Tomatoes get more forum questions and misdiagnoses than almost any other garden crop, because five of their most common diseases all start the same way: a spot on a lower leaf. Below is the comparison chart pathologists use, plus what a photo-based AI model actually looks at to tell them apart faster than the human eye can.
Early blight (Alternaria solani)
Look for concentric rings inside a brown lesion — like a target or bullseye — usually starting on the oldest, lowest leaves first. A yellow halo often surrounds the spot, and heavily infected leaves yellow and drop entirely, working up the plant over several weeks.
Late blight (Phytophthora infestans)
No rings here — instead you get irregular, water-soaked, greasy-looking patches that spread fast in cool, wet weather. Check the underside of the lesion in early morning for a faint white fungal fringe. Unlike early blight, late blight can destroy a plant within a week and also attacks stems and green fruit.
Septoria leaf spot
Small, circular spots with a distinctive dark border and a tan or gray center, often with tiny black dots (fruiting bodies) visible in the middle under a hand lens. It spreads from the bottom of the plant upward and, unlike early blight, rarely shows concentric rings.
Bacterial spot
Smaller, more angular, water-soaked spots that turn dark brown to black, often with a yellow halo. Because it is bacterial, not fungal, copper-based fungicides only partially help and standard fungicides do nothing — this is the mix-up that wastes growers the most money.
Leaf mold (Fulvia fulva)
Pale yellow patches on the leaf's top surface paired with an olive-green to grayish-purple velvety mold on the underside directly beneath them. It is mostly a greenhouse and high-humidity problem — improving airflow often solves it before any spray is needed.
How an AI scanner separates them in one photo
A trained vision model does the same comparison a pathologist does, but simultaneously and at pixel resolution: lesion shape (circular vs irregular vs angular), the presence or absence of concentric rings, halo color, texture on the underside of the leaf, and which leaves on the plant are affected first. PlantWatch AI weighs all of these cues together, cross-checks them against the reported region and recent weather, and returns a ranked diagnosis with a treatment plan — so you are not guessing between five look-alikes from a single blurry photo.
Key takeaways
- Concentric 'bullseye' rings point to early blight; greasy, irregular patches point to late blight.
- Septoria leaf spot has a dark border with tiny black dots in the center; bacterial spot is smaller and more angular.
- Leaf mold is a humidity problem first — better airflow often fixes it without spraying.
- Bacterial diseases don't respond to standard fungicides, so a correct diagnosis saves money, not just time.
- AI comparison tools weigh lesion shape, halo, texture and plant position together, the same cues a pathologist uses.
Related in the pathology library
Products you can buy for this problem
Typical registered products for the diseases covered in this article, with their active ingredient, mixing rate, spray interval and pre-harvest interval. Rotate FRAC/IRAC groups to avoid resistance.
Mancozeb 80% WP
Active ingredient: Mancozeb
Copper oxychloride 50% WP
Active ingredient: Copper oxychloride
Ridomil Gold MZ (metalaxyl-M + mancozeb)
Active ingredient: Metalaxyl-M 4% + Mancozeb 64%
Cymoxanil + famoxadone
Active ingredient: Cymoxanil 30% + Famoxadone 22.5%
Chlorothalonil 75% WP
Active ingredient: Chlorothalonil
Azoxystrobin 23% SC
Active ingredient: Azoxystrobin
Doses shown are typical registered field rates. Always read the product label, confirm the product is registered for your crop and country, respect the pre-harvest interval (PHI), and wear the protective equipment the label specifies. PlantWatch AI provides agronomic guidance, not a legal spray recommendation.
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