Avocados are not keen on chilly nights or hard sunlight. Or wind.

If your day temps are higher than 80F and your plantlings have been living the soft life indoors they could have suffered sunburn.

As a first move try some shelter - a burlap-covered frame like a tomato frame with one side open until they toughen up. You'll have some idea of whether it was a temperature problem when the plants start putting out new growth. Start with morning sun for a few weeks, give protection over noon, and allow some western light as they get older. You'll have to protect the trunks from heat - similar to citrus. It might also help if your usual fertiliser has some calcium in it as avocados appreciate a sweet soil.

If there's no positive response then the next call could be to the local tree-crops advisor for information on local diseases.