Answer: There are a number of possible caused for rust colored needles, ranging from seasonal needle loss to pests and disease issues. Over or underwatering can also cause discoloration. It is difficult to diagnose this type of symptom long distance and the treatment depends on what is causing it. I would suggest you consult with your local county extension and/or professionally trained nursery staff to try to determine the cause of the change and based on knowing that, decide how to proceed. If it is something that requires a chemical control, your county extension will have the most up to date information on what to use and how/when is best to apply it. It is always better to check into this type of problem as early as possible in case it is something that can be controlled, so don't wait. Good luck wiht your mugo pine!
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