When my tomatoes are approximately one metre, they wilt from the top and on inspection there is rot on the stem above the root system. What causes this and how can it be cured?
The symptoms you mention suggest that your tomatoes may well have a fungal disease called blight. It can arrive and infect a plant very, very quickly. Unfortunately, once the symptoms of blight are obvious there is no way to ‘heal’ your plant.
It’s important to remove the plant in its entirety from the garden and either burn or remove from your property. It is essential that next year you do not grow tomatoes or potatoes in the area in which the infection has occurred. This does not necessarily preclude you from growing tomatoes in your home veggie garden; however, a new site as far away from the infected area as possible is critical to the success of your future cultivation of tomatoes.
Next year it would be worth regularly spraying your tomato plants every 3-4 weeks with oxychloride spray until they are well established and young fruit is forming. This should help prevent any possible infections. But do note that it is a preventative, not a cure.
Garden hygiene is also very important, clearing up at the end of the season and crop rotation will help to avoid blight spores. Regularly feed your tomato plants with Daltons Tomato Fert to help grow a nice healthy plant.