In relation with the biocontrol of aflatoxins on peanuts using soil fungi, the effects of fungi isolated from soil of peanut fields on the growth and aflatoxins production of Aspergilus flavus were investigated in vitro.
Soil samples were taken from peanut fields at harvest time up to ± 2 weeks after harvest during the dry season (the end of May 2000) at Wonogiri regency, Central Java. The number of samples from 52 peanuts fields was 52. The dilution method followed by pour plate method on Aspcrgillus flavus and parasiticus agar (AFPA) and peptone dextrose agar (PTDA) media were used for enumerating and isolating A. flavus and other fungi in the soil, respectively. Toxigenic and non-toxigenic A. flavus isolates were determined by culturing each isolate on fresh coconut agar medium 10%. The presence of aflatoxin was observed under UV lamp with wave length of 365 nm, and then it was further confirmed using Thin Layer Chromatography method. The dominant fungal isolates obtained were tested for their antagonistic property against five toxigenic A. flavus isolates using the direct opposition method on Potato Dextrose Agar (PDA). One isolate of toxigenic A. flavus and each of the promising antagonist were grown in dual culture on SMKY (200 g Sucrose, 0.5 g MgSO4 7H2O, 3 g KNO3 and 7 g Yeast Extract) liquid medium to determine the effect of the antagonists on aflatoxin production.
A. flavus was found on 98.1% of 52 soil samples. Its population was 11 - 1768 cfu/g. If the fluorescent isolates were assumed toxigenic, then 7951 (43%) out of 18 380 colonies of A. flavus were considered toxigenic. Thirty two fungal species were isolated from all soil samples. The predominant fungi were Aspergillus niger (its presence was 100% of all samples), followed by Trichoderma harzianum (98.1%), A. flavus (98.1%), Nigrospora orizae (86.5%), Penicillium sp.1 (84.6%) and P. funiculosum (78.8%). The test fungi were selected based on 25 - 100% of their presence, and they were not human and plant pathogens. Toxigenic A. flavus isolate 102 was the most resistant to the test fungi. Test fungi that inhibited more than 20% the growth of A. flavus isolate 102 were non-toxigenic A. flavus isolates 26, 181, 361, 612, A. niger, A. tamarii and Trichoderma harzianum. Aspergillus niger was the most promising fungal antagonist, because it caused the highest percent inhibition (80%) (P=0.05) of aflatoxin production of A. flavus isolate 102, followed by non-toxigcnic A. flavus isolate 612 (61%), A. tamarii (60%) and nan-toxigenic A. flavus isolate 361 (59%).A. flavus was found on 98.1% of 52 soil samples. Its population was 11 - 1768 cfu/g. If the fluorescent isolates were assumed toxigenic, then 7951 (43%) out of 18 380 colonies of A. flavus were considered toxigenic. Thirty two fungal species were isolated from all soil samples. The predominant fungi were Aspergillus niger (its presence was 100% of all samples), followed by Trichoderma harzianum (98.1%), A. flavus (98.1%), Nigrospora orizae (86.5%), Penicillium sp.1 (84.6%) and P. funiculosum (78.8%). The test fungi were selected based on 25 - 100% of their presence, and they were not human and plant pathogens. Toxigenic A. flavus isolate 102 was the most resistant to the test fungi. Test fungi that inhibited more than 20% the growth of A. flavus isolate 102 were non-toxigenic A. flavus isolates 26, 181, 361, 612, A. niger, A. tamarii and Trichoderma harzianum. Aspergillus niger was the most promising fungal antagonist, because it caused the highest percent inhibition (80%) (P=0.05) of aflatoxin production of A. flavus isolate 102, followed by non-toxigcnic A. flavus isolate 612 (61%), A. tamarii (60%) and nan-toxigenic A. flavus isolate 361 (59%).