Molecular identification of a sexual interloper: The pear pathogen, Venturia pirina, has sex on apple

Venturia pirina (the pear scab pathogen) and V. inaequalis (the apple scab pathogen) were detected as ascospores discharged from apple leaf litter in New Zealand (spring 1998). Pseudothecia of both species were located on dea d apple leaves: however, only those of V. inaequalis were associated with s cab lesions, V. pirina was identified by rDNA sequence analyses, because mo rphological characters could not distinguish this fungus from V. asperata ( a rare saprophyte on apple) and other Venturia spp. pathogenic on rosaceous fruit trees. Species-specific polymerase chain reaction primers designed t o the 18S end of the internal transcribed spacer 1 region differentiated Ve nturia fruit tree pathogens reliably. V. pirina field isolates were pathoge nic on pear, but only weak saprophytes on apple. In rare instances, when ap pressoria of V. pirina appeared to penetrate the cuticle of apple leaves, e pidermal cells responded with a localized hypersensitive response (HR). To our knowledge, this is the first report of induction of HR-like events by I I pirina on its nonhost, apple, and also the first record of sexual reprod uction of V. pirina on apple. It is assumed that V. pirina pseudothecia for med from saprophytic lesions in senescing apple leaves when active defense mechanisms such as HR were no longer induced.