Degroot Spire Pine trees turning brown
Thank you for the question. Diagnosing the brown areas on your arborvitae is difficult as there are many possible causes and it may be due to more than one factor. Some browning is normal but in such newly planted trees, what you see is a bit worrisome. We can't tell from the photo if disease, pest, or environmental factors are the cause of the browning but since they were only planted 2 months ago it's logical to suspect environmental issues. Over or under watering or compacted soil near the root ball is often a factor in browning such as you see. Your trees are planted in a rock mulch. Do you know what's under the rock? Landscape plastic can severely limit the amount of water the trees receive. On the other hand, too much water could be pooling near the roots for a variety of reasons. Chemical sprays for insect pests can also cause browning.
If your trees were planted by a service, you may want to check with them on your options.
Here are a few links for you to learn more about self diagnosing the problem, proper growing conditions for arborvitae, and stress related dieback:
Thank you for contacting Extension.