Avocado leaves going brown

BJ11 says...
Leaves can go brown for several reasons (and a picture will help identify why).
Cold winds and sunburn can damage leaves and turn them brown - but I presume your sheltered area would protect the tree from these things.
Similarly if you say it is Hass then it is probably grafted and not a seedling (so isn't just a 'dud' seedling).
Old leaves browning (typically from the tip) might be due to salt burn. This would suggest there is too much fertiliser in the soil.
Spots of brown might be due to salty water, or fungal infection (although the latter are often black).
Make sure mulch is away from the trunk of the tree to prevent any risk of collar rot impacting - this may also show up as sick leaves.
If the leaves are discoloured as well then it could be a deficiency - consider calcium and potash.
It is also possible the tree is going through a natural 'shed' of leaves - if there is loads of new growth and the leaves just look 'old' then it might be dropping and replacing its leaves.

Daleys has a simple but real informative video about growing avocado trees successfully. it'll certainly shed a bit of light to why your trees are having problems. hope it helps! http://youtu.be/W_Vx86SCX4w