The thuja green giant evergreen is an extremely popular tree for landscaping, shade and privacy because it grows very quickly, is nearly entirely disease-free and is highly drought-resistant once established. The thuja green giant is often promoted as an alternative to hemlocks and pines, which are very prone to disease and easily weakened by drought. However, while thujas are pretty tough, there are a few diseases that can bring your tree down, although it is fairly unusual. Keeping an eye out for warning signs will help you treat the problem, should one arise, and keep your giant healthy and strong.
Thujas are not prone to bagworms, unlike their counterparts, hemlock and pine trees. However, they can get bagworm infections on rare occasions. Bagworms will eat the foliage on a thuja, should they manage to infect the tree, and will contribute to dieback, weakening the tree and slowing growth. The best way to deal with bagworms--particularly on thujas, which generally are not sprayed for this infection--is to remove the white, silky bags that hold the pupae when you spot them and clip any infected foliage. Since bagworms do not consider a thuja prime real estate anyway, you should be able to control the infection in this manner. If you cannot, then use a pesticide to fight the problem on a larger scale and spray your thuja again before the bagworms have a chance to emerge in the following early spring.
Like any other plant, thuja green giant evergreen trees can develop rot through a variety of circumstances. Particularly when the trees are young, make sure that if you are watering, you only do so in the morning so that the tree does not sit with moisture on its foliage throughout the afternoon and evening. Make sure that the soil around the tree is well drained to prevent root rot as well. If your tree has rot issues, you will notice that it is dropping leaves and may appear to be "thirsty" even when it is getting plenty of water. This indicates that root rot has invaded, and generally the tree is lost at this point. However, if the problem manifests as mold or powdery mildew on leaves or branches, then you can remove them using sterile pruning technique (rubbing the clippers with rubbing alcohol after every clip and disposing of the debris in a garbage bag) and you should be able to help your thuja get through the problem.
Scale insects are small bugs that ooze a sweet, viscous liquid that mold and mildew--not to mention ants--love. These insects are very difficult to spot since they are about the size of a grain of pepper, so usually the first sign of trouble will be mold or mildew growing on your tree. However, scale can also stunt needle growth or lead to browning and defoliation. You will have to treat your thuja with an insecticide to get rid of this problem, since it is difficult to impossible to visually identify and remove every affected area of the tree.