Control of onion white rot using beneficial microorganisms and soil solarisation

The fungal isolates Chaetomium globosum (A53), Coniothyrium minitans (A69), Trichoderma harzianum (C52), Trichoderma koningii (C60, C62) and Trichoderma viride (D73) were confirmed as antagonists of Sclerotium cepivorum, the causal agent of onion white rot. In dual culture with S. cepivorum, these isolates produced inhibition zones and colonised pathogen hyphae. When agar was amended with culture filtrates of C. Globosum and T. harzianum, the growth of S. cepivorum was distorted or unusual, indicating the production of antibiotics. The ability of these isolates to reduce the incidence of onion white rot was tested in a glasshouse trial in artificially infested (1 sclerotium per gram soil) Wakanui silt loam soil. A fungal homogenate of each of the fungal antagonists was cultured in a sand:bran mix and added to the soil in polystyrene boxes (59 x 33 x 19cm), at two different application times. Application 1 was applied to half of the polystyrene boxes, two weeks before onion seed planting and application 2 was applied to the remaining soil boxes at the time of onion seed planting. T. harzianum (C52) applied to the soil two weeks before onion seed planting resulted in disease control equivalent to the fungicide (procymidone) seed treatment (82.9%). When the fungal treatments were applied to the soil at the time of planting, all treatments afforded disease control equivalent to the procymidone control. A significantly lower seedling emergence resulted with the application of the fungal treatments compared with the procymidone control treatment; irrespective of fungal application time, however, a significantly greater number of seedlings emerged when the fungal treatments were applied at the time of planting rather than two weeks before planting. T. harzianum and T. Koningii (C62) were selected for further investigation as these species established and grew well on a range of media, unlike Chaetomium globosum and Coniothyrium minitans. In a second glasshouse trial, T. harzianum and T. koningii were applied to the soil at the time of planting as onion seed coatings or alginate pellets. Mixtures of the two fungi were also prepared and a combination of Trichoderma spp. (Trichopel) was also included as a treatment. Unfortunately, only low numbers of white rot diseased seedlings were evident and no significant treatment effects were observed for the duration of the trial. No disease developed in a field trial established to compare commercially prepared pellets of T. harzianum, T. koningii and combinations of the two Trichoderma spp. The lack of treatment effects was a result of low amounts of pathogen inoculum rather than the failure of the fungal antagonist delivery system. S. cepivorum sclerotial viability was greatly reduced when sclerotia were incubated in soil and exposed to continuous temperatures ranging from 20 to 50+ºC in incubators. Temperatures above 40°C reduced sclerotial viability to 0% after a 1 day exposure period, whereas at 20°C, sclerotial viability was reduced to 10.7% after 28 days. Temperatures above 20-25°C adversely affected sclerotial viability, although a lower temperature for a longer period did not cause the same extent of sclerotial degradation as a higher temperature for a shorter period of time. A four week soil solarisation trial did not reduce sclerotial viability when artificially produced S. cepivorum sclerotia were buried in the soil at 10 and 20cm depths under polythene. The solarisation treatment raised the average soil temperature by 7°C. While there was a significant difference in the maximum and minimum temperatures recorded at 10 and 20cm in both solarised and non solarised soil, the increase in temperature at the shallower depths did not affect sclerotial viability. Increases in temperature adversely affected sclerotial viability, although in Canterbury the use of soil solarisation alone did not reduce sclerotial viability. Beneficial microorganisms could be combined with soil solarisation, either following soil solarisation at the time of onion seed planting or heat tolerant microorganisms could be incorporated into the soil before soil solarisation treatment. An integrated control approach could provide a commercially viable programme for control of onion white rot.