Blight

What you are looking at

Dark brown or blackish patches appear at the tips and edges of the leaves, usually with a pale yellow-green border where the killed tissue meets the living. They are not neat spots. They spread outwards, run down the leaf stalk, and turn up as brown streaks on the stems.

Go out early on a humid morning and turn a leaf over. Around the edge of each patch there is a fine white bloom, and that is the organism producing spores. By midday it has dried and gone, which is why so many people never see it.

Then it moves. A potato bed that showed a few marks on Saturday can be a collapsed brown mass by the following weekend, with a distinct smell of decay over it. Nothing else in a vegetable garden works at that speed.

On tomatoes the fruit is where it hurts. Firm, dark brown, blotchy areas develop, often starting at the shoulder near the stalk, and the flesh underneath goes leathery. A fruit picked before the mark shows will develop one in the fruit bowl.

It is not a fungus

Phytophthora infestans is an oomycete, a water mould. Its ancestors are closer to brown algae than to anything in the fungal kingdom, and the resemblance to a fungus is convergence rather than kinship.

That has two practical consequences. Products developed against true fungi are frequently poor against oomycetes, so a general garden fungicide is doubly useless here. And oomycetes produce swimming spores that need a film of liquid water to move in, which is why blight is a disease of wet warm weather rather than merely humid weather, and why keeping the leaves dry is the whole prevention strategy.

The genus is the same one behind Phytophthora root rot on rhododendrons and sudden oak death. This species is the one that emptied Ireland in the 1840s.

The two days that decide it

Blight infection is predictable enough to be forecast. In Great Britain the current standard is the Hutton Criteria, which replaced the older Smith Period in 2017 and better matches the strains now in circulation.

A Hutton Period is two consecutive days with a minimum temperature of at least 10 degrees C and at least six hours each day at 90 per cent relative humidity or above. Free forecast services publish them by postcode.

Knowing this changes what you do with your July. A warning means going out to look at the crop rather than assuming the weather has been fine. Two warm muggy nights after a wet day is the pattern, and it is why blight arrives in the same week across a whole county.

Airborne spores travel miles. An allotment site where one plot has blight will have it everywhere within a fortnight, so a neighbour’s crop matters as much as your own care.

Why the page says there is no cure

Once the organism is inside the plant, nothing available to a gardener will get it out. Amateur fungicide approvals in Great Britain no longer include an effective blight product, and the protectant most people remember was mancozeb, which had its approval withdrawn in 2021 and cannot be used.

That is worth stating plainly, because a great deal of what still ranks for this search recommends a spray programme that is no longer legal to follow.

What remains is prevention and salvage, and both work well.

What to do about it

Choose the plant. A Sarpo potato or a Crimson Crush tomato in an ordinary British summer will still be standing when everything around it has gone, and that decision is made once, in a catalogue, in January.

Grow tomatoes under cover and water them at the base. A greenhouse tomato is close to safe from this.

Lift early potatoes before blight season. First and second earlies are usually ready by the end of July, and a crop that is out of the ground cannot get blight.

Then, when it does arrive, act on the day rather than the week. Cut every potato stem off at soil level and take the foliage away. The tubers stop growing, and you lose the last few weeks of bulking, but the spores never wash down to them. Leave the bed undisturbed for two or three weeks so the spores on the soil surface die, then lift on a dry day and sort carefully. Anything with a firm brown patch under the skin goes in the bin rather than the store.

For tomatoes, pick everything that has begun to colour and ripen it indoors, discarding any fruit already marked.

Bag the infected material. A home compost heap will not kill this.

What it gets mistaken for

Early blight, Alternaria solani, produces smaller brown spots with concentric rings in them, like a target, on older lower leaves. It moves slowly and does not flatten a crop in a week.

Grey mould enters through wounds and dead flowers and grows a fuzzy grey coat that puffs spores. Blight lesions are firm and dry with no fuzzy growth except the fine white ring at the margin.

Blossom end rot marks a tomato only at the base, opposite the stalk, in a flat leathery sharply bounded patch, and it leaves the foliage untouched. Blight marks the shoulder and the leaves at the same time.

Magnesium deficiency yellows potato foliage between the veins from the bottom of the plant upwards, with no brown dead patches and no smell. It looks alarming in August and matters very little.