Blackleg disease (phoma stem canker) caused by the fungus Leptosphaeria maculans is a major disease of canola (oilseed rape, Brassica napus) worldwide. Canola plants in pots were exposed to blackleg-infested stubble of canola with different complements of resistance genes and then assessed for disease. Plant mortality was reduced when plants were exposed to stubble from a cultivar with a different complement of resistance genes compared to stubble of a cultivar with the same resistance gene. These findings were consistent with 7 years of field surveys, which showed that changes in selection pressure as a result of extensive sowing of cultivars with major-gene resistance, termed 'sylvestris resistance', dramatically influenced the frequency of virulent isolates in the population towards particular resistance genes, and therefore disease severity. All these data were supported by PCR-genotyping surveys of fungal populations whereby the frequency of virulence alleles of avirulence genes AvrLm1 and AvrLm4 changed significantly depending on the resistance gene present in the cultivar from which the isolates were cultured. This is the first example of a study showing that sowing of canola cultivars with different complements of resistance genes in subsequent years, i.e. rotation of resistance genes, minimizes disease pressure by manipulating fungal populations. This approach provides a valuable disease management strategy for canola growers and is likely to be applicable to other plant diseases.

The timing of flowering in canola (Brassica napus) is an important determinant of adaptation to its environment. Cultivars of canola varying in maturity are grown over a wide range of photoperiod and temperature conditions in Australia. A quantitative understanding of the genotypic and environmental control of time to flowering can be used to improve breeding programs and crop management strategies. Controlled environment and field studies were used to determine the responses of 21 cultivars of canola and breeding lines of Indian mustard to vernalisation, temperature, and photoperiod. The number of days to flowering in all genotypes was reduced in response to vernalisation and long days, due to a reduced duration between sowing and buds visible. The vernalisation response was saturated with c. 25 days at 3°C. Base and optimum temperatures for development were confirmed at 0 and 20°C, respectively. The photoperiod response occurred between 10.8 and 16.3 h, and plants responded to photoperiod from emergence. A simulation model incorporating these effects was developed, which predicted days to flowering with a mean deviation of c. 5 days. Later flowering genotypes had model parameters that indicated greater responses to vernalisation and photoperiod than early-flowering genotypes.

The frequency of hybridisation between Brassica napus L. and Raphanus raphanistrum L. under agronomic conditions was assessed in field experiments, where R. raphanistrum were randomly planted at two different densities into large plots of B. napus. An acetolacate synthase (ALS)-inhibiting herbicide-resistant trait was used to detect potential hybrid individuals. No hybrids were detected amongst 25,000 seedlings grown from seed collected from R. raphanistrum plants. Two hybrids were obtained from more than 52-million B. napus seedlings. Both hybrids were characterised as amphidiploids (AACCRrRr, 2n = 56) and were fertile. The frequency of hybridisation into B. napus in this experiment using male-fertile B. napus was 4 × 10 -8 .

Nitrogen (N) contributed by legumes is an important component of N supply to subsequent cereal crops, yet few Australian grain-growers routinely monitor soil mineral N before applying N fertiliser. Soil and crop N data from 16 dryland experiments conducted in eastern Australia from 1989–2016 were examined to explore the possibility of developing simple predictive relationships to assist farmer decision-making. In each experiment, legume crops were harvested for grain or brown-manured (BM, terminated before maturity with herbicide), and wheat, barley or canola were grown. Soil mineral N measured immediately before sowing wheat in the following year was significantly higher (P < 0.05) after 31 of the 33 legume pre-cropping treatments than adjacent non-legume controls. The average improvements in soil mineral N were greater for legume BM (60 ± 16 kg N/ha; n = 5) than grain crops (35 ± 20 kg N/ha; n = 26), but soil N benefits were similar when expressed on the basis of summer fallow rainfall (0.15 ± 0.09 kg N/ha per mm), residual legume shoot dry matter (9 ± 5 kg N/ha per t/ha), or total legume residue N (28 ± 11%). Legume grain crops increased soil mineral N by 18 ± 9 kg N/ha per t/ha grain harvested. Apparent recovery of legume residue N by wheat averaged 30 ± 10% for 20 legume treatments in a subset of eight experiments. Apparent recovery of fertiliser N in the absence of legumes in two of these experiments was 64 ± 16% of the 51–75 kg fertiliser-N/ha supplied. The 25 year dataset provided new insights into the expected availability of soil mineral N after legumes and the relative value of legume N to a following wheat crop, which can guide farmer decisions regarding N fertiliser use.

A range of Brassica species was screened for resistance to Leptosphaeria maculans, the causal agent of blackleg. The lines were assessed in 8 disease nurseries in 4 canola growing regions of Australia and in 1�glasshouse trial, with a view to identifying alternative sources of resistance to L. maculans for Australian breeding programs. Lines were screened for degree of internal and external blackleg symptoms during both the seedling and adult plant growth stages. Correlation for resistance with ranking between disease nurseries was very strong (0.41-0.98). Brassica carinata and B. nigra were the most resistant species in the disease nurseries, being even more resistant than B. juncea. The 7 European winter B. napus lines tested were significantly more resistant than the 7�Australian spring B. napus lines, with another crucifer, Sinapis alba, being intermediate in resistance between the European and Australian B. napus lines. The same ranking of lines from most to least resistant was also seen when cotyledons and stems were inoculated in the glasshouse with 2 well-characterised Australian isolates. With the exception of the B. napus susceptible control Westar, all lines had similar frequencies of seedling survival in the nurseries. However, mature plants of these lines varied significantly in their degree of resistance. This indicates that screening for seedling survival is not useful in selecting L. maculans resistant lines in Australia. The Brassica lines with the B genome, especially B. carinata, and the winter B. napus types are now being used as sources of resistance in Australian breeding programs.