The fungi Sclerotinia minor and S. sclerotiorum are the causal agents of two similar diseases of peanut (Arachis hypogaea L.). Both diseases cause significant losses in the Australian peanut industry. Chemical and cultural control methods will not provide complete control. Development of cultivars with resistance to Sclerotinia will be an important component of integrated control. The capacity to breed and select for such resistance efficiently must be established before serious investment of resources is made to develop Sclerotinia resistant cultivars. The aims of this project are: (1) to generate information that will assist in the improvement of Sclerotinia resistance in peanut; (2) to develop screening techniques; (3) to identify Sclerotinia resistant peanut germplasm; (4) to understand the inheritance and estimate heritability of resistance to S. minor; (5) to assess response to selection for resistance; and (6) to test the effectiveness of identified sources of resistance against both S. minor and S. sclerotiorum.
Experiments were conducted to develop screening techniques applicable for this project and a full scale breeding program. A previously unpublished technique for evaluating physiological resistance was described and modified. The artificial inoculation technique using colonised bean pod segments was found to be more robust than using colonised agar disks: discriminating among the physiological resistance of peanut genotypes to Sclerotinia whether used in controlled environment cabinets or the simple tent structures, and working well with both whole plants and detached stems. Cultivars and lines with the shortest lesion length in this test have demonstrated resistance to S. minor in field experiments in both Australia and in the USA.
The use of a tent structure in place of a controlled environment cabinet (CEC) to create a high humidity environment allowed larger numbers of individuals to be tested at one time. With this technique, glasshouse space and labour costs, rather than the size of the available CECs, are the limits to the number of individuals that can be tested at one time. This will allow mass screening of segregating populations or replicated testing of progenies in a breeding program context that would not be possible with limited CEC facilities.
Calculating a Moderated Lesion Length (MLL) by eliminating the failed or very small lesions was found to improve precision in measuring resistance responses of genotypes. Compared to average lesion length, the MLL had a stronger association with foci count (FC), a measure of resistance in the field. In cases where the small lesions do not occur independently of peanut genotype, a small lesion count (SLC) can be used to describe that variation.
The detached stem technique examined in this thesis was another useful tool for screening, particularly in situations where seed production by the plants is deemed critical, or where the seed quantity available would not provide sufficient replication for the pot-based glasshouse test.
This study has clearly established that material which shows resistance to S. minor in the USA, is resistant to S. minor and quite likely to be resistant to S. sclerotiorum in Australia. The high level of resistance to both S. minor and S. sclerotiorum in germplasm from Texas, particularly TxAG-4, was confirmed. The component lines of Virginia 93 Bunch showed good resistance in the field, which is primarily architectural resistance. Physiological resistance to S. minor was also identified in a cultivar and a landrace from Indonesia and three rust resistant breeding lines from Queensland. All germplasm found to have high physiological resistance to S. minor belonged to the Spanish type.
Inheritance of physiological resistance to S. minor was studied using a Generation Means Analysis (GMA) of the cross TxAG-4/VA 861102 and its reciprocal. The broad-sense heritability of physiological resistance on a single plant basis was estimated at 47%, much higher than earlier estimates obtained in field studies. The average gene action of Sclerotinia resistance genes from TxAG-4 was found to be additive. No dominance effects were detected in the GMA. A small but significant reciprocal effect between TxAG-4 and VA 861102 indicated that VA 861102 passed on some physiological resistance maternally.
Selection of single F2 and F3 plants successfully achieved an improvement in physiological resistance as measured by MLL. Selection was based on both MLL and seed production and further work should be conducted to quantify the comparative contribution of these two criteria. To estimate a realised heritability of physiological resistance in early generation selection it may be necessary to use a detached stem technique so that seed production occurs independently of expression of disease resistance. Successful selection of highly resistant genotypes from small F2 and F3 populations was taken as a possible indication of oligogenic control of resistance.
An experiment was conducted to confirm the value of resistance against both S. minor and S. sclerotiorum. TxAG-4 was found to have physiological resistance to both S. minor and S. sclerotiorum. This resistance was expressed against both Sclerotinia species by progeny that were selected for resistance to S. minor.
On the basis of the information obtained, the comparative advantages of three strategies for Sclerotinia resistant cultivar development are described: (1) introduction of germplasm; (2) recurrent backcrossing with screening and crossing in the BCnF1 generation; and (3) pedigree selection. At present introduction and backcrossing are recommended as the preferred strategies. Pedigree selection for Sclerotinia resistance is expected to become increasingly valuable after the development, by introduction and backcrossing, of Sclerotinia resistant cultivars with other desirable combinations of traits.