Hot + wet = rot. Water molds can be problematic anywhere this condition is present. This is why most kinds of rhododendrons can't be grown in the Deep South even in shaded positions with good drainage.

You might find this thread interesting: http://forums.gardenweb.com/discussions/4015261/are-you-damaging-your-tree-roots?n=33

Great thread Vladimir, I must have missed that one this past summer. My poor results in dark red fabric pots in full sun makes a lot more sense now. I'm glad everything I have is now in light-colored plastic or cedar containers!

Carl Whitcomb mentioned on the Evergreen Garden Works page you linked to has his own web site. However I clicked on one of the links to his articles under The Info and it opened with an Under Maintenance, Come Back Later statement.

Anyway there is much more material in his books. Libraries of colleges with horticulture programs have them in my area.

http://www.drcarlwhitcomb.com/Home_Page.html

Brent from evergreen gardenworks is a very well known in the bonsai circle and his articles are always spot on. His site existed before even google came to be. And it has not changed much since then.

Nurseries prefer black plastic pots because they do most of their selling in spring when it is advantageous to keep the pots warm. In summer they are absolutely a terrible thing to do to the plants. But most dark coloured pots will become hot in summer sun. In ground the temperature is very stable and changes extremely slowly with ambient temperature. That is what the roots like - very stable temperature. Leaves and stems can tolerate a much wider swing as long as the roots are happy.

These are some of the things one can do mitigate that:

a) dig in the pot in soil or mulch. I do this to some but by fall the plant would have sent out roots into the soil and needs to be chopped. Plus the pots will be dirty. It is a chore we always end up doing on a rainy cold day in fall. Having a bottle of cuban rum handy is a good idea.

b) pot in a pot: We have some large decorative clay pots and we put some plants in the pot with a bit of mulch to adjust the height. Another fall task is to clean the pots and store them for winter.

c) add trailing plants: My favourite method. Plant some trailing annuals in the pot that will engulf the sides of the pot and keep it shaded. Downside: it will compete for space and nutrients depending on how vigorous the guest plants are.

d) location: locate the pot so that the pot is shaded from the afternoons sun. A plant in ground, a low garden wall, wooden barrier, other annual pots can all serve as mechanisms to shade the pot. Avoid black-top surfaces and even hardscapes can heat up the pot pretty quickly.