First Report of Aspergillus flavus Colonizing Naturally Dispersed Seeds of Oxandra acuminata, Pseudomalmea diclina, and Unonopsis matthewsii in Peru
Authors: Alvarez-Loayza P. , White J. F. , Giraldo C. Cadenas 
Published: May 2008

Tree species Oxandra acuminata, Pseudomalmea diclina, and Unonopsis matthewsii (Annonaceae) are sources of wood for people of the Amazonian Region where the trees are harvested from natural populations. With increased human population and agriculture in the Amazonian Region, forest diversity is affected. To manage the forest communities, it is necessary to understand the dynamics of regeneration of forest plants. Diseases that affect seed and seedling survival are critical in determining the ultimate species composition. During the dry season in June of 2006, rotten seeds of U. matthewsii (approximately 90% of 380 seeds) were observed in seven natural locations over an area of 150 ha in lowland tropical forest in Manu National Park, Peru. Colonized seeds were open and covered with yellow, dry, powdery, easily liberated conidia. One month later, seeds of O. acuminata in the same locations showed the same symptoms. In August of 2007, P. diclina seeds were naturally dispersed (by animals) in these plots and showed similar symptoms. The disease affecting O. acuminata was found in two other sampling sites along the river at the Los Amigos Research Station located 80 km southeast of Manu National Park. In all cases, the pathogen was identified as Aspergillus flavus based on morphological characteristics (1,2). Isolation of the pathogen was made on potato dextrose agar (PDA) amended with chloramphenicol (100 mg/l) and incubated at 28°C for 5 to 7 days. Single-spore isolations were made from each plant species and maintained as stock cultures. Colonies grown on PDA were granular, flat, and yellow at first, but quickly became bright to dark yellow-green. The radiate conidial heads measured approximately 400 μm in diameter. Older globose vesicles measured from 28 to 45 μm in diameter. Conidia were globose or subglobose with roughened walls and measured 3 to 5.5 μm in diameter. For pathogenicity tests, healthy seeds from the three species obtained from several trees, were previously surface sterilized by dipping in a 0.1% chlorine solution and allowed to dry. To inoculate seeds, a small scalpel was used to make a superficial cut on the seeds, after which a conidial suspension (3 × 105 conidia/ml of distilled water) was pipetted over each wound. For each plant species, 20 seeds were inoculated and 20 were used as control. This procedure was repeated twice. Each seed was maintained in a petri plate at ambient temperature in a field station lab and evaluated daily for 10 days. Inoculated seeds of all three species showed symptoms identical to those seen in field populations. Colonized seeds died after 2 to 7 days, and dry, yellow conidia were produced inside the open seed after approximately 1 to 2 days of decay. A. flavus was reisolated from colonized seeds. Control seeds remained healthy. A. flavus is ubiquitous and has been reported on numerous host plants worldwide (1,2). However, to our knowledge, this is the first report of A. flavus causing high seed mortality of species of Annonaceae in Peru. Our observations suggest that A. flavus is an important fungus affecting survival of seeds of O. acuminata, P. diclina, and U. matthewsii in the natural plant communities where we conducted this study.

References: (1) B. W. Horn. Mycologia 97:202, 2005. (2) K. B. Raper and D. I. Fennell. The Genus Aspergillus. Williams and Wilkins, Baltimore, MD, 1965.