Seedling issues - what is this a sign of?

Hello there!

A couple of weeks ago I transplanted a bunch of seedlings: 2 local varieties of tomato, peppers, cucumbers and courgette. I last saw them 4 days ago (don't live in the property) and they were fine, but today I found they had either disappeared amongst the mulch or looking like the images attached.

I was worried there was a problem with the manure bed in which the tomatoes are planted, but the peppers have the same symptoms and are planted in a different bed.

My guess is this is the result of a drop in temperatures, it'a been getting colder at night.

I would love any input anybody can give, I am looking forward to learn from these events.

Thank you!

The symptoms in the pictures look like early blight to me. It's a soil borne fungal disease and the spores overwinter. It is more prevalent if old infected plant matter is left in place instead of removed, burned, or composted in a hot (140f+) compost pile. It can affect plants of poor vigor, plants that are stressed (like transplant stress) and can occur more often in excessively wet conditions. Have you had too much rain in your region, with cloudy damp days and not much sunshine? Did you plant these plants in the same spot as last years tomatoes and peppers? The plants may have already been experiencing a nutrient deficiency before you purchased them, and deficiencies cause stress. The pictures definitely look like the plants succumbed to a disease infection, and my best guess is early blight, though it may be something entirely different. The best way to deal with diseases is prevention, as they are generally everywhere, but intense pockets can exist (like planting the same thing over and over in the same soil year after year without crop rotation). Having healthy populations of good bacteria and fungus in the soil is the best way I've found to manage diseases. This can be done with healthy, quality compost, and purchasing soil inoculants like Effective Microorganisms (EM), Subculture-B, and mycorrhizal inoculants. If you do your own composting, you can go to nearby woods and and take a shovel full of soil from the forest floor and put that in your compost and that will inoculate your compost with the native biota in your area. In addition to these techniques, there's a product called Serenade, which I have used with good results, as a foliar spray to help manage disease pressure. I mix Serenade and EM together as a foliar spray and it is most effective if it is applied before any disease symptoms show so the good bacteria are populating the surface of the plants and make it more difficult for disease to get a foothold and cause infection. If one waits to apply foliar sprays until symptoms arise, the plant is already infected. This is a biological technique to manage disease pressure, but there are other ways such as using copper or sulfur foliar sprays to manage disease. These are all methods that are approved for organic gardening. Excessive use of copper and sulfur, year after year, can sometimes cause problems in the soil, but that's really another discussion. Hope this helps!