Asian pear tree (Pyrus pyrifolia) is an important fruit crop in Asian countries. Between the autumn of 2008 and the summer of 2009, stem cankers and twig diebacks of Asian pear trees were observed in middle Taiwan. Necrotic lesions extending from branch scars progressed with age, resulting in darkened vascular discoloration. Two cultivars of Asian pear, Taichung No. 2 grown in Changhwa County and Heng-shan grown in Taichung County, showed the same symptoms. Disease incidence increased rapidly after a rain or storm event, eventually exceeding 50%. Pycnidia on severely infected branches contained one-celled, fusiform to ellipsoidal, smooth- and thin-walled hyaline conidia, with an average length (L) and width (W) of 19.1 (11.3 to 24.8) × 5.9 (4.5 to 8.0) μm and a L/W ratio of 3.2 (n = 44). Diseased branch tissues collected from the two locations were surface sterilized in 0.6% NaOCl, rinsed with water, and plated on potato dextrose agar (PDA). Fungal isolates, recovered from both locations, produced white, aerial mycelium and became dull gray within a week after incubating plates at 25°C. To confirm the identities of the isolates, the internal transcribed spacer (ITS) regions amplified with primers ITS1/ITS4 were deposited in GenBank (Accession Nos. GU395186 and GU395187). Both of the sequences were 99% identical to that of Neofusicoccum parvum (Accession No. EU882162) over a 534-bp alignment. Thus, both morphological and molecular characters confirmed this species as N. parvum (3), reported as the anamorph of Botryosphaeria parva (1). The two voucher isolates (BCRC34605 and BCRC34609) were deposited in Bioresource Collection and Research Center, Hsinchu, Taiwan. Pathogenicity tests were first conducted on 2-year-old greenhouse-potted Asian pear trees utilizing N. parvum isolate BCRC34605. Ten plants of the cv. Mi-li were stem wounded with a 5-mm cork borer at a depth of 2 mm. Inoculation consisted of inserting 5-mm mycelium plugs of the pathogen into the wounds and wrapping with Parafilm. Sterile PDA plugs applied to an equal number of plants with the same methods served as the controls. After 2 months incubation at an average temperature of 21°C, all inoculated plants exhibited necrotic lesions with a mean length of 23.5 mm and the control plants remained symptomless. The pathogen was reisolated from lesions of inoculated stems, thus fulfilling Koch's postulates. Pathogenicity tests were repeated by inoculating the other N. parvum isolate (BCRC34609) on pear cv. Taichung No. 2, resulting in similar results. N. parvum has been reported causing dieback and canker in a wide range of fruit trees, including grapevine (4) and mango trees (2). To our knowledge, this is the first report of N. parvum associated with stem canker and dieback on Asian pear trees. In addition, this is a newly recorded species for the mycobiota of Taiwan.
References: (1) P. W. Crous et al. Stud. Mycol. 55:235, 2006. (2) J. Javier-Alva et al. Plant Dis. 93:426, 2009. (3) S. R. Mohali et al. Fungal Divers. 25:103, 2007. (4) J. R. Urbez-Torres and W. D. Gubler. Plant Dis. 93:584, 2009.
