Avocado (Persea americana Mill.) is grown in some areas of southern Italy. In spring 2011, a previously unknown rot was observed on fruit that was marketed in Torino (northern Italy). The decayed area started from the stalk, appeared irregular and soft, and was surrounded by a dark brown margin. The internal decayed area appeared rotten, brown, and surrounded by bleached tissue. Fragments (approximately 3 mm) were taken from the margin of the internal diseased tissues, cultured on potato dextrose agar (PDA), and incubated at temperatures between 21 and 25°C under alternating conditions of light and dark. Colonies of the fungus initially appeared whitish, later turning mouse gray to black. Mature mycelium was septate and produced a dark pigment. The fungus, grown on oat agar (2) and incubated at temperatures between 21 and 25°C under alternating light and darkness, produced grayish colonies with a fluffy aerial mycelium that became dark with age and produced black pigments. After 18 days of incubation, such colonies produced pycnidia aggregated into stromatic masses, emerging from decayed tissues, and up to 3 to 4 mm in diameter. Conidia produced in the pycnidia were initially unicellular, hyaline, granulose, ovoid to ellipsoidal, and measured 20.8 to 26.9 × 12.5 to 16.1 (average 24.4 × 13.5) μm. After 7 days, mature conidia became darker, uniseptate, and longitudinally striate. Paraphyses produced within the tissues of pycnidia were hyaline, cylindrical, nonseptate, and up to 63 μm long. Morphological characteristics of mycelia, pycnidia, and conidia observed with a light microscope permitted identify of the fungus as Lasiodiplodia theobromae (3). The internal transcribed spacer (ITS) region of rDNA was amplified using the primers ITS1/ITS4 and sequenced. BLAST analysis (1) of the 488-bp segment showed a 100% similarity with the corresponding sequence (GenBank Accession No. GQ502453) of L. theobromae Pat. Griffon & Maubl. The nucleotide sequence of the strain used for pathogenicity tests was submitted to GenBank (Accession No. JN849098). Pathogenicity tests were performed by inoculating 10 avocado fruits after surface disinfesting in 1% sodium hypochlorite and then wounding. Mycelial disks (8 mm in diameter) obtained from PDA cultures of one strain were placed on wounds. Ten control fruits were inoculated with plain PDA. Fruits were incubated at 15 ± 1°C. The first symptoms developed 4 days after the artificial inoculation. After 7 days, the rot was evident and L. theobromae was consistently reisolated. Noninoculated fruit remained healthy. The pathogenicity test was performed twice. To our knowledge, this is the first report of the presence of L. theobromae causing postharvest fruit rot on avocado in Italy, as well as in Europe. The occurrence of postharvest fruit rot on avocado caused by L. theobromae was described in many avocado-producing areas such as the United States (4), South Africa, and Israel. In Italy, the economic importance of avocado cultivation is currently limited.
References: (1) S. F. Altschul et al. Nucleic Acids Res. 25:3389, 1997. (2). P. Narayanasamy. Microbial Plant Pathogens. Detection and Disease Diagnosis: Fungal Pathogens. Springer, Dordrecht, 2011. (3) E. Punithalingam. Sheet 519. CMI Description of Fungi and bacteria, 1976. (4) H. E. Stevens and R. B. Piper. Circular No. 582, USDA, 1941.