03 July 1992
Vol 257, Issue 5066

About The Cover

Sectors of a tobacco leaf infiltrated with the plant pathogenic bacterium Erwinia amylovora or harpin, a protein isolated from it, show collapsed tissue (dark spots); sectors infiltrated with E. amylovora mutants that lack harpin have not collapsed. Erwinia amylovora causes fire blight, a severe disease of apple and pear trees; harpin appears to be responsible for the collapse of infiltrated tissue and is required for the development of the disease. See page 85. [Photograph: K. Loeffler]