Can you post some pics of the whole plant? I've grown outside using
aeroponics and have never had that problem. Looking at your pics the hydroton looks "very" wet. Mine are usually dry on top. I'm wondering if the stems are getting hit with some spray and the sun is burning them. My plants are in direct sunlight all summer so I don't think that would be a problem unless the stems are wet while the sun is beating down on them.
At this link
http://www.hydroponicsonline.com/for...my-system.html you can see my first
aeroponic grow and you will see the hydroton is dry.