Biphenyls and dibenzofurans - Fire blight-induced phytoalexins of pear

After fire blight inoculation, shoots of apple and pear formed biphenyls and dibenzofurans as defense compounds. The phytoalexins were only present in the transition zone of stems, whereas leaves were devoid of these compounds. In shoots of the apple cultivar 'Holsteiner Cox', four biphenyls and two dibenzofurans accumulated in the downward advancing transition zone six weeks after Erwinia amylovora inoculation. In the pear cultivar 'Conference', three biphenyls and one dibenzofuran were detected. The total phytoalexin content in the transition zone of pear was much lower than that in apple. In addition to 'Conference', two other pear cultivars, 'Alexander Lucas' and 'Harrow Sweet' were investigated for their phytoalexin formation in response to fire blight inoculation. Differences in the morphological changes as well as in the biphenyl and dibenzofuran concentration and pattern were observed between these three genotypes. 'Harrow Sweet', classified as most resistant to fire blight, showed the highest phytoalexin concentration when compared to 'Alexander Lucas' and 'Conference' and its transition zone stopped the downward migration 12 days after inoculation. Five biphenyls and one dibenzofuran were detectable in this pear cultivar.