Hardy, Adam (2007) New midge resistance for Australian grain sorghum. [Thesis (_PhD/Research)] (Unpublished)
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[Abstract]: The development and commercial release of midge resistant grain sorghum hybrids in Australia has been one of the real success stories of the sorghum industry. Almost all the current commercial sorghum hybrids grown contain a significant baseline of midge resistance that has greatly reduced that pest status of this insect. However while breeding efforts have been successful, it seems only one mechanism of resistance remains present in commercial hybrids. This mechanism of resistance known as ‘ovipositional-antixenosis’ is polygenic in nature and has contributed to the gradual reduction in the genetic base of commercial sorghum hybrids, potentially limiting the advancement of other agronomic traits. In order to expand both the level of resistance and the genetic diversity within Australian commercial sorghum hybrids, research was undertaken to capture new sources of midge resistance. Initially the goal of this work was to isolate and characterise the most promising new antibiosis sources of midge resistance previously documented within international breeding lines. However as the work progressed the discovery of a previously undescribed tolerance source of midge resistance led to more detailed studies of midge larval biology within a narrow range of sorghum germplasm lines to better characterise the resistance mechanism. To complete this work a new water-injection technique was developed to allow more precise studies of midge larval biology where previous traditional screening methods were deemed inadequate. Using this method several antibiotic lines were shown to contain diverse modes of action, while the tolerance mechanism of resistance was confirmed and more accurately characterised. In all cases plant characters within the developing spikelet were associated with each resistance mechanism. Overall, one source of resistance, found in lines derived from the Indian land race line DJ6514, was identified as superior to the others tested. Germplasm derived from this source was found to confer inter-related antibiosis and tolerance mechanisms. Both mechanisms were indirectly shown to be caused by the unique antibiotic properties of the developing caryopsis. Larval mortality (antibiosis) increased in this line, in line with increased feeding against the developing caryopsis. A second tolerance mode of action was also linked to anti-feeding properties of the developing caryopsis. Larvae were recorded at higher than normal rates feeding away from the caryopsis resulting in the survival of both larvae and grain in a significant proportion of spikelets. This escape ‘tolerance’ mechanism of resistance was confirmed indirectly in several studies and could be artificially induced in other genotypes when larval feeding against the caryopsis was delayed. As such the resistance is better described as antixenosis to caryopsis feeding. When both resistance mechanisms are present, the resistance isolated from DJ6514 was found to cause a three to four fold increase in seed set in resistant lines. This source of resistance has been directly selected for incorporation into the Australian breeding program and has been shown to be inherited simply as a single gene that needs to be deployed on both sides of the breeding program to ensure stable expression. When breeding efforts are complete a new suite of highly resistant and genetically diverse midge resistant sorghum hybrids will emerge into the marketplace.
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