Identification of sources of disease resistance in Chinese faba bean germplasm for incorporation in Australian cultivars.

Abstract

A total of 115 Chinese accessions from six provinces with diverse climate, topography and farming systems were screened for reaction to three fungal diseases Ascochyta blight, (Acochyta fabae), chocolate spot (Botrytis fabae) and Cercospora leaf spot (Cercospora zonata). Not all the accessions were included in three disease screening trials due to a limited number of seeds for some accessions. However, a majority of lines were included in all three disease screening trials and the remaining at least for two trials. In the Ascochyta blight screening trial 96 Chinese accessions were evaluated and while the majority of lines were susceptible a number of lines including 1688, 1689 (Gansu), 1729 (Guangxi), 1750 (Jiangsu), 1998 and 1999 (Qinghai) were identified with a significant degree of heterogeneity including a low proportion of resistant plants. Single plants with a disease score of either 1 (resistant) or 3 (moderately resistant) could be used for the development and selection of resistant lines. The effect of three concentrations of B. fabae inoculum was observed on four cultivars 1714-1, Nura, Fiesta and Icarus. The disease development was proportional to the amount of inoculum irrespective of cultivars but the relative responses of cultivars varied between treatments, consistent with partial resistance. The lines 1714-1 and Icarus were most resistant at all treatments. A medium dose of inoculum provided the opportunity for cv. Nura to express resistance compared to cv. Fiesta, while at a low and high inoculum concentration, there was no significant difference between the two cultivars. A total of 99 Chinese accessions from six provinces were evaluated for chocolate spot resistance. Germplasm from Qinghai, Guangxi, Jiangsu and Gansu were more resistant as compared to germplasm from Yunnan and Guangdong. Qinghai Province lines 1982, 1983, 1988, 1994, 1998 and 1999 were the least susceptible among all the Chinese lines and could be valuable sources of resistance against the chocolate spot disease. A total of 45 faba bean lines, 22 from six different regions of China, identified in the preliminary screening as having some degree of resistance, 13 regional lines previously reported as resistant to chocolate spot and 10 Australian cultivars were evaluated for chocolate spot resistance. Germplasm from Qinghai, Jiangsu and Yunnan regions were comparatively resistant and particularly accessions 1983 and 1985 from Qinghai province showed a moderately resistant reaction. Accession 1714-1, a selection from Acc 1714 from Gansu, was the most resistant among all lines including Icarus and Farah. None of the other Chinese lines was equivalent to Australian resistant cultivars Icarus and Farah. In Cercospora leaf spot screening trial, a total of 72 Chinese accessions and two local check lines were screened for disease resistance. The only resistant line was the local check, however, three lines 1673, 1727 and 1890 from Gansu, Guangxi and Yunnan, respectively expressed a minor level of resistance and there is potential to develop resistant lines through single plant selection from these heterogeneous accessions. There was significant defoliation due to disease. The genetics of inheritance of resistance to A. fabae (isolate 86/03) was investigated in five faba bean lines. The resistance of cvs. Farah and Ascot was under the control of different genes in the two cultivars. The F₂ populations of the crosses of these two cultivars with susceptible cv. Icarus segregated in different distribution patterns for resistance and susceptibility that indicated a dominant gene controls resistance in Farah while resistance in Ascot is controlled by a recessive gene. The cross 1783/2*Ascot (Acc 1783/2 of Tunisian origin) did not segregate in the F₂ generation, with the exception of a single plant rated 5, indicating resistance in both cultivars is controlled by the same gene(s). Two crosses out of three from 1689/1*Ascot (Acc 1689/1 susceptible and of Chinese origin) segregated in the ratio 1:3 indicating the resistant gene of Ascot is recessive to 1689/1. Seventeen isolates of A. fabae from southern Australia were characterised for a number of characters including pathogenicity on cv. Icarus, growth on medium, growth patterns, pycnidial formation and distribution, size of spores and colour. The isolates were variable in most of the features. Pycnidial formation and growth on media were positively associated with disease severity, but there was no association between the other traits in culture with disease severity. This project has identified variation within Chinese germplasm in response to the fungal diseases Ascochyta blight, chocolate spot and Cercospora leaf spot. Lines with a degree of resistance to Ascochyta blight and Cercospora leaf spot were heterogeneous and screening of larger populations of identified lines and selection of individual resistant plants could lead to the development of new resistant lines. Genetic studies of resistance to Ascochyta blight comparing Ascot and Farah confirmed that there are alternative genes controlling resistance, and resistant Chinese lines might provide additional resistance genes. Identification of chocolate spot resistance in Acc 1714-1 is a significant finding as the majority of previously identified sources of resistance to this disease originated from the Andean region. Further characterization and utilization of these new sources of disease resistance should contribute to a long-term strategy of breeding disease resistant faba bean cultivars.