ABSTRACT
Winter pears (Pyrus communis) in the United States are produced primarily in the Pacific Northwest. Potebniamyces pyri (anamorph Phacidiopycnis piri) is the causal agent of Phacidiopycnis rot, a recently reported postharvest disease on pears in the United States. Infection of fruit by P. pyri occurs in the orchard, and symptoms develop during storage. P. pyri was observed to be associated with cankers, dead bark, and twig dieback of pear trees. P. pyri was isolated from 40 to 50% of the twig samples exhibiting dieback symptoms from three commercial d'Anjou pear orchards, and 35% of dying bark samples from one orchard. However, little information is available regarding pathogenicity of P. pyri on pear trees. To determine the distribution of P. pyri, dying and dead bark samples were collected from pear orchards in various pear-producing areas in Oregon and Washington, and examined for presence of fruiting bodies (pycnidia or apothecia) of P. pyri. In the orchard, 2-year-old twigs were wounded using a sterile cork borer with or without spraying with a commercial aerosol tissue-freezing product at the wound sites. Wounds were then inoculated with either mycelial plugs from an agar medium or conidial suspensions of P. pyri. In a separate experiment, freshly made pruning wounds were inoculated with conidial suspensions of P. pyri. Canker development was monitored approximately monthly for up to 6 months after inoculation, at which time reisolation of P. pyri was attempted. P. pyri was found to be widespread in the Pacific Northwest. Incidence of trees infected by P. pyri based on presence of viable pycnidia in pear orchards ranged from 0 to 100%. Monthly tree inoculations in the orchard indicated that P. pyri in general did not cause cankers on non-cold-injured, wound-inoculated twigs, but apparently became established on cold-injured, wound-inoculated twigs and caused small cankers. Minor dieback developed on twigs inoculated at pruning wounds. At 6 months after inoculation, P. pyri was recovered from the majority of inoculated twigs. Thus, P. pyri appears to be a weak canker-causing pathogen on pear trees.
