blackened tips of avocado leaves

Giles starts with ...
Hello. I have a reasonably well established albeit young avocado tree- about 6 feet tall. lots of healthy green leaves and new budding red leaves as well as some buds with also some a few small 1cm in length avocados starting to show. However this week a few of my leaves have started to turn black at their tips and crunchy to the point that they are easy to break off. have added powerfeed over weekend and it gets a regular water via the reticulation system about 10 mins 3 times a week. in good sunny spot. based in Booragoon, Perth. WA. pls refer to the photos. hope it is not root rot, but thinking it may be too much salt or simply burnt but strange it is only a few leaves at the mo. thanks.

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