It seems in general that the trees need to have -12 to -15 degrees centigrade for fairly long periods to actually kill them (as seen in central
What can I do?
Unfortunately Phytophthora is a widespread disease and quite hard to control. The addition of green compost into the growing media or soil can help. To our knowledge there are no chemicals available on the retail market to help control or prevent the disease. The Mycorrhizal fungus (Rootgrow) supplied with your tree also helps the root system to fight off Phytophthora but it will not prevent it. For further information below is the Wiki link to the form of Phytophthora that Wollemi Pines suffer with:
Alternatively the Wollemi Pine is known to coppice so if you think the top of the tree is dead or dying but the roots are OK, the best thing to do is remove the top of the tree either 3 inches above soil level or above the healthiest looking branch using sterile secateurs or a pruning saw. The tree will produce a white exudate which should seal the wound. Hopefully if the roots are still alive green buds will develop on the remaining trunk producing a bushy tree. If you want a more upright tree in the future it would be best to choose the strongest leader and remove any side shoots.