Marike Stucke

June 9, 2025, 12:32 pm | Read time: 4 minutes

In wet summers, hobby gardeners often face a familiar problem: brown, rotten spots on tomato plants and fruits. In many cases, the so-called late blight is to blame–a fungal disease that can quickly destroy the entire harvest. But how exactly does it develop? How can you recognize it? And how can the damage be limited?

How to Recognize Late Blight on Tomatoes?

The first signs usually appear on the lower leaves of the plant: They initially develop gray-brown spots that quickly spread. Over time, the affected leaves wilt and die. Later, the infection spreads to stems and fruits until the entire plant is affected. Typical are brown, hardened spots on the tomatoes, often near the stem. The flesh underneath remains hard and inedible. If late blight spreads unchecked, the entire plant can die within days or weeks–resulting in the loss of the entire harvest.

How Does Late Blight Develop on Tomatoes?

The disease is caused by the fungus “Phytophthora infestans,” originally known for potato blight. It thrives in moist weather and can spread particularly quickly when the plant leaves remain wet for extended periods. This is especially true in rainy summers but can also occur with the frequent use of irrigation systems.

Typical Causes:

• Rain-soaked leaves on outdoor tomatoes
• Excessive humidity in the greenhouse
• Dense plant growth that doesn’t allow enough air circulation
• The fungus often spreads from nearby potato plants to tomatoes. If you grow both in your garden, ensure enough distance between the beds. Spores from a nearby potato field can also be carried several miles by the wind.

Related: The 5 Most Common Tomato Diseases and How to Combat Them

Are All Brown Spots on Tomatoes Late Blight?

No, not every brown or gray spot necessarily indicates late blight. Especially in unripe tomatoes, less harmful discolorations occasionally occur:

• Blossom end rot appears as a dark spot on the blossom end and is caused by calcium deficiency, not fungal infection. A suitable tomato fertilizer can help here.
• Sunburn leaves light, later brownish spots on the sun-exposed side of the fruit and on the leaves. This often affects plants that were previously grown in a greenhouse and are suddenly exposed to much more direct sunlight. If you plant young plants from the greenhouse, they should be hardened off against the outdoor climate first.

In contrast, late blight spots are usually deep brown, hardened, and spread rapidly. The foliage also shows discoloration and drying at the same time.

More on the topic

Blossom End Rot, Mildew, and More The 5 Most Common Tomato Diseases and How to Combat Them

Plant Diseases Identifying and Combating Boxwood Blight in Your Garden

What to Do About Late Blight?

If late blight is detected early, the damage can sometimes be contained. The following measures help:

• Immediately remove affected leaves and fruits and dispose of them in the trash (not on the compost. From there, the spores can spread again in the garden).
• Thin out tomatoes to allow more air to reach the plant and reduce spore transmission through direct plant contact.
• Water only in the morning and only the root area to keep leaves as dry as possible. If you want to water automatically, it’s best to use a drip system that waters close to the ground and keeps the leaves dry.
• In persistently wet weather, a protective cover can help (e.g., a tomato roof outdoors).
• Tomato varieties with high resistance to late blight are usually less frequently affected.
• If the infestation is already advanced, often the only solution is to remove the entire plant to prevent spreading to neighboring plants.

Conclusion

Brown spots on tomatoes should always be closely examined. If late blight is the cause, quick action is necessary to prevent greater damage. With proper care and a protected location, tomato plants can be well protected from the fungus–and the harvest remains secure and plentiful.