Association of Microascus cirrosus (Microascaceae, Ascomycetes) with Brown Leaf Spot of Pistachio in Iran.

Iran is the largest pistachio-producing and exporting country in the world with an annual production of approximately 200,000 t. In July 2009, brown spots were noticed on the leaves of an unknown cultivar of pistachio (Pistacia vera L.) in the Mazhan Region of Birjand, South Khorasan, Iran. Nearly 20% of the trees in the surveyed area (1 ha) showed irregularly shaped leaf spots that were mainly on the abaxial surface, measured 4 to 6 mm in diameter, and were initially pale brown but progressively became dark brown. Lesions were cut from diseased tissues, surface sterilized, and placed on acidified potato dextrose agar (APDA). An anamorphic fungus was consistently isolated from the tissues. The fungus was identified as Microascus cirrosus Curzi (1,4). Colonies on PDA at 25°C reached 9 mm in diameter in 7 days and appeared olivaceous brown and zonate, and perithecia developed in 14 days. Perithecia were black, superficial or partially immersed, spherical, 110 to 200 μm in diameter, with cylindrical necks measuring up to 60 μm long by 25 to 35 μm broad; asci were globose to ovate, eight spored, and 9 to 11 × 8 to 10.5 μm; ascospores were pale reddish brown, variable in shape, ranging from concavo-convex, sometimes plano-convex to oval, and 4.5 to 6 × 3 to 4 μm. The associated anamorph on PDA was a Scopulariopsis sp. with annellophores borne singly or in groups of two or three, 10 to 16 × 2.5 to 3.5 μm, with abruptly narrowed apices; conidia were produced in chains, globose to subglobose, basally truncate, pale olive to fuscous, and 4 to 5 × 3 to 4 μm; chlamydospores were globose to pyriform, terminal or intercalary, and 8.5 to 17.5 μm in long axis. Pathogenicity of two isolates was tested using the filter-paper method of Takahashi et al. (3). Mycelial plugs (5 mm in diameter) as well as filter papers containing conidia were placed on five needle-wounded or nonwounded attached leaves of pistachio cv. Kale-Ghuchi in triplicate. The leaves were covered with a translucent plastic bag for 4 days. Inoculated detached and inoculated wounded attached leaves showed symptoms after 6 and 9 days on 5-month-old and 2-month-old leaves, respectively. Control leaves (five in triplicate) remained symptomless. The pathogen was frequently reisolated from the margins of the lesions. To our knowledge, this is the first record of M. cirrosus occurring on pistachio as well as the first report of a member of this genus from Iran. The same fungus has been reported as the causal agent of stored corn deterioration in the United States (1). M. cirrosus also produced a dimorphic yeast phase on PDA but not on malt extract agar or water agar. Cultures with partial reversion developed chlamydospore-like cells. Higher temperatures (25 to 30°C) and surface-sterilized pistachio leaves placed on PDA accelerated the reversion of the yeast phase to hyphal phase. Although dimorphism has been reported in other plant pathogenic fungi (2), this is the first report of dimorphism in this fungus and of a dimorphic fungus attacking pistachio. Further investigations are needed to determine the relation between dimorphism and pathogenicity. A culture (IRAN 1497 C) of the fungus is preserved by R. Zare. References: (1) G. L. Barron et al. Can. J. Bot. 39:1609, 1961. (2) M. Nadal et al. FEMS Microbiol. Lett. 284:127, 2008. (3) W. Takahashi et al. J. Plant Pathol. 91:171, 2009. (4) J. A. von Arx. Persoonia 8:191, 1975.