School of Agriculture, Food and Ecosystem Sciences - Theses
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ItemDevelopment of management practices for control of Rhizoctonia solani Kuhn (AG 8) in Victorian cereal crops( 2003)The studies reported in this thesis showed that Rhizoctonia solani Anastomosis Group 8 was widespread in all Victorian cropping soils. In barley, the incidence of Rhizoctonia root rot at anthesis, was 54%, 70%, 93% and 97% on grey clay, red clay, red duplex and Mallee soils respectively. In wheat, the incidence at anthesis was 70%, 69%, 74% and 68% on the grey clay, red clay, red duplex and Mallee soils respectively. Over the years 1990-1992, the disease caused an average grain yield loss of 12% in barley and 5.5% in wheat, with the highest losses occurring on the Mallee and red duplex soil types. It was shown that farmers were not achieving adequate control of Rhizoctonia root rot, and that cultivation per se was not effective in reducing its incidence or severity. In barley, the average incidence of Rhizoctonia root rot at anthesis was over 90% in crops grown on Mallee or red duplex soil types, regardless of tillage practice. Other researchers have reported tillage to reduce the incidence and severity of Rhizoctonia root rot. In wheat, incidence at anthesis was over 60% in crops grown on all soil types, regardless of tillage practice. The distribution of R. solani (AG 8) was related to both biological and physical factors of the different soil types. The disease was more severe in soils where organic matter levels were low, and there was little natural suppression of the fungus by other organisms. It is postulated that increasing the amount of organic matter in the soil would lead to a higher microbial mass in the soil and the resulting competition would reduce the survival of R. solani (AG 8). The root and hypocotyl rot strain of R. solani AG 2-1 (ZG 6) that occurs on pulses, was found on wheat and barley roots, and the cruciferous strain AG 2-1 (ZG 5) was found on both wheat and lupin roots and hypocotyls. These strains of R. solani could therefore increase under intensive rotations in the Wimmera and North-Central areas. Results from the field experiments demonstrated that the use of "cultivate deep - sow shallow" methods of crop establishment in conservation farming systems are as effective in controlling Rhizoctonia root rot, as traditional cropping systems. However, control could be improved with a strategic cultivation in years with an early autumn break to maximise destruction of the fungus networks of the fungus in the soil. Deep banding fertiliser, 5-10 cm under the seed, with the depth-modified combine at sowing reduced the severity of Rhizoctonia root rot and increased grain yield in 5 out of 9 field experiments. This indicates that deep banding fertiliser is viable option for farmers, and further studies in this area with a range of fertilisers would confirm these results. As a consequence of the studies represented in this thesis it was concluded that further research into the management of R. solani (AG 8) should concentrate on developing host plant resistance with transgenes. Other diseases: Cereal cyst nematode (Heterodera avenae) was detected in all soil types, and caused annual yield losses of 6.0% in barley and 6.6% in wheat. The nematode was most common on Wimmera grey clays, and was recorded for the first time on the red duplex soils of the North-Central district. Further studies are required to determine if changes in cultural practice through the use of gypsum, stubble retention and direct drilling to improve soil structure will increase the incidence of cereal cyst nematode. The re-introduction of susceptible cultivars into the Wimmera could lead to major outbreaks of cereal cyst nematode. Take-all (Gaeumannomyces graminis var. tritici) was detected in crops growing on all soil types. Yield losses were 4.9% in barley and 5.1% in wheat, and the pathogen was most prelevent in Mallee and red duplex soils. It is recommended that further studies should be undertaken to determine whether the adoption of conservation farming practices to improve soil structure will reduce the incidence and severity of take-all.
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ItemEffects of plant growth regulators on the efficacy of metsulfuron methyl in controlling Australian bracken [Pteridium esculentum Forst. f. (Cockayne)]( 2002)Bracken is a problem weed in pastures, forest clearings and nature reserves. Control methods such as defoliation and the use of herbicides are only effective in suppressing the weed for a limited period. The difficulty of controlling this plant is attributed to the large rhizome network that supports its continuous regrowth. The effect of using growth regulating factors, specifically the ethylene-releasing chemical ethephon and nitrogen (N) fertiliser, as pre-treatments to the application of the sulfonylurea herbicide metsulfuron methyl was investigated as a means of promoting the growth of rhizome buds and improving the delivery of herbicide to sites of regrowth in bracken. Aftermath effects of this integrated chemical control approach, including herbicide residual activity and potential allelopathic effects on associated vegetation, were also studied. Ethephon increased the translocation of 14C deoxyglucose, a surrogate for herbicide, from source fronds to the rhizoine system. However, field studies in pastures found little evidence of enhanced control of bracken frond density when ethephon or N were used with the herbicide. The herbicide was highly effective on it's own at the rate used (40 g a.i. ha-1), and this may have masked interactions with the growth factors. Differences between seasons, sites, and bracken populations may also have been important. There was some evidence for reduced bracken bud density 6 months after ethephon treatments were applied, but the results were not consistent across the ethephon rates used. Controlled environment studies revealed a moderate increase in crozier development when ethephon was applied to clonal plants, and this coincided . with increased concentrations of ethylene gas evolution measured on enclosed fronds. The well-known allelopathic effect of bracken was confirmed and herbicide treatment was found to enhance it. The potential for injury to crops like corn was noted at higher rates of application of metsulfuron methyl than those recommended for use in pastures. The suppression of frond growth for one year from herbicide application confirmed the effectiveness of metsulfuron methyl. However, this period is only temporary as frond regrowth would be expected after a year or two and has the potential to return to pre-treatment levels without follow-up treatment. Further research is required to find more long-lasting control methods that reduce the need for repeated, expensive herbicide treatment to achieve control of this weed.