Garden
Plant Problems
Septoria Leaf Spot: The Tell Is Inside the Spot
Look closely at one spot. Septoria makes many small round spots, none bigger than a quarter inch, with dark edges and pale centers — and tiny black dots inside each pale center. Those dots are the giveaway. It starts on the lowest leaves after the first fruit sets and works upward.
The tell is inside the spot
Plenty of things put spots on a tomato leaf. This one signs its work.
Missouri Botanical Garden: spots are "circular, about 1/16 to 1/4 inch in diameter with dark brown margins and tan to gray centers with small black fruiting structures." Those small black structures are the fungus's own spore cases, sitting in the middle of each pale spot. Get close, or use your phone camera zoomed in. Once you have seen them you cannot unsee them.
Wisconsin describes the same thing from the outside — "whitish spots with dark borders characteristic of the disease" — and puts the size at about a quarter of an inch.
The pattern is as diagnostic as the spot. Missouri Botanical: "Characteristically, there are many spots per leaf." Not three or four. Dozens, on a single leaf, all about the same size.
It starts low and climbs
Where it appears, and when, is the second half of the identification.
Missouri Botanical: Septoria "usually appears on the lower leaves after the first fruit sets", and "This disease spreads upwards from oldest to youngest growth." Wisconsin agrees on the starting point — symptoms "first appear at the base of affected plants".
So the shape of the problem over a few weeks is a plant going bare from the ankles up. Wisconsin is blunt about where that ends: multiple spots merge, and it "can lead to total defoliation of lower leaves and even the death of an infected plant."
One piece of good news, and it is worth knowing before you panic. Missouri Botanical: "Fruit infection is rare." What you are protecting is the leaf area that ripens the fruit, not the fruit itself.
Septoria or early blight?
These two get mixed up constantly, and Wisconsin puts them in the same sentence: Septoria is "one of two common fungal diseases that can devastate tomatoes", and "The second common tomato blight is early blight."
They start in the same place and travel the same direction, so the spot itself is what separates them. Missouri Botanical describes early blight's mark as spots that "enlarge into a series of concentric rings surrounded by a yellow area" on the "lower, more mature leaves", progressing "from the base of the plant, upward" — and the cost as "extensive defoliation, exposing fruit to sunscald and reducing yields."
One aside that is more useful than it looks. The same page notes that "occasionally, flea beetles transmit this disease" — so if you have been watching shot-holes appear in the same bed, the two problems may not be unrelated.
| What you are looking at | Septoria leaf spot | Early blight |
|---|---|---|
| Number and size of spots | Many per leaf, and characteristically so. One sixteenth to one quarter of an inch, fairly uniform. | Circular or irregular dark spots that enlarge, rather than many small ones that stay small. |
| What the spot becomes | Stays a spot. Tan to gray center with small black fruiting structures in it. | Enlarges into a series of concentric rings surrounded by a yellow area. |
| Where it starts | Lower leaves, after the first fruit sets. | Lower, more mature leaves. Both start at the bottom, which is why the spot decides it. |
| Which way it travels | Upwards from oldest to youngest growth. | Also progresses from the base of the plant upward. |
| What it costs you | Total defoliation of lower leaves and even plant death. Fruit infection is rare. | Extensive defoliation, exposing fruit to sunscald and reducing yields. |
Why it matters which one you have
Both are managed the same way at the level of airflow and watering, so an honest answer is that the first steps do not change much. What changes is what you expect: our page on early and late blight exists because one of those two is a season-long nuisance and the other kills a plant in days and threatens the neighborhood. Septoria belongs firmly in the first category. If you have talked yourself into late blight, the spot count and the black specks are how you talk yourself back out.
What to do, and where to stop
Missouri Botanical publishes a numbered plan. These are the parts that cost nothing and the one caveat almost nobody prints.
- Take off the lowest infected leaves — but only those. Missouri Botanical says that if caught early, the lower infected leaves can be removed and burned or destroyed. Then the limit: removing leaves above where the fruit has formed will weaken the plant and expose the fruit to sunscald. Strip from the bottom, stop at the fruit.
- Stop watering from above. Missouri Botanical is direct about it — overhead watering facilitates infection and spreads the disease. A soaker hose at the base, early in the day, so leaves have the day to dry.
- Mulch, once the soil has warmed. The spores live in debris on the ground and travel up in splashed soil. Mulch is there to stop the splash, and the timing matters because mulching cold soil keeps it cold.
- Pull nightshade and horsenettle. The fungus overwinters on weeds in the nightshade family, the same family as the tomato. Those weeds are a reservoir sitting next to your crop.
- Clean the stakes and cages. This is the step people skip. Missouri Botanical: the fungus can also survive on equipment such as plant stakes and cages. Last year's hardware is a way for the disease to arrive before the plant has a single leaf.
We are not printing a spray schedule. Both sources name fungicide options and both are clear about the limit — Missouri Botanical: "Fungicides will not cure infected leaves, but they will protect new leaves from becoming infected." Wisconsin adds that they work "if they are carefully applied very early in the course of the disease". Which is a real constraint: by the time most people identify this, the early window has gone.
Septoria questions people actually ask
How do I tell Septoria leaf spot from early blight?
Count the spots and look inside one. Septoria makes many small spots per leaf, an eighth of an inch or so, with pale centers holding tiny black specks. Early blight makes fewer, larger lesions marked with concentric rings. Both start on the lowest leaves, so the spot itself is what decides it.
Will Septoria leaf spot kill my tomato plant?
It can. Wisconsin says merging spots can cause total defoliation of lower leaves and even the death of an infected plant. That is the bad end of it, and it takes weeks rather than days. Fruit infection is rare, so what you are losing is the leaf area that ripens the crop.
Should I remove leaves with Septoria leaf spot?
The lowest infected ones, yes, if you catch it early. But Missouri Botanical gives the limit almost nobody prints: removing leaves above where the fruit has formed will weaken the plant and expose the fruit to sunscald. Strip from the bottom and stop where the fruit starts.
Does Septoria leaf spot come back every year?
It can, because it overwinters nearby. Missouri Botanical names three reservoirs: infected tomato debris, weeds in the nightshade family, and equipment such as plant stakes and cages. Clearing the debris and cleaning the hardware over winter removes two of the three.
Sources
Every planting date, depth, and temperature on this page traces to these university extension guides. Where they differ, the ranges above hold the whole spread.
- University of Wisconsin–Madison Division of Extension — Septoria Leaf Spot
- Missouri Botanical Garden — Early Blight of Tomato
- Missouri Botanical Garden — Septoria Leaf Spot of Tomato
We are building GardenTrack to remember what grew where and what turned up on it last year, so a disease that overwinters in your own garden is not a surprise twice — the app is in beta now, invite-only.