Kenneth G. Wilson (1923–). The Columbia Guide to Standard American English. 1993.
disastrous (adj.)
has just three syllables (diz-AS-truhs or dis-AS-truhs); a frequent misspelling (disasterous) by the inexperienced gives it four, probably reflecting a mispronunciation (diz-AS-tuhr-uhs or dis-AS-tuhr-uhs). Both misspelling and mispronunciation are shibboleths.
