Answer: Browning on arborvitae can be caused by a range of stress factors including over or underwatering, planting too deeply, transplant shock, poor rooting, chemical damage from accidental herbicide contact or overfertilizing, or pest problems such as bagworm and disease problems such as canker, among others. Generally poor growing conditions in a location with insufficient light, poor air circulation, and excess heat and humidity could also contribute to stress.
This plant really needs at a minimum a half a day of direct sun to stay healthy, otherwise in too much shade it will become thin and sparse. It also normally loses some foliage at the interior each fall, this can be a somewhat noticeable annual browning process. If your plant is in too much shade, each year it will grow thinner meaning less dense and you would see it losing foliage. Lack of moisture can also cause color change and foliage loss -- this plant likes an evenly moist but well drained soil. If the neighboring trees are stealing moisture from the root area of the arborvitaes, or if they have not rooted well into the native soil, then this might be part of the problem. Unfortunately, it is very difficult to diagnose this type of problem long distance.
If the browning is related to the location, spraying will not help. If it is a pest or disease issue then spraying might help. But you want to make sure that anything you spray will be effective on whatever is causing the problem. You have to have a precise cause first, then figure out how to treat that specifically. In a quick check I find Malocide contains pyrimethamine as the active ingredient and is listed as an antimalarial drug (for both human and veterinary use) so I am not sure why you would use that on your plants.
Before spraying again, I would strongly suggest you consult with your local county extension to try to obtain a specific diagnosis and then once you know that, determine how to proceed. If it is something that requires a chemical treatment, they will be able to make up to date recommendations on what to use and how/when is best to apply it for the desired results with the minimum of spraying.
I'm sorry I can't be more specific for you long distance. I hope you can save the rest.
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