Answer: It's hard to pinpoint exactly what you have as there are many leaf diseases that tomatoes can get. The symptoms of blight or leaf spot are very similar, with lower leaves getting brown or black spots, yellowing and then dying. Eventually this fungal disease will spread up the entire plant. It's usually more common during warm, wet weather. To control,
Clean up all diseased foliage and destroy it.
Rotate where you plant crops each season.
Mulch tomato seedlings with plastic, hay or straw after planting and be careful when watering, so as to not splash the fungal spores from the soil onto the plant.
As a last resort and if the condition is severe, you can spray with a preventive fungicide, such as copper, in the early season.
There is also a disease called curly top virus, which is spread by leafhoppers. The leaves curl under and die. There is no cure for curly top once it strikes. However, if the plant survives to bear fruit, it may not be curly top. If the plants yellow and die, this is the likely cause. Cover them with floating row cover early in the season to prevent leafhoppers from feeding. Leafhoppers don't like shade, so they should go elsewhere to feed.
As soon as you see the problem, you may want to take a sample to your County Cooperative Extension office for diagnosis, to make sure what it is.
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