Agriculture and Agri-Food Canada, Cereal Research Centre, 195 Dafoe Road, Winnipeg, MB, Canada R3T 2M9

Background and objectives
Fusarium head blight (FHB), ear blight or scab, caused by Fusarium graminearum Schwabe., is endemic in the spring wheat growing area of the north central plains of North America. Since 1991, it is estimated that the cost of the disease to the cereal industry has exceeded US$2.6 billion [1]. Two types of resistance are generally recognised: Type I, resistance to initial infection, and Type II, resistance to spread within the head. The FHB-resistant Chinese hexaploid cultivars such as Sumai 3 have Type II resistance. Single floret injection (SFI) inoculation is thought to be a relatively reliable method for screening for this type of resistance [2].

The durum wheat class is amongst the most susceptible to the disease. In Canada, the principal durum-growing area of Saskatchewan so far has not been directly threatened by FHB, but there are concerns that environmental/global changes may be such that conditions promote the spread of the disease. Attempts to cross resistance into durum lines from Chinese hexaploids were unsuccessful, therefore the objectives of the study were to identify FHB resistant germplasm in a tetraploid background, and to cross the resistance into an adapted durum wheat background. Spray inoculation was used for field screening and results compared to reactions obtained from both spray and SFI inoculation methods under controlled conditions.

Materials and methods
Ninety-seven tetraploids, mostly Triticum dicoccoides, were screened for reaction to FHB with a spray inoculation. Six T. dicoccoides accessions were moderately resistant, of which 2, Td160 and Td161, were used in crosses with the susceptible durum cultivar AC Morse. A limited number of F3 lines, were planted in an artificially inoculated, irrigated nursery in 1996. The F4-derived F5 lines were subsequently screened under controlled conditions using both spray and SFI inoculation methods. The F4-derived F6 lines were again tested in 1997. All heads were inoculated at anthesis. In the field, inoculum was first applied when 50% of the heads were in flower, and a second application was made 4 days later. For all experiments, a concentration of 10,000 F. graminearum conidia/ml was used. In field tests, 50 ml of inoculum was sprayed per 1.5 m row, and irrigation was applied in the evening and the following morning of each inoculation. Under controlled conditions, 10 Ál were injected into single florets or 3 ml sprayed onto individual heads, and plants were kept in 100% humidity for 24 h. Infected spikelets were counted 18-21 days later. For field tests, an FHB index was calculated from percentage heads infected x percentage infected spikelets / 100.

Results and conclusions
In both years of field testing, a higher percentage of lines from the cross with Td161 were resistant (FHB index <30), and more susceptible lines were observed in Td160 progeny (FHB index >50). Under controlled conditions average number of infected spikelets from progeny of Td161 crosses was lower than in progeny of Td160 crosses, 38 and 52%, respectively. After spray inoculation, average number of infected spikelets was higher in lines from Td161 crosses than from progeny of Td160 crosses. There were more infected spikelets following spray inoculation than single floret injection in progeny of both Td160 and Td161, 61 and 70% respectively. SFI inoculation under controlled conditions was highly correlated, P <0.002, with the FHB index obtained from field screening averaged over 2 years for progeny from the more resistant cross with Td161. Results were mixed for progeny from the cross involving Td160. SFI inoculation was correlated with 1996 field FHB index, but spray inoculation was correlated with 1997 field index, P<0.05. SFI appeared to be useful for screening resistant lines with applicability to the large-scale screening required by breeding programs.

1. Johnson D, Flaskerud G, Taylor S, Satyanarayana V, 1997. Proceedings National Fusarium Head Blight Forum, St. Paul, Minnesota, pp. 5-6.
2. Bai G-H, Shaner G, 1996. Plant Dis. 80, 975-979.