Avocado Dieback Caused by Neofusicoccum parvum in the Andalucia Region, Spain

Abstract

From 2002 to 2006, adult avocado trees, Persea americana Miller cv. Hass, located in the subtropical-fruit-producing area of Andalucia (southern Spain) developed symptoms of dieback characterized by death of twigs and branches in the tree canopy. Sections of surface-disinfested, necrotic branch tissues were plated on Difco potato dextrose agar (PDA) (Sparks, NV) and a Neofusicoccum-like fungus was isolated. On PDA, the isolates had white, appressed mycelium that turned dull gray as the colony aged, although conidia were not formed. Abundant pycnidia and conidia developed when isolates were cultured on 2% water agar with sterilized pine needles as substratum at 25°C under near-UV light for 2 weeks. Conidia were hyaline, unicellular, ellipsoid with an obtuse apex and subtruncate base, averaged 16.2 μm long by 5.8 μm wide and ranged from 12.0 to 20.0 by 4.0 to 8.0 μm, and becoming brown with one or two septa with age. Sequenced rDNA fragments (ITS1, 5.8S rDNA, and ITS2, amplified with ITS1 and ITS4 primers) of two avocado isolates were 100% homologous with Neofusicoccum parvum (Pennycook & Samuels) Crous, Slippers, & A.J.L. Phillips (1) (GenBank Accession Nos. AM410965 and AM410966). Morphological and molecular results confirmed this species as N. parvum, reported as the anamorph of Botryosphaeria parva (1). A pathogenicity test was conducted using two isolates on sets of five 2-year-old avocado plants produced from seeds of cv. Topa-Topa growing in 5-liter pots with soil. Unwounded and wounded plants were used for inoculations. Plants were wounded 2 to 3 cm below the apical tip with a lance (4 mm long and 1 mm deep). For inoculation, 4-mm 2-week-old PDA culture plugs were placed in contact with wounded tissues and covered with Parafilm. Five noninoculated plants treated similarly served as controls. Plants were maintained in the greenhouse with a temperature range of 18 to 26°C, and 1 month later, brown stem lesions, as much as 5 cm, originating from the inoculation site followed by dieback of branches were observed. Reisolations from necrotic branches were successful, and both isolates with identical morphology to those used for inoculations were recovered. Pathogenicity tests of seedlings using the same methods also caused stem lesions on unwounded plants and the pathogen was reisolated. To our knowledge, this is the first report of N. parvum causing dieback of avocado trees in Spain. Previously, B. parva has been reported causing stem-end rot of avocado fruit in New Zealand (2). In Spain, since diseased orchards are increasing rapidly, this pathogen could be efficiently distributed by pruning activities (tools and vegetal debris) as observed with other diseases (3). The presence of N. parvum in this subtropical area presents a serious disease problem not only to avocado but also to mango (Mangifera indica L.), which is another susceptible host (4). References: (1) P. W. Crous et al. Stud. Mycol. 55:235, 2006. (2) W. F. T. Hartill et al. N. Z. J. Crop Hortic. Sci. 30:249. 2002. (3) A. J. L. Phillips. Phytopathol. Mediterr. 41:3, 2002. (4) B. Slippers et al. Mycologia 97:99, 2005.