Question
my new avacado tree about 3 foot tall has developed brown rust like leaves that are curling up after growing well to this height, what could be the problem?

Answer
Hi Sharon

I would check for spider mites.  They tend to suck the leaves dry and make them turn rusty colored.  There are two types.  Red Spider mites and two spotted spider mites.  These insects are very small and you may need a magnifying glass in order to see them.  If you find them, you will need to treat the tree with an insecticide.  A safe product is called Safer's Insecticidal Soap.  It is available at most garden centers.  You will need to treat about every 3-4 days for a total of 3-4 applications.  This product will suffocate the bugs and kill them them. Make sure to treat the undersides of the leaves where most of them are located.  I hope this helps.  Thanks.