Through some discussions on the Avro mailing lists, there are use cases that need the ability to inject a custom SpecificDatumReader. Crunch does not make it easy to inject the custom reader as it is currently implemented causing consumers to copy three classes (e.g. AvroFileSource, AvroFileSourceTarget, AvroFileReaderFactory) to gain this functionality. With a few minor changes to AvroFileSource and AvroFileReaderFactory, consumers can inject their custom reader fairly easily.
 - http://search-hadoop.com/m/98HA2DitiF/avro+enum+passivity&subj=Avro+enum+and+passivity+