Posted: 26/03/2015 at 22:38
Sounds like "damping off" - a fungal disease than can affect young wallflower seedlings and others too. Below is a quote from the Marshallseeds website re damping od of wallflower seedlings:
Wallflower English Bedder Blend
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Perennial Bedder Blend has dense clusters of fragrant, 1/2", velvety, richly colored flowers in shades of yellow, orange, brown, and rusty red. A must for any English garden or rock garden, these very fragrant and showy flowers are also especially nice in containers and as a ground cover for spring blooming bulbs. Does well in coastal and mountainous regions where it thrives in cool summers.
Full sun to light shade. Blooms spring.
Height: 10" - 12" tall
Days to Emerge: 5 - 10 Days
Seed Depth: 1/4"
Seed Spacing: 1"
Thinning: When 1/2" tall, thin to 1 every 8"
When to sow outside: 2 - 4 weeks before average last frost or in early fall, 2 months before average first frost.
When to start inside: 6 - 8 weeks before average last frost.
Special Germination Instructions: Seedlings are very susceptible to damping off (fungal seedling disease); use a very light, sterilized soil mix and water from the bottom.