Give your greenhouse a good wash down before planting to maximise light transmission and get rid of disease spores.
Late (potato) blight can damage or even kill outdoor tomatoes and spoil the fruits before you can harvest them, but it is less of a problem if you grow your plants in a greenhouse or polytunnel. If you have to grow outside, choose the sunniest spot you can find (e.g. in front of a south-facing wall). There are also a few varieties which are more blight-tolerant e.g. Delfland’s Blight-tolerant Outdoor Tomato Collection or Sutton’s Crimson Crush
Tomatoes will not stand a frost – keep your seedlings indoors or in a heated greenhouse until the risk of frost has passed – sometime in May for most areas, but check your local weather forecast.
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?
Did You Know?
British Tomato growers use an army of over 100 million beneficial predatory bugs to help balance glasshouse ecosystems where pests are controlled by predators and so are present at a level below those causing economic damage and thus reducing, to virtually zero, the need to use any pesticides which if they are will be primarily biological in origin.