First Report of Fusarium avenaceum Causing Postharvest Decay on ‘Conference’ Pears in the Netherlands

Abstract

Pear (Pyrus communis) is an important fruit crop in the Netherlands, with a total production of 349,000 tons in 2014. ‘Conference’ is the main pear cultivar that occupies 75% of the total pear production area in the Netherlands. In the Netherlands, pears are kept in controlled atmosphere cold storage for 9 to 11 months after harvest. Lesions were observed on pears of the cultivar Conference in a survey carried out from 2012 to 2014 in packing houses in the Netherlands. In general, low incidences of 1 to 5% were recorded. Lesions showed brown and watery circular necrosis, were slightly sunken, often with visible whitish, yellowish, or pink mycelia covering the lesions. Fruits were rinsed with sterile water, and lesions were sprayed with 70% ethanol until droplet runoff. The skin was removed aseptically with a scalpel, and tissue under the lesion was isolated and placed onto potato dextrose agar (PDA). The PDA plates were incubated at 20°C in the dark, and single spore isolates were propagated on fresh PDA plates. These isolates produced fast-growing colonies with extensive aerial mycelium which was initially white, then turning yellow to pink and a dark pink to red reverse. Macroconidia were slightly falcate, thin-walled, usually 5 septate, with a tapering apical cell, and measured 40 to 80 × 3.5 to 5 μm. Both cultural and morphological characteristics of the pathogen were similar to those described for Fusarium spp. The identity of two representative isolates (Fu5-44261 and Fu6-44280) from different pear lots was confirmed by means of multilocus gene sequencing. Genomic DNA was extracted using the LGC Mag Plant Kit (Berlin) in combination with the Kingfisher method (Waltham, MA). Sequences of ITS region, translation elongation factor 1-alpha (TEF1), and histone H3 (HIS3) loci were amplified and sequenced. The sequences have been deposited in GenBank under accession numbers KT350588 and 89 (ITS), KT350605 and 06 (TEF1), and KT935569 and 70 (HIS3). MegaBLAST analysis revealed that our ITS sequences matched with 99.8 to 100% identity to Fusarium spp. belonging to the Fusarium tricinctum species complex. The TEF1 sequences of both isolates were 99 to 100% identical with F. avenaceum culture collection sequences in GenBank (JQ429374 and GQ915502). The HIS3 sequences of both isolates were 97 to 99% identical with the sequences of accessions JQ435857 and GQ915469, confirming the identity of these isolates as F. avenaceum. Koch’s postulates were performed on 15 ‘Conference’ pears per isolate. Surface sterilized fruit were inoculated with 20 μl of a suspension of 105 macroconidia ml–1 prepared from a 15-day-old PDA culture, after wounding with a needle. Inoculated fruits were sealed in a plastic bag and incubated in darkness at 20°C. Symptoms appeared after 4 to 6 days on 100% of the fruits while mock-inoculated controls with water remained symptomless. Fungal colonies isolated from the lesions and cultured on PDA morphologically resembled the original isolate from the infected pears. Symptoms observed on artificially inoculated ‘Conference’ pear fruit were identical to the decay observed on ‘Conference’ pears that were obtained from cold storage. Only few reports describe symptoms associated with F. avenaceum on apple in Europe and the United States (Kou et al. 2014; Sever et al. 2012; Sorensen et al. 2009), while F. avenaceum infections of pears have not been reported. Thus, this is the first report of storage decay caused by F. avenaceum on pear. F. avenaceum infections may constitute a safety issue due to the potential production of mycotoxins such as moniliformin (Sorensen et al. 2009).