Control of white rot of garlic by antagonistic fungi under controlled environmental conditions

Abstract

Several fungi were isolated from soil samples collected in areas with a high incidence of white rot disease of garlic at Amarantina county, Minas Gerais, Brazil. After the screening in vitro for antagonists to Sclerotium cepivorum, three fungi were found to be highly inhibitory to the pathogen. These fungi were identified as Trichoderma harzianum Rifai, Paecilomyces lilacinus (Thom.) Samson, and a Penicillium sp. The three antagonists produced in vitro nonvolatile antibiotics towards Sc. cepivorum. These substances were thermolabile (120 °C, 15 min) and significantly inhibited the growth of Sc. cepivorum. Interactions between the pathogen and each antagonist, studied by a dual slide-mount technique, showed that Pa. lilacinus and Penicillium sp. caused an inhibition halo to Sc. cepivorum although hyphal contact never occurred. Trichoderma harzianum showed inhibitory activity at a distance, and after contact with Sc. cepivorum, caused hyphal cytoplasm disintegration and cell wall collapse. This detrimental effect occurred either after direct penetration of the cell wall or after the formation of "coilings." The efficacy of the three antagonists in the biocontrol of white rot was tested under controlled environmental conditions in a nonsterilized, artificially infested soil (1 sclerotia per gram of soil). Trichoderma harzianum VL1 applied as a conidial suspension (106 conidia/mL) during the transplanting period significantly reduced the severity of white rot and increased the number of healthy plants when compared with the untreated control. Trichoderma harzianum gave significantly better protection to the plants than Coniothyrium minitans, a known biocontrol agent of sclerotia-forming fungi. Paecilomyces lilacinus, Penicillium sp., and the mixture of the three antagonists gave no protection against the disease.