In message , Dvd
writes
Neighbour planted tomatoes, peppers and cosmos in a new bed. In less than a
week everything has died. There appears to be some accelerated growth, such
as occurs with herbicides, but there have been no chemicals used anywhere in
or near the garden.

Sounds like there have been and a fast acting one too. Try growing
mustard and cress from seed in the suspect bed. They are very sensitive
to trace herbicide. If they will grow then something else killed your
neighbours plants. Slugs or rough handling or inadequate watering in.

Could toxic chemicals from a brand new plastic hose have
leached into the water that was used on the plants? (Husband says no, wife
says water appeared milky).

Water from the tap often appears milky from dissolved air. There isn't
much in a new hose pipe that could leach and affect plants.

Anyone experienced anything like this? Weather
has been fine -- no frost, not particularly hot, and sufficient rainfall.

It's very unlikely to be the hose. Look elsewhere.

Regards,
--
Martin Brown