I also experience some 'diseases' with Aloe arborescens and one of it's hybrids ANZAC. I have watched my plants and several large banks of them in old plantations in parks and gardens on the Adelaide Plains for some years now. My conclusions are that the plants are susceptible to cold and excessive damp in Winter as well as frost. Sometimes the leaves develop large black spots but sometimes whole plants die. This can take a very long time to happen and by the time I've noticed the severity of the damage it is too late to save the plant. I think the stem gets damaged to the point of rotting through but as it is comparatively woody it doesn't collapse and an observer is easily led to think things are OK. But gradually the leaves begin to show signs of die-back, blackening spots and eventually complete 'death'. I live approx. 350m above Adelaide and the altitude seems to make the difference as it is colder and much damper here in Winter; not just rain and hail but endless days of fog and deep cloud cover which I think are probably alien to the natural conditions in which this aloe grows. There are aloes that grow at high altitudes in Sth Africa and get regular Winter snows but they are not the 'arborescent' tree-form species, rather compact, ground hugging rosettes such as Aloe polyphylla. Hope this helps.
