, shiny brown spots look as if painted. In early blight, a dark leathery spot at the stem end.
Other diseases that may look like it
- Septoria leaf spot: many small spots with light gray centers and tiny black dots, without rings.
- Bacterial spot: small dark spots with an oily look and no rings.
- Gray mold: spots covered by a velvety gray layer that releases dust.
The disease cycle and the conditions that ignite it
The spore reaches the lower leaf from the soil or residue, and needs free water to germinate and penetrate the leaf. After a few days the spot appears and starts producing new spores on its surface, which travel by wind and splash to the upper leaves and neighboring plants, and the cycle repeats many times during the season.
- Temperature: the best temperature for infection is roughly between 24 and 29 degrees Celsius.
- Wetness: night dew is enough, and alternating night wetness with day dryness suits the fungus perfectly, because spores form and spread more with dryness after wetness.
- Plant stress: heavy fruit load, nitrogen deficiency, thirst, and heavy nematode or other pest pressure.
- Overhead irrigation: and water splash that carries spores from the soil to the lower leaves.
- Repeated planting: of nightshade crops in the same field.
Solutions: biological, then chemical with smart rotation
Early blight is a disease managed over the whole season, not cured with one spray. The foundation is a multi-site protectant, with systemic fungicides added in rotation, starting early from the lower leaves.
Biological control
Biological products registered worldwide for early blight rely on strains of the bacteria Bacillus subtilis and Bacillus amyloliquefaciens, which work preventively by suppressing spore germination on the leaf surface. Their effectiveness is moderate and they give the best results in a rotation program with chemical fungicides and in early infections. There is currently no specialized biological fungicide for early blight in our store.
Fungicides and FRAC group rotation
Early blight control programs worldwide rely on multi-site protective fungicides as the base, with systemic fungicides added from the strobilurins (FRAC 11), triazoles (FRAC 3) and SDHI inhibitors (FRAC 7). Alternaria isolates have been recorded with resistance to strobilurins, and SDHI fungicides are also prone to resistance, so rotation is not a luxury.
Start spraying at the first spots on the lower leaves, or preventively if your field has a history with the disease and the weather is warm with dew, and cover the lower leaves well because that is where it starts.
Suitable fungicides from the Orkida store
At Orkida we offer a copper protectant for early blight that forms the base of the program, with systemic fungicides from different groups to rotate between. These are the options and when we use each:
- Score 250 EC: from Syngenta, difenoconazole (FRAC 3), a systemic active ingredient that inhibits ergosterol formation in the fungal cell membrane. We consider it one of the strongest options against early blight and Stemphylium spot on tomato, and it is also used on potato, pepper and cucurbits.
- Bellis 38% WG: from BASF, boscalid (FRAC 7) with pyraclostrobin (FRAC 11), two active ingredients that stop fungal respiration at two different sites, covering early blight on potato and tomato, along with gray mold and powdery mildew. An ideal partner for Score because it shares no group with it.
- Ortiva Top 325 SC: from Syngenta, azoxystrobin (11) with difenoconazole (3), covering Alternaria and Septoria leaf spots, early blight and powdery mildew.
- Miravis Duo 200 SC: from Syngenta, pydiflumetofen (7) with difenoconazole (3), for blights, leaf spots and powdery mildew.
- Delta Copper 850 WP: from the Delta factory, copper oxychloride 85% (FRAC M1), the multi-site protectant that forms the base of the program, preventing Alternaria spore germination on the leaf surface, and no resistance develops to it. Alone it is weaker than systemic fungicides in an active infection, so we spray it preventively at the start of the season and add it to the Score or Bellis tank when pressure rises. It is used on tomato, potato, pepper and eggplant at the dose and pre-harvest interval written on the label.
Note: Score, Ortiva Top and Miravis Duo all contain difenoconazole. If you spray one and then another, you have repeated group 3. And Bellis and Ortiva Top share group 11, while Bellis and Miravis Duo share group 7.
A practical sequence in which no group repeats in two consecutive sprays: Bellis (7+11), then Score (3), then Bellis, then Score, and Delta Copper accompanies the whole program because it is from a completely different group (M1).
And if you want Ortiva Top or Miravis Duo, make the spray that follows them one that shares no group with them. Stick to the number of sprays and the pre-harvest interval written on each product's label.
And if you find the disease is late blight, do not settle for these fungicides, since it needs fungicides specialized for oomycetes and fast action.
Prevention: cut the path between the soil and the leaves
The fungus starts from the soil and residue and reaches the lower leaves by splash, so effective prevention cuts this path and keeps the plant unstressed all season.
General points
- Crop rotation: do not plant tomato, potato, eggplant or pepper in the same place where one of them grew the previous season.
- Remove residue: after harvest and do not leave it on the soil surface, and get rid of nightshade weeds and volunteer plants.
- Cut the splash: drip irrigation instead of overhead spraying, and covering the soil with mulch or plastic stops spores flying from the soil to the leaves.
- Balanced nutrition: do not deprive the plant of nitrogen at fruiting time, since its lack speeds up the disease, and balance it with phosphorus and potassium.
- Healthy seed and seedlings: from a trusted source.
- Tolerant varieties: there is no fully resistant variety, but some varieties are more tolerant.
And these are tips specific to the two most affected crops:
- Tomato: lift the plant off the ground by staking, and remove lower leaves that touch or are near the soil, especially if the first spots appear on them. But do not strip the plant too much, since fruit needs leaf cover to protect it from the sun.
- Potato: harvest after full maturity and handle tubers gently to reduce wounds. Store tubers in a well-ventilated place with moderate humidity, discard spotted ones before storage, and keep new potato fields away from infected tomato or potato fields.
Frequently asked questions about early blight
Should I cut off infected leaves?
Yes, at the start of infection: removing spotted lower leaves reduces the source of spores. Take the cut leaves out of the field and do not leave them on the soil.
Is tomato fruit from an infected plant edible?
Healthy fruit from an infected plant is fine after the fungicide's safety interval has passed. A fruit with a rotting spot at the stem end is better discarded, since it is not marketable and rots quickly.
Is early blight transmitted through seeds?
Yes, it can be, and it may cause collar rot in seedlings. Use seeds and seedlings from a trusted source.
Why does early blight appear every year in the same field?
Because the fungus survives in plant residue and soil for years, and on nightshade weeds. The solution is crop rotation, removing residue and mulching the soil, not more sprays.
What is the difference between early blight and late blight in brief?
Early blight is a warmth-loving fungus that starts on the lower leaves with ring spots and a yellow halo and advances slowly. Late blight is a fungus-like organism that likes cool, humid conditions, with large water-soaked spots and white fuzz under the leaf, and it destroys a field quickly.
Explore other crop diseases and pests in the pest library.