Grow mustard as a green manure crop outside, then cut it up and dig it into your soil as a soil cleanser. (Planting it outside enables you to continue using your greenhouse). Wait 3-4 weeks after digging it in before planting the next crop.

Supportive measures:

It is helpful to know what type of soil you have to begin with so the first step is to get your soil tested. Tomatoes require a soil pH of around 5.5 to 6.8. Organic compost of six months or more should be dug into the soil to improve quality and will add the necessary organisms to assist in fighting any diseases that have built up in the soil.

If you have clay soils then add peat moss or sand to improve the texture, or for sandy soil, use compost to assist in conditioning it.

Good ventilation in the greenhouse is another essential ingredient in the prevention of diseases, as is cleaning the greenhouse of any fungal build-up.

A good mulch such as straw is great for providing a physical barrier between soil and plant to help protect from diseases, and to keep the soil moist and prevent weed buildup. It also keeps the roots cool.

If you are purchasing plants or seeds, ensure they are disease free. Some varieties of tomato are more disease resistant than others - ask your supplier.

Grow mustard as a green manure crop outside, then cut it up and dig it into your soil as a soil cleanser. (Planting it outside enables you to continue using your greenhouse). Wait 3-4 weeks after digging it in before planting the next crop.

A couple of handy webpages are Organic Greenhouse Vegetables and Organic Greenhouse Vegetable Production.