Late (potato) blight
Late blight (Phytophthora infestans
) affects both tomatoes and potatoes. The disease survives over winter on volunteer potatoes (plants growing from tubers left in the ground). Spores can also blow in from nearby infected crops. In warm, wet seasons the disease spreads rapidly – it’s less of a problem in fine weather. Commercial potato growers use ‘Smith periods’ to predict when their crops are at risk (two successive days with temperatures above 10°C and relative humidity above 90% for more than 11 hours each day). You can see when these are forecast here: Blightwatch
. The only materials available to gardeners for the control of blight contain copper and are ‘protectants’ which means they have to be applied before the plants are infected, with coverage of all surfaces. Copper is toxic to aquatic organisms and it accumulates in the soil so we don’t use it in our garden.
If you are in a high-risk area, choose some blight-tolerant varieties; we have selected six of the best for our Blight-tolerant Outdoor Tomato Collection.
Other leaf diseases
Many modern F1 varieties e.g. Shirley F1 are resistant to the most common leaf diseases such as Leaf Mould (Cladosporium) and Tomato Mosaic Virus (TMV) (Gardener’s Delight is NOT resistant to TMV). Powdery mildew can also be a problem in some years.
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.