pilosocereus azureus problem

I was hoping someone could tell me whats attacking this large pilosocereus azureus (Blue Torch) cactus which is over 3m tall. Its a light silvery colour and is mostly on the bright sunny side of the plant, the rear of the plant looks almost untouched so i suspect a fungus or virus, more recently horizontal cracks have begun to form in the skin. I suspect the plant maybe doomed but thought i'd atleast see if anyone knows whats attacking it.

Looks like sun damage, has it been exposed hot weather/sun ?

Thanks for your reply but I fear its not sunburn. The palnt was thriving in its current location for about 6 years without issue, also when we had the 2009 heatwave and bushfires in victoria it didn't effect the plant at all, temp was over 40 for most of a week! I first noticed the discolouration in early spring and its gotten much worse since, starts off with a slight pink plemish on the skin and then goes powdery white before a more brown and cracked appearance.

Another reason why I don't think its sun burn is the tender top of the plant is still fine and it gets even more sun being above the apex of the shead. Here is another pic of the same part a few weeks ago. Much less white and no horizontal cracks.

I have damage like this on a few of my pilos in the garden this year too. I think it is a combination of sun damage and lack of nutrients. Kinda like when the old leaves turn yellow on plants that grow leaves so that they can transport the nutrients which are in that part of the plant to the growing points. I only just noticed it recently and have decided to fertilize to see if that improves the situation.
The same plants survived serious heat in the past including the black saturday heatwave. The only things that are different this year is I haven't fed them for a very long time, also thisn year we've had lots of cold cloudy weather followed by ridiculous heat and bright sun, the swing between extremes often causes sunburn more than the heat itself.
Course it could always be disease but then why is only the afternoon sun side being affected?

I wondered that myself about only sunny side effected. I don't think i've ever fetilized this plant as it was putting on a lot of growth each year. Still if its sunburn there's a good possibility it will live through this which is honestly better than i was thinking, though its probably never going to be the impressive specimin it once was. Now i have to figure out how to give it a little shade.

Thanks for your reply.