Why are my newly planted trees wilting and turning yellow?

The symptoms you have described sound like Phytophthora root rot attack. Symptoms of which include sudden wilting, smaller yellow foliage that eventually turns brown, dead bark and plant death. Control of this disease relies heavily on either improving the vigour of the tree by the use of mycorrhiza, irrigation, mulching and fertilising or removing the dying tree or shrub and planting with a more root rot resistant species. Consult your local Bartlett arborist for further advice.