Tomato Blight is a serious disease that affects tomatoes in Scotland. Here is what it looks like, the conditions it needs, and the controls that actually work.
What does it look like?
On leaves: Rapidly spreading, watery rot of leaves, which soon collapse, shrivel and turn brown.
On stems: Brown lesions that can cause collapse.
On fruit: Watersoaked patches turning brown on green fruit; mature fruits decay rapidly.
Tomato blight and potato blight are the same organism, Phytophthora infestans. If you also grow potatoes, a wet spell that starts an outbreak here is a real risk to them too — check them as well. See Potato Blight →
Controls: what actually works
Every method below is graded by evidence, not by how often it gets repeated. Where a claim has been tested and failed, we say so plainly.
Chemical control
RHS: 'There are currently no fungicides available for use by gardeners against blight.'
What actually works: The same is true here: no fungicide helps, so keep greenhouse foliage dry, grow a resistant variety such as 'Crimson Crush' or 'Mountain Magic', and destroy infected material rather than composting it.
Keep foliage dry and manage greenhouse humidity
Keep greenhouse foliage dry. That is the mitigation for the high-humidity risk that follows once spores get into a greenhouse.
Grow blight-resistant varieties
'Crimson Crush' and 'Mountain Magic' both carry strong resistance, and both 'performed very well at Wisley in 2021, when blight was extremely widespread'.
Cultural controls: dispose of infected material by deep burial, green waste collection or burning — never compost; minimum four-year rotation; monitor blight forecasts
The Hutton Criteria forecast applies here identically — same organism, same trigger conditions. Cross-link to the potato blight page's Hutton section rather than duplicating it.
Use affected fruit promptly; make unripe green fruit into chutney or sauce rather than trying to ripen it indoors
RHS: 'Use the fruit from affected crops as soon as possible. Once a plant has developed high levels of the disease on the leaves and stems it is likely that many of the fruit will also have been infected, even if they don't yet show symptoms.'
This answers the question most gardeners actually have once blight appears: can I still use the fruit? The advice is not to try ripening green fruit indoors, since much of it may already be infected without showing it — a large share is likely to rot instead. Using it straight away, for example as chutney or green tomato sauce, is the safer option.
Frequently asked questions
What should I do first if I find tomato blight in my garden?
Start with keep foliage dry and manage greenhouse humidity. After that, grow blight-resistant varieties. The full list of evidenced controls is above, each graded by how much evidence supports it.
Is there a spray that works?
No. There are no amateur fungicides available for tomato blight. The controls that work are cultural: the measures listed above.
Sources
2 sources, recorded with what each was used for
- RHS1: organism, leaf/stem/FRUIT symptoms, warm wet conditions, greenhouse reduces spore arrival but high humidity accelerates symptoms if breached, keep foliage dry, no amateur fungicides, 'Crimson Crush'/'Mountain Magic' Wisley 2021, disposal and four-year rotation
- RHS1: corroborates organism and the no-amateur-fungicides finding