The patch that changes overnight
Late blight is the tomato problem I worry about when a plant looks noticeably worse between one evening and the next morning. A leaf that seemed healthy now has a broad, irregular brown patch. Another looks dark and limp, as though it has been soaked, even though the rain stopped hours ago. If the weather has been cool and wet, that combination deserves a closer look right away.
Picture checking tomatoes at 7 a.m. after two rainy days. Yesterday, one plant had a few dull green areas on its leaves. Today, those areas are brown, several leaf edges have collapsed, and a dark streak has appeared on a stem. That speed is a useful clue. Late blight, caused by Phytophthora infestans, can move through tomato foliage quickly when leaves stay wet.
What to look for before you pull anything out
Leaves: large, messy patches rather than neat spots
Early leaf lesions may look pale green or water-soaked. They turn brown to dark brown and expand into irregular patches, often crossing veins instead of staying within a tidy circle. The tissue can look greasy or limp before it dries. A patch may begin at a leaf tip or edge, but it does not have to.
On a damp morning, turn a suspect leaf over. Along the boundary between healthy green tissue and the brown patch, you may see a fine white, fuzzy growth. It is easiest to spot while the leaf is still humid; it may disappear from view once the sun dries it. The absence of white growth at noon does not rule out late blight.
Stems and fruit: the clues people miss
Check leaf stalks and main stems, not just the leaf blades. Late blight can leave elongated, dark brown to black lesions that look almost greasy. Affected fruit develops firm, brownish patches that may look slightly sunken or oily rather than soft and moldy at first. Green fruit can show these marks too.
A quick walk around the plant is more useful than staring at one damaged leaf. Look for:
- Large, irregular brown leaf patches that have expanded since yesterday
- Dark lesions on leaf stalks or stems
- Fine white growth at a lesion’s edge during damp weather
- Firm brown patches on fruit, especially alongside leaf symptoms
- Similar new damage on nearby tomatoes or potatoes
No single item proves the diagnosis. Several appearing together after sustained wet weather make it much more convincing.
Don’t mistake every brown leaf for late blight
The most common mistake is calling an ordinary lower-leaf disease “late blight” because the name sounds right for the end of summer. Early blight typically starts on older, lower leaves with more defined spots. Those spots often develop concentric rings, like a tiny target, and yellowing around them. Septoria leaf spot usually produces many small spots rather than a few rapidly expanding patches. Neither comparison is perfect, but pattern and pace help.
Fruit damage has look-alikes as well. Blossom-end rot forms a dark, leathery area at the blossom end of a tomato; by itself, that is not evidence of late blight. Sunscald tends to leave a pale, bleached area on fruit exposed to strong sun. A single crispy leaf touching a hot fence, with no spreading lesions elsewhere, is not a reason to remove a productive plant.
If the damage is confined to one old leaf and looks unchanged after a couple of dry days, watch it. If new, water-soaked patches appear across the plant overnight, act.
There is one less obvious complication: late blight may not start at the bottom of the plant. Gardeners who inspect only the oldest leaves can miss lesions higher in the canopy or on stems. Give the whole plant a look, including the shaded side.
What to do when the signs add up
Check first, then handle the plant
Take clear photos of the top and underside of a fresh lesion, plus the stem and any affected fruit. Note when you first saw it and what the weather has been like. Your local extension service or plant diagnostic clinic can help confirm a doubtful case, which matters because removing a healthy tomato is frustrating—and leaving a blighted one in place gives the disease more opportunity to spread.
Until you know what you have, avoid brushing through wet plants and moving straight to healthy tomatoes. The organism produces spores on infected tissue in humid conditions, and nearby tomatoes and potatoes are worth checking. Water at soil level rather than wetting the foliage, and give crowded plants enough airflow to dry.
If the symptoms are unmistakable or a diagnosis confirms late blight, remove the infected plant promptly. Bag foliage and fruit as you work so you are not dragging diseased material through the garden. Follow local guidance for disposal; do not leave it on an open compost pile. Check neighboring plants daily for new lesions, including volunteer potatoes and tomatoes.
Discard fruit with suspicious brown lesions. Sound fruit from a plant you are removing can be harvested and inspected carefully, but do not store fruit that is already developing spots. Fungicides, including products labeled for late blight, are chiefly protective: they are not a way to turn brown, infected tissue green again. If you decide to protect still-healthy plants, use only a product labeled for tomatoes and the disease, and follow its application and preharvest directions.
A calm second look is often the best tool
Late blight deserves urgency, not panic. One dry, yellowing leaf on an otherwise healthy plant does not need a dramatic fix. Mark or photograph the suspicious area, then compare it the next morning. Fresh lesions, dark stem streaks, and a fast change after wet weather tell a very different story from an old spot that stays put. That short, deliberate check can save a good plant—or help you catch a serious problem while there is still time to protect the rest of the row.
