First Report of Asian Brown Rot Caused by Monilia polystroma on Apricot in Switzerland.

Abstract

A survey for Monilia fructicola (G. Winter) Honey on apricots (Prunus armeniaca L.) was conducted in July and August 2009 and 2010 in Canton Wallis, Switzerland. Mummies of fruits showing brown rot were collected and isolations were conducted. Nearly 200 fungal isolates, tentatively identified as M. fructigena, were retested with a multiplex PCR (1). With the Agilent 2100 Bioanalyzer (Agilent Technologies, Basel, Switzerland) instead of 1.5% agarose gels, the 23 bp difference between the diagnostic fragments of M. fructigena and M. polystroma van Leeuwen (1) could clearly be scored. M. polystroma was diagnosed in 3 of 65 and 1 of 132 isolates collected in 2009 (13 orchards) and 2010 (10 orchards), respectively. The internal transcribed spacer (ITS) regions of four isolates (09-G4, 09-P16, 09-S5, and 10-C6) were amplified and sequenced (4). The four sequences (GenBank No. JN128835) as well as those of the Hungarian isolate UFT (AM937114 [3]) were identical and highly similar to the type sequence for M. polystroma (Y17876 [2]). The type sequence had a "T" at position 414, which was lacking in the other five sequences. The genomic region of unknown function used by Côté et al. (1) to develop their PCR diagnostic tool was sequenced for isolate 09-G4 with primers MFG.for (3) and M Poly rev 5'-CCACTTACATTTTTGGCTATTG-3'. The Swiss isolate (GenBank No. JN128836) and the Hungarian isolate UFT (AM937120) sequences were identical. The pathogenicity of isolate 09-G4 was tested on Golden delicious apples. Six apples were surface sterilized (70% ethanol), halved, and placed in sterile plastic boxes cut-side down. Further, six half apples were wounded in the center with a round scalpel with a diameter of 1 cm and inoculated with a round, potato dextrose agar (PDA) plug (1-cm diameter) of actively growing mycelium (5- to 7-day-old culture). Control apples (six halves) were treated with a PDA plug without mycelium. All fruits were incubated at 20°C with a 12-h light 12-h dark cycle. Seven days after inoculation, typical brown rot symptoms were visible on all inoculated fruits. Mock inoculated fruits remained healthy. Three inoculated halves, in addition to the brown rot symptoms, also produced sporodochia and around the inoculation point the tissue become black. With the multiplex PCR (1), M. polystroma was identified as the pathogen causing brown rot symptoms on the inoculated apples. The ellipsoid single-cell hyaline conidia of isolate 09-G4 grown on the Golden delicious apples averaged 15.2 ± 4.0 × 8.97 ± 1.1 μm and were the expected size for M. polystroma conidia (14.9 to 9.1 μm [4]). The first evidence of a new Monilia species was reported by Fulton et al. (2). They found that M. fructigena isolates from Japan were distinguishable from European isolates by five base substitutions in the ITS region (four in ITS1 and one in ITS2). Later, van Leeuwen et al. (4) found that the two groups of isolates could also be distinguished by morphological differences and described the new species as M. polystroma. According to the Centre for Agricultural Bioscience International, the impact of M. polystroma in a new area is presumed to be the same or very similar to that of M. fructigena. To our knowledge, this is the first report of M. polystroma in Swiss orchards.