Answer: I don't think the extension office was too far off track. There are several possible causes. Allowing the soil to get too dry between waterings is one. You might want to try watering a day or two sooner and see if that makes a difference.
Using hard water can cause leaf tip burn. If your tap water is hard, use distilled or filtered instead.
Too much fertilizer. If you are using fertilizer, dilute it to half-strength and apply it no more than once per month.
The tips that have already turned brown will not recover, so you may as well trim them off with scissors. This will make the plant look nicer and it will help you monitor this problem in the future.
One final thought. Plants in containers can sometimes develop air pockets within the roots and even though you think you are watering throughly, there may still be some bone dry soil within the root system. Try immersing the container in a larger container of water to saturate the soil. Leave it there until no more air bubbles rise to the surface and you are sure the soil is completely wet. Then drain the excess water. If you do this every 4-6 weeks you'll be assured that the soil is able to take up needed water.
Best wishes with your philodendron!
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