Luteoforol, a flavan 4-ol, is induced in pome fruits by prohexadione-calcium and shows phytoalexin-like properties against Erwinia amylovora and other plant pathogens Francesco Spinelli 1 , John-Bryan Speakman 2 , Wilhelm Rademacher 2 , Heidi Halbwirth 3 , Karl Stich 3 and Guglielmo Costa 1 1 Department of Arboriculture, University of Bologna, viale Fanin 46, 40127Bologna, Italy; Phone: +39-51-2096447; Fax: +39-51-2096401; E-mail: spinelli@agrsci.unibo.it); 2 BASF Agricultural Center, Global Research Crop Protection PR/HB, Li 470 67114 Limburgerhof, Germany; 3 Institute of Technical BioScience, Technical University of Vienna, Getreidemarkt 9, 1060Vienna, Austria Accepted 11 February 2005 Key words: Acylcyclohexanediones, fire blight, induced resistance, Pantoea agglomerans, phytoalexins, Pseudomonas fluorescens, Venturia inaequalis Abstract Treatments with prohexadione-calcium led to lowered incidence of fire blight, scab and other diseases in pome fruit trees and other crop plants. In addition to acting as a growth regulator, prohexadione-calcium interferes with flavonoid metabolism and induces the accumulation of the 3-deoxycatechin luteoliflavan in shoots of pome fruit trees. Luteoliflavan does not possess any remarkable antimicrobial activity. Therefore luteoforol, its unstable and highly reactive precursor, has been tested in vitro for its bactericidal and fungicidal activities. Luteoforol was found to be highly active against different strains of Erwinia amylovora, the causal agent of fire blight, and all other bacterial and fungal organisms tested. Phytotoxic effects were also observed in pear plantlets. The results obtained indicate that prohexadione-calcium induces luteoforol as an active principle with non-specific biocidal properties. It is proposed that luteoforol is released upon pathogen attack from its cellular compartment and inhibits further disease development by destroying pathogen cells as well as by inducing a hypersensitive-like reaction in the host plant tissue. This mechanism would be closely analogous to the one known for structurally related phytoalexins in sorghum. Abbreviations: FHT – flavanone 3-hydroxylase; ProCa – prohexadione-calcium; TrixE – trinexapac-ethyl. Introduction Fire blight, caused by the bacterium Erwinia am- ylovora, is the most devastating bacterial disease of apples (Malus domestica) and pears (Pyrus com- munis) (Vanneste, 2000). The bacterium penetrates the plant through the nectaries of flowers, but it may also enter leaves via wounds and natural openings in the cuticle. Erwinia amylovora can in- vade all parts of a tree solely by internal progres- sion through the host tissues. Thus, a single infection can potentially kill a whole tree (Vanne- ste, 2000). Economic damages caused by fire blight are substantial and a severe outbreak can disrupt fruit production of an orchard for several years. Most of the economically interesting apple and pear varieties are very susceptible to fire blight. As a result, the disease significantly limits the area for European Journal of Plant Pathology (2005) 112: 133–142 Ó Springer 2005 DOI: 10.1007/s10658-005-2192-x