As its name suggests, bacterial wilt is a disease caused by bacteria. I this case, the species is called Ralstonia solanacearum.
How It Spreads
Bacterial wilt is easily spread via the soil, water and sometimes when roots touch. The bacteria mostly infects plants by entering through wounds on roots.
Tomatoes and other nightshade vegetables are susceptible to this disease and both tomatoes and potatoes are particularly vulnerable. Many other plants can be attacked by this disease though including peanuts, beans, bananas and custard apples. All of the above ground parts of these plants can be affected.
The most obvious symptom of this tomato disease is wilting leaves and foliage. Many other diseases cause wilting however. To help determine whether bacterial wilt is the culprit, remove a stem from your plant using sharp, clean secateurs. If a white fluid drips or oozes out of the cut section when you squeeze the stem, bacterial wilt is likely the cause of your tomato plants' problem. Leaves may also turn yellow.
There is no effective treatment for bacterial wilt, prevention is the only real option.
Good prevention strategies are vital to ensuring your tomato plants remain healthy. Start by purchasing certified disease resistant seeds or seedlings if available. If you're saving your own seed, only collect seed from plants that appear healthy and soak them in warm (but not boiling) water prior to sowing to help kill any bacteria that is present on the seeds. Tomato varieties are available that are resistant to bacterial wilt so try to purchase these varieties if they're available in your area. Practicing crop rotation is vital to preventing this disease. It's also very important to separate tomatoes from other plants that are susceptible to this disease - potatoes in particular. Companion planting with antibiotic plants such as chives and mustard is also a useful method for preventing this disease. Since plant roots are the primary entry point for the bacteria that causes bacterial wilt, try to minimise the amount of digging you do near tomatoes to ensure you don't damage the roots. Some weeds can contract bacterial wilt so you'll also need to keep areas around tomatoes well weeded. When watering tomato plants or other potential bacterial wilt carriers, don't allow water to run off the soil onto other beds - pour only a small amount of water on the soil and wait until it has been absorbed before adding more. You should also try to water at the base of all plants rather than from the top - this reduces splashing which can spread the bacteria and it also prevents the foliage from staying wet which promotes bacterial growth. A dripper irrigation system is great for this. If your garden beds are situated on a slope, consider digging trenches on the low side to help catch rainwater runoff. This also helps direct water to your plant roots rather than it being wasted on garden paths.