Acremonium Brown Spot, a New Disease Caused by Acremonium sclerotigenum on Bagged Apple Fruit in China
Authors: Li B. H. , Wang C. C. , Dong X. L. , Zhang Z. F. , Wang C. X. 
Published: March 2014

In China, covering developing apple (Malus × domestica) fruit with paper bags is a standard production practice. The fruit are usually covered from May to October to exclude pests and rain-dispersed pathogens and reduce pesticide residue at harvest. From 2010 to 2012, a fruit spot disease was observed on bagged fruit and caused 1 to 30% annual yield losses in most orchards in Shandong Province. Affected fruit were covered with red-brown, sunken, circular lesions 2 to 20 mm in diameter with dark violet edges often surrounded by a red halo. In many cases, the lesion cracked and pinkish mycelium was observed within the cracks. Isolations were made from bagged fruit from 12 orchards in October 2010 to 2012. Fungal isolations were made onto potato dextrose agar (PDA) medium. Two strains were consistently obtained from isolates. Strain 1 produced conidia assembled in head. Conidia were ellipsoidal to ovoid and 2.1 to 7.5 × 1.1 to 3.0 μm. Colonies were whitish with some pink and powdery on PDA. String 2 produced conidia in a long chain. Conidia were spindle-shaped with apiculate at both ends and 2.1 to 6.6 × 1.3 to 3.8 μm. Colonies were whitish at the beginning and grayish later and powdery on PDA. To further confirm the identity of the isolated fungus, the large subunit (LSU), the small subunit (SSU), and the internal transcribed spacer (ITS) sequences of ribosomal DNA, and the β-tubulin gene (β-tubulin), were amplified and sequenced with the primers V9G/LR5, NS1/NS24, ITS1/ITS4, and Bt1a/Bt1b, respectively. LSU (GenBank Accession Nos. KJ194115 and KJ194116), SSU (KJ194117 and KJ194118), ITS (KF225143 and KF225144), and β-tubulin (KF225145 and KF225146) sequences didn't have any variation between the two strains sequenced. Phylogenetic analyses of each of the examined genes indicated a high similarity (>99%) with Acremonium sclerotigenum (CBS 384.65 HQ232129). Based on the sequence data and the morphology, we identified the fungus as A. sclerotigenum (1,2). To confirm pathogenicity, a spore suspension (1 × 104 conidia per ml) was made from each of the strains isolated. Strains were subsequently inoculated on to 10 mature apple fruit by wounding them to a depth of 2 mm with an acupuncture needle. Inoculation with sterile distilled water was included as a control. Prior to inoculation, all fruit were surface-sterilized with 75% alcohol. Lesions developed on fruit inoculated with the putative pathogen 10 days after incubation in >90% humidity chamber at 25°C. The fungi that were isolated from the infected fruit were identical to the inoculated strains. No lesions developed on the control fruit. This is the first report of brown spot disease caused by A. sclerotigenum in apple and in bagged fruit production. Given that brown spot disease symptoms were usually observed in September after long periods of rain, management efforts need to focus on protecting bagged fruit before harvest.

References: (1) H. Perdomo et al. J. Clin. Microbiol. 49:243, 2011. (2) R. C. Summerell et al. Stud. Mycol. 68:139, 2011.