I have 6 Amaranthus varieties growing from seed. They are doing extremely well and growing like crazy. The Elephant heads are about 18 inches tall and producing several small flowers. However the leaves on the leading shoot are shrivelling up, not dieing. This is happening to all of them. I can't believe it is a disease, Could it be that in our warm weather, low 80s during the day they are "bolting" and cannot cope with the climate.
Looks to me the damage from chili thrips. http://www.doacs.state.fl.us/pi/enpp/ento/chillithrips.html
I don't know if they're "officially" in your country. I had similar damage on jute plants here in Florida.
Pick a few damaged leaves and seal them in a plastic baggie. After a few hours you can see them moving in the moisture droplets on the insides of the baggie.
They're best identified with a microscope.
