H. V. Silva-Rojas, and
S. García-Morales, Colegio de Postgraduados en Ciencias Agrícolas, Km. 36.5 Carr. México-Texcoco, Montecillo, Texcoco, Edo. de México, México 56230; and
G. Avila-Quezada, Centro de Investigación en Alimentación y Desarrollo A.C. Ave. 4 Sur no. 3820. Fracc. Vencedores del desierto, Cd. Delicias, Chihuahua, México 33089
Avocado (Persea americana L.) production for export markets has increased in Mexico during the past 10 years. The production system, however, is affected by several sanitation factors, including diseases. During the spring of 2009, smooth, black, circular spots were noted on the surface of avocado fruit. A study was conducted during the winter of 2010 to ascertain the etiology and identify the fungus associated with black spot symptoms on avocado fruit in orchards of Nuevo Parangaricutiro County (19°25′00″ and 102°07′43″) in Michoacan, Mexico. Several fungal isolates were obtained on potato dextrose agar (PDA) from the margin of lesions on immature fruit. The internal transcribed spacer region (ITS) of the rDNA from representative isolates was sequenced with universal primers ITS5 and ITS4 (2). BLAST searches in GenBank showed 100% similarity of the nucleotide sequences with Neofusicoccum parvum (Pennycook & Samuels) Crous, Slippers & A.J.L. Phillips, GenBank Accession Nos. GU188001 to GU188007 and GU187985 to GU187987. A representative nucleotide sequence of this region was deposited in GenBank under the Accession No. JN203129. Strains of N. parvum produced aerial and compact mycelium on acidified PDA, the anamorph state of Botryosphaeria parva. Mycelium was initially white, turning gradually gray to black. Conidia were one celled, hyaline, ellipsoidal to fusiform, externally smooth, thin walled, nonseptate, with one or two septa with age, and an average length and width of 14.5 (9.5 to 19) × 5.8 (4.0 to 7.2) μm (n = 100). Pathogenicity tests were conducted with six avocado fruit cv. Hass. Fruit were inoculated at three evenly spaced locations on the fruit surface, either by wounding the tissue with a needle that had been dipped in a conidial mass from an 8-day-old monoconidial culture of N. parvum strain CIAD-021-11 or by placing 5 μl of 1 × 106 conidia ml–1 suspension on each inoculation site. Inoculated fruit were maintained in a moist chamber at 25°C for 2 weeks. Black lesions appeared on all wounded sites 2 days postinoculation (dpi) and on unwounded sites 4 dpi. The delay of symptom development was likely due to penetration through the lenticels, which took longer to initiate infection. No symptoms were observed in the control fruit. The pathogen was reisolated from the lesions of all inoculated fruit, thus fulfilling Koch's postulates. The results confirmed the pathogenic potential of this fungus and indicated its possible involvement in the etiology of black spot on avocado fruit. N. parvum is a cosmopolitan, plurivorous pathogen causing disease in several hosts of economic importance, such as grapes and kiwi, as well as causing stem-end rot of avocado fruit in New Zealand (1) and avocado twigs in Spain (3). To our knowledge, this is the first report of N. parvum causing black spots on avocado fruit in Mexico.
References: (1) W. F. T. Hartill et al. N.Z.J. Crop Hortic. Sci. 30:249. 2002. (2) 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. (3) T. Zea-Bonilla et al. Plant Dis. 91:1052, 2007.