Biological effects of two Trichoderma harzianum isolates and mint oil on some postharvest fungal pathogens of sugar beet

Abstract

Sugarbeet (Beta vulgaris L.) stored in outdoor piles prior to processing, during this storage period the crop is subject to multiple postharvest rots. Some fungal pathogens (A. niger, A. flavus, Fusarium sp. and Rhizopus sp.) were isolated and identified from rotted sugarbeet. Pathogenicity test of the isolated fungi had different pathogenicity rates using different concentrations of spore suspension at different incubation periods. A. flavus showed little number of lesions and negative effect on sugarbeet when tested for pathogenicity. Two antagonists of Trichoderma harzianum were tested for their effectiveness on sugarbeet rot fungal pathogens and compared with mint oil. In dual culture, size of clear zone is not correlated with the occurrence of pathogen overgrowth by antagonists. The antagonistic action of the two tested isolates of T. harzianum against the isolated phytopathogens indicated different degrees of antagonism. The non-volatile and gaseous volatile organic compounds by the antagonists inhibited growth of phytopathogens on agar plates, and no inhibition was investigated for Rhizopus sp. and Fusarium spp. by the tested volatile and non-volatile compounds, respectively. Moreover, mint oil showed inhibition degrees as well as volatile compounds of antagonists for all of the tested pathogens except against Rhizopus. Sugarbeet treated with mint oil as well as culture filtrate of T. harzianum suppressed pathogen spore germination on the surface of sugarbeet root resulted in decreasing rot incidence. This work provides strong evidence that T. harzianum and mint oil are competitors to control sugarbeet rot.