Alternaria leaf blight

What you are looking at

Dark brown to black spots on the lowest, oldest leaves of a carrot, a potato, a tomato or a brassica. Each spot has a yellow halo around it, and the yellow spreads until the whole leaf goes.

Look inside a spot. Concentric rings, one inside the next, like a target or the rings of a tree. That is the Alternaria signature and the other leaf spot diseases do not produce it.

The spots run together, the lower leaves collapse while still attached, and the damage works steadily up the plant. The growing tip usually stays green longest.

On carrots the effect is at the crown, where the foliage blackens and gives way so the tops break off when you pull. On potatoes and tomatoes there are also dark sunken lesions on the tubers and on the fruit at the stem end.

Telling it from late blight

On a potato or a tomato this is the distinction worth getting right, because the two diseases move at completely different speeds and call for different responses.

Late blight makes greasy grey-green patches with no internal structure and no sharp edge. In damp weather there is a fine white fringe of mould on the underside at the margin of the patch. It spreads outward fast and can take a crop in a week.

Alternaria makes discrete round brown spots with rings inside them and a yellow halo. It starts on the oldest leaves at the bottom and climbs slowly, over weeks.

The pace is the practical difference. A crop going down to late blight needs the haulm off today. A crop with Alternaria on its lower leaves in July usually finishes the season.

The other reason to separate them is that late blight is an oomycete and this is a true fungus, so what works against one disappoints against the other.

It comes in on the seed

Most foliage diseases arrive on the wind or survive in the soil, and this one does both. What sets it apart is that it also travels inside the seed.

Carrot, brassica and tomato seed all carry Alternaria, which is how a bed with no history and clean ground goes down to it in the first season. Treated or certified seed avoids that, and saving your own seed from an affected plant guarantees the opposite.

The rest of the carryover is ordinary. The fungus survives on crop debris left on the ground, on volunteers, and on related weeds including fat hen, the wild nightshades and charlock, so clearing the tops at the end of a crop and keeping the weeds down does real work.

Leaf wetness is the lever

Spores need several hours of continuous moisture on the leaf to germinate and get in. Everything that shortens that period reduces infection, and a gardener controls more of it than they might think.

Space the plants so air moves between them. Thin the carrot row rather than leaving it dense. Water the soil rather than the foliage, and do it in the morning so anything that does land on a leaf dries before nightfall. Mulch under tomatoes and potatoes so rain does not splash spores up from the debris in the soil.

None of that stops a wet fortnight. It does decide whether the crop loses its lower leaves in July or in September, which on a carrot or a maincrop potato is most of the yield.

What to do about it

Buy treated or certified seed for carrots, brassicas and tomatoes.

Never save seed from an affected plant.

Rotate three years off the family, and keep fat hen, nightshades and charlock down.

Clear the old tops off the plot at the end of the crop.

Space generously and thin the rows so the leaves dry.

Water the soil in the morning rather than the foliage in the evening.

Mulch under tomatoes and potatoes to stop the splash.

Feed evenly, since hungry plants lose their lower leaves first.

Pick off the first spotted leaves in dry weather and take them away.

Cut and remove potato haulm early on a bad crop, and leave the tubers a fortnight before lifting.

What it gets mistaken for

Late blight makes greasy grey-green patches with no rings, spreads fast, and often shows white mould on the underside.

The general leaf spots, Septoria and the bacterial ones, make spots without concentric rings.

Downy mildew is bounded by the veins into angular patches with furry growth underneath.

Scorch marks the leaf edges evenly and stops when the weather changes.

Carrot fly reddens and stunts the tops and tunnels the root.

Magnesium shortage yellows between the veins with no spots at all.