BIOCONTROL OF AFLATOXINS IN FIGS

Abstract

For several years, we have investigated the use of atoxigenic strains (strains not able to produce aflatoxins) of Aspergillus flavus as biocontrol agents to reduce aflatoxin contamination of figs in California, USA. The atoxigenic strain AF36 has been successful in reducing aflatoxin contamination of cottonseed in commercial cotton fields in Arizona. Approximately 6% of the A. flavus isolates obtained from commercial fig orchards in California belonged to the atoxigenic strain AF36. In 2003 and 2004, AF36 was applied in 'Calimyrna' fig orchard. In early summer, wheat seeds infected with AF36 were applied at the rate of 11.2 kg/ha to the orchard floor. Almost all A. flavus isolates obtained from noncaprified figs collected from the orchard floor belonged to the applied strain AF36, indicating that AF36 had colonized debris in the orchard. In late summer, leaf and soil samples were collected. The soil had a higher density of A. flavus/A. parasiticus in the areas under the drip lines where infected wheat had been placed (72.6 to 113.2 colony forming units (cfu)/g soil for areas treated with AF36 in 2003) than in the middles (5.5 to 11.9 cfu/g) or under the drip lines in the untreated areas (0.9 cfu/g). In 2003, all of the A. flavus isolates obtained from the soil under the drip lines in the treated areas belonged to AF36 compared to only 18.7% in the untreated areas. In contrast, in the middle the incidence of AF36 was approximately the same in the treated areas (21.5 and 33.7%) and in the untreated areas (20.8%). The density of A. flavus/A. parasiticus on leaves did not differ significantly between treatments, even though higher levels of AF36 were on leaves in the treated areas than in the untreated areas. Atoxigenic strains continue to show promise for the reduction of aflatoxin contamination of figs.