Plum (Prunus domestica) and peach (P. persica) are widely grown, often in alternate rows with citrus, in the Mazandaran Province of Iran. In June 2011, a dry fruit rot of plum was observed in several production regions in Mazandaran Province (35°47′N, 50°34′E). Initial symptoms at pit-hardening stage appeared as dark brown, circular, necrotic spots from 2 to 5 cm in diameter. They later developed into a dry fruit rot. Severe symptoms occurred during June and July when warm weather (temperature around 28°C) and high relative humidity (RH) (>85%) were present. Marketable yield losses reached 50% to almost 100% in many orchards. To isolate the causal organism, symptomatic fruits were surface disinfested for 1 min in 0.5% active chlorine, washed thoroughly with sterile distilled water, and segments were plated on potato dextrose agar (PDA) amended with 50 mg/liter of streptomycin sulfate and incubated at 25°C for 3 days. The fungus Hyphodermella rosae (Bresadola) Nakasone was consistently isolated (37 isolates from 79 samples) and identified on the basis of morphological characteristics on PDA. Basidiomata were effuse, resupinate, 15 × 10 mm, crustaceous, tubercules small with apical bristles, and light orange to grayish orange. Subhymenium was up to 30 μm thick, composed of vertically arranged, short-celled, nonagglutinated hyphae; subhymenial hyphae were 3 to 4 μm in diameter. Basidiospores were ellipsoid, 7.5 to 8.5 × 4.5 to 5.5 μm (100 determination), and their cell walls were thin, hyaline, and smooth (1). Genomic DNA was extracted from mycelium with a DNA extraction kit (Qiagen, Hilden, Germany) according to the manufacturer's directions and grown on potato dextrose broth for 4 days at 28°C. The rDNA region was amplified with the primers ITS4 (5′-TCCTCCGCTTATTGATATGC-3′) and ITS5 (5′- GGAAGTAAAAGTCGTAACAA-3′) (4) and the PCR product was sequenced. Nucleotide BLAST analysis of the amplified 627-bp fragment confirmed a 99% similarity with the sequence of H. rosae (GenBank Accession No. JN593086). A pathogenicity test was conducted with isolate MA4099 by placing 5-day-old mycelial plugs grown on PDA at the surface of healthy fruit (n = 6) incubated under >85% RH at 25°C for at least 4 days until the appearance of symptoms, which were similar to those displayed under orchard conditions. Control fruits, inoculated with blocks of PDA plugs, remained intact and symptomless. Reisolation from inoculated fruit samples consistently yielded the inoculated fungus, completing Koch's postulates. The genus Hyphodermella has been reported to be causing wood rot on apricot (2) and sweet and sour cherry (3). To our knowledge, this is the first report of H. rosae causing dry fruit rot on a stone fruit species in the world.
References: (1) K. K. Nakasone. Mycologie, 29:231, 2008. (2) J. M. Ogawa et al. Diseases of Apricot (Prunus armeniaca L.). The American Phytopathological Society, St. Paul, MN, 2003. (3) J. K. Uyemoto et al. Diseases of Sweet Cherry (Prunus avium L.) and Sour Cherry (P. cerasus L.). IS-MPMInet, http://www.ismpminet.org/resources/common/comment/cherry.asp, accessed June 2012. (4) T. J. White et al. Page: 315 in: PCR Protocols: A Guide to Methods and Application. M.A. Innis et al., eds. Academic Press, San Diego, CA, 1990.