On Wed, 27 Feb 2013 18:22:49 -0800, Jeff Liebermann wrote:

Take some dilute hydrochloric acid (Muriatic Acid or pool chlorine), and
smear it on the plastic rot. If it fizzes and belches gases, it's
calcium carbonate.

I'll try that tomorrow as I have tons of pool acid (HCl).

Like you, I'll take a relevant picture, and let the world know the
results.