A new postharvest fruit rot in apple and pear caused by Phacidium lacerum

Authors: WISEMAN, M - Washington State University; KIM, Y - Pace International, Llc - Usa; Dugan, Frank; ROGERS, J - Washington State University; Xiao, Chang-Lin

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/26/2015
Publication Date: 1/1/2016
Citation: Wiseman, M.S., Kim, Y.K., Dugan, F.M., Rogers, J.D., Xiao, C. 2016. A new postharvest fruit rot in apple and pear caused by Phacidium lacerum. Plant Disease. 100:32-39. doi: 10.1094/PDIS-02-15-0158-RE.

Interpretive Summary: During surveys for postharvest diseases of apple and pear in Washington State, an unknown fruit rot disease was observed on stored apples and pears collected from commercial fruit packinghouses. The disease appeared to originate from infection of the stem and calyx tissue of the fruit or wounds on the fruit. An unknown pycnidial fungus was consistently isolated from the decayed fruit. Isolates from apple and pear were identified as Phacidium lacerum based on molecular phylogenetic analysis and morphology. In laboratory and/or field tests, the fungus was able to cause decay at 0ºC on apple and pear fruit inoculated with the fungus and re-isolated from decayed fruit, proving that Phacidium lacerum is a postharvest fruit-rotting pathogen of apple and pear. This is the first report of a fruit rot in stored apples and pears caused by P. lacerum. We propose Phacidium rot as the name of this disease.

Technical Abstract: During surveys for postharvest diseases of apples and pears, an unknown postharvest fruit rot was observed in Washington State. The disease appeared to originate from infection of the stem and calyx tissue of the fruit or wounds on the fruit. An unknown pycnidial fungus was consistently isolated from the decayed fruit. Isolates from apple and pear were characterized and identified by molecular phylogenetic analysis and morphology. Pathogenicity of representative isolates on apple and pear fruit was tested under laboratory or field conditions. A BLAST search in GenBank showed that isolates differed from Phacidium lacerum and its synonym Ceuthospora pinastri by only 0-4 base pairs in sequences within part of the combined large ribosomal subunit + internal transcribed spacer + small ribosomal subunit regions. The phylogenetic analysis confirmed the taxonomic placement of the unknown fungus in the genus Phacidium with the highest match being C. pinastri (formerly anamorphic P. lacerum) and with closely related taxa from GenBank forming congeneric clades. The fungus grew at 0 to 30°C and formed unilocular to multilocular pycnidial conidiomata on artificial media after ca. 5-7 days at room temperature. On potato dextrose agar incubated for a 12-h photoperiod, semi-immersed globose to subglobose pycnidial conidiomata were 250-1000 µm in diameter (mean=350), with 1-3 non-papillate to slightly-papillate ostioles and a buff conidial matrix. Conidia produced on phialides were 8-13 × 1.5-2.5 µm, hyaline, aseptate, cylindrical, with an abruptly tapered, typically slightly protuberant base, 2-3 guttules, and sometimes with a mucilaginous, flexuous, unbranched appendage which is attached to the apex of the conidium and disappears with age. Conidiogenous cells were flask shaped and 6-15 ×1.5-3 µm. Colony characteristics included: felt-like aerial white mycelium, gray olivaceous at the center becoming greenish to colorless toward the margin, in concentric rings, with pycnidia forming in 5-7 d originating from the center of the plate. Morphological characteristics of the fungus had the greatest conformity with the description for C. pinastri. Based on molecular and morphological data, the fungus is identified as P. lacerum. ‘Fuji’ apple fruit and ‘d’Anjou’ pear fruit that were wounded, inoculated with representative isolates, and incubated at 0°C yielded the same symptoms as seen on decayed fruit collected from commercial fruit packinghouses. Stem-end rot, calyx-end rot, and wound associated rot developed on fruit inoculated in the orchard after 3 months of cold storage. The fungus was re-isolated from the diseased fruit. This is the first report of a fruit rot in apple and pear caused by P. lacerum. We propose Phacidium rot as the name of this disease.