Getting the watering right in a grow bag or pot is always more difficult than if you grow tomatoes in soil. If you have a small greenhouse and always grow your tomatoes in the same place, then there can be a build up of soil-borne diseases such as Fusarium (wilt; crown and root rot), Verticillium wilt and Corky root rot. You can get round this either by not growing in the soil or by using grafted plants: see To graft or not graft for tomato growing?
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