There are few sights more disheartening on the allotment than a row of tomato plants that were lush and green a week ago, now collapsing under brown, greasy patches. Blight — whether early blight or the far more devastating late blight — can wipe out an outdoor crop in days. The good news is that prompt action and a few changes to how you grow can tip the odds back in your favour.

What blight looks like — and why speed matters

Late blight (Phytophthora infestans) is the one that strikes fear into British growers. It often appears first on the leaves as dark, water-soaked patches that quickly turn brown or purplish-black. In humid conditions, a fuzzy white mould may develop on the undersides. The stems can develop dark lesions, and green fruit turns brown and firm before rotting. Early blight (Alternaria solani) is slower, causing concentric brown rings on lower leaves, but it still weakens plants and reduces yield.

The key is speed. Blight spreads by airborne spores and splashing water. Once you see it, the clock is ticking. A single infected leaf can release thousands of spores that will infect neighbouring plants — and potatoes — within hours.

Blight or something else? Quick checks

Not every brown patch is blight, so check before you panic. Blossom end rot appears as a sunken, dark patch at the base of the fruit, not on the leaves. Magnesium deficiency causes yellowing between the veins of older leaves, often with no browning. Leaf spot diseases tend to have a defined margin and may not spread as fast. If in doubt, look for the classic blight pattern: large, irregular brown patches that cross the veins, often starting on lower leaves, with a pale green halo. If the plant collapses rapidly, assume blight.

Immediate action: triage and removal

As soon as you confirm blight, act. Do not wait for the whole plant to turn. Remove affected foliage promptly, cutting back to healthy tissue with clean secateurs. If the plant is badly infected, remove it entirely. Bag the waste — do not compost it, as blight spores can survive on infected material. Take it to your council green waste collection or burn it. Wash your hands, tools, and gloves afterwards. If fruit is still green but the plant is going down, you can pick it and ripen it indoors; blight-infected fruit will rot, so discard any with brown patches.

• Remove all infected leaves and stems, plus any fallen debris.
• Do not compost blighted material — bag and bin it.
• Harvest healthy green tomatoes to ripen indoors.
• Clean tools and hands before touching healthy plants.

Improving airflow and growing conditions

Blight thrives in still, humid conditions. Spacing plants generously — at least 60cm apart — and pruning lower leaves to improve airflow can make a real difference. Water at the base, not over the foliage, and do it in the morning so leaves dry before evening. A mulch of straw or grass clippings keeps soil moisture even and reduces splash. If you grow under a polytunnel or cloche, ventilate well. Avoid overhead irrigation, and consider a temporary shelter of clear plastic over a frame to keep rain off the leaves during wet spells — but leave the sides open for airflow. On the allotment, don't plant tomatoes right next to potatoes; both are susceptible, and potatoes are often the first source of infection.

Treatment options that can help

Once blight is established, no spray will cure it. But protectant sprays can slow the spread if applied before infection takes hold. Copper-based fungicides are the main option for organic growers, though they work best as a preventative rather than a cure. Apply according to the label, and reapply after rain. Some gardeners use a dilute solution of bicarbonate of soda or a milk spray, but these are not reliable in a bad blight year. The most effective treatment is still removal and hygiene. If you catch it early, you might save part of the crop; if not, cut your losses and focus on next year.

Planning ahead: resistant varieties and next season

Next season, choose blight-resistant varieties. Look for those with known resistance to late blight, such as 'Crimson Crush', 'Mountain Magic', 'Lizzano', or 'Losetto'. These are not immune, but they often survive longer and crop better in wet summers. Rotate your crops so tomatoes don't follow potatoes or tomatoes. Remove volunteer potato plants and any leftover tomato debris. Keep a diary of when blight strikes on your site — it often arrives at the same time each year, so you can plan to cover plants or harvest early. With a bit of vigilance and the right varieties, you can still enjoy a decent harvest even in a blight year.