I'm with Tris, above. Look at the blackening in the leaf joint.
If it is blight it can rip through the crop in 24-48 hours, sorry to be a pessimist.
If it is blight, there is little you can do. Remove diseased parts as soon as you can, right back beyond the affected parts. Fastidious hygiene might help, wash secateurs and hands when you go from plant to plant. That applies for any fungal or viral disease.
Blight happens, airborne spores and high humidity in some years increases the chance. If you do lose some plants and want to try again next year, look carefully at varieties. Some F1 hybrids are advertised as blight resistant. Seeds cost a bit more, but when you add up values of compost, fertiliser and time spending a bit more on seed isn't a big thing.
I hope it's not blight, but fear that it is. Some of my plants are showing signs, started yesterday, the 3 F1 hybrids might be OK but I think I will lose 4 of the 7 plants I have.
The way it looks today, I will be pleased if I have any by the end of the month.