Garden diseases - Tomato greenback

greenback on tomatoes on a vine
Hard, unripe patches on tomatoes signal tomato greenback
Image: University of Connecticut Home and Garden Education Center

Tomato greenback is a physiological problem caused by stress to the plant, rather than a pest or disease. It usually affects greenhouse-grown tomatoes and causes hard, green areas to form on the fruit that make it unpleasant to eat. 

What is tomato greenback?

When tomatoes begin to develop and ripen, the fruits, stems and leaves require a sensitive balance of water, light and nutrients. Greenback is usually caused by excessive heat in the greenhouse, exacerbated by a lack of potassium. The result is hard green patches that fail to ripen properly on the stalk end of the tomato. 

How to prevent tomato greenback

Tomato 'Nimbus' from Thompson & Morgan
You can grow greenback resistant varieties
Image: Tomato 'Nimbus' from Thompson & Morgan

There’s no treatment for tomato greenback once it develops, but there are several things you can do to prevent your tomatoes suffering from stress: 

  • • Apply shading to the glass of your greenhouse in early summer. 

  • • Damp down your greenhouse paths first thing in the morning on hot summer days. 

  • • Make sure there is adequate ventilation.

  • • Water regularly.

  • • Feed tomato plants with a high potash feed - follow the directions carefully.

Should your tomatoes develop greenback, the ripe parts of the fruit can still be eaten. Simply remove the dark, unripe sections and discard. You can also try growing greenback resistant varieties such as ‘Alicante’, ‘Cristal’, ‘Nimbus’ and ‘Craigella’.

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