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

Tomatoes are among the most popular vegetables in home gardens. It’s all the more frustrating when brown spots suddenly appear on leaves or fruits. myHOMEBOOK explains how to identify late blight—and what steps to take next.

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 leaves of the plants 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 does not allow sufficient air circulation
• The fungus often spreads from neighboring potato plants to the tomatoes. Those who grow both in the garden should maintain sufficient distance between the beds. However, 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 a late blight infection. 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 remedy this.
• Sunburn leaves light, later brownish spots on the sun-exposed side of the fruit and on the leaves. This often affects plants that previously grew in a greenhouse and are suddenly exposed to much more direct sunlight. Those who transplant young plants from the greenhouse should harden them 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.

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 in the compost, as the spores can spread again in the garden from there).
• 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. Those who wish to water automatically should opt for 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.

More on the topic

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

Danger in the Vegetable Patch 6 Common Cucumber Diseases to Recognize, Prevent, and Treat

Garden Editor Explains Three Important Aspects of Tomato Location

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 shielded from the fungus–ensuring a secure and abundant harvest.