Impact of competitive mycoflora on Aspergillus flavus and A. parasiticus populations in stored peanut pods

PASSONE, M.A.; PONZIO, V.; NESCI, A.; ETCHEVERRY, M.

Cairns Convention Centre, Queensland, Australia

Congreso; 8th International Mycological Congress; 2006

Organising Commitee of the 8th International Mycological Congress

The aflatoxigenic fungi, A. flavus and A. parasiticus infect a wide variety of crops, all of which produce oil-rich seed. A histological study of the host-pathogen interaction between stored peanut (Arachis hypogaea L.) and A. parasiticus demonstrated that the fungus colonized all tissues of the peanut pod and appeared to grain ingress through the corky layer of the pericarp. GUS activity was found in hyphae infecting the pericarp, embryo and cotyledons, indicating expression of aflatoxin biosynthetic genes in these tissues (Haixin et al, 2000). This fact appears to turn to peanut pods more susceptible to be infected by other fungal pathogens. Thus, the object of the present work was to evaluate the behavior of Aspergillus section Flavi population relative to other natural competing mycoflora in peanut pods over a 5-month storage period. Two experimental polyethylene big bags were used to store 200 kg of peanut pods from July to December 2004. One of these was previously homogeneously inoculated with a spore suspension (104 spores ml-1) of a mixture of A. flavus CHG46 and A. parasiticus CHG24. Ten samples (250 g) were randomly collected at 30 days intervals using a sampling device. Temperature and humidity were monitored using a distance reading thermometer and hygrometer. To determine fungal colonization on peanut pods, 50 g of milled pods were shaken with 450 ml of peptone / water and spread on dichloran / rose Bengal / chloramphenicol (DRBC) and dichloran / glycerol 18% (DG18) media. The results were expressed as colony-forming units (CFUs) per gram of peanut pods. The fungal genera were identified according to Pitt and Hocking (1997) and Samson et al (2002). The results on mycobiota composition present in nature peanut pods corroborate that the predominant genera of potentially toxigenic fungi, during the five storage months, were Penicillium spp., Aspergillus section Flavi, Aspergillus section Nigri and Fusarium spp. In minor grade the genera isolated were Eurotium spp., Trichoderma spp., Mucor spp., Absidia spp., Monascus spp. and Alternaria spp. Whereas, during the 1th storage month, mycological analyses of inoculated peanut pods showed predominance of Penicillium spp. (4.5x104 UFC g-1) following by Aspergillus section Flavi (1.8x104 UFC g-1), Fusarium spp. (2.2x103 UFC g-1) and Aspergillus section Nigri (4x102 UFC g-1), a high prevalence of Aspergillus section Flavi (7.7x104 UFC g-1) following by Penicllium spp and Aspergillus section Nigri with count of 6.8x103 and 3x103 UFG g-1, respectively and absence of Fusarium spp was observed at the end of the storage period. Between the 1th and 5th month of storage, fungal count in both containers significantly increased (1 log unit) according enhanced the temperature (7.1 to 19.8°C), while the humidity to be maintained relatively constant about  of 8.2%. Significantly differences of 0.5 and 1 log units was observed when compared total fungal count between nature and inoculated peanut pods. This study showed the prevalence of Aspergillus section Flavi from 2th to 6th sampling in inoculated peanut pods demonstrating the high competition capacity of this populations between Aspergillus section Nigri, Penicillium spp., Fusarium spp. and other fungal species for niche occupation and other interspecific interaction that could exert selection pressure on the mycota influencing the dominance of species.