P. A. Salisbury

Major gene and polygenic resistance to Leptosphaeria maculans in oilseed rape (Brassica napus)

The most common and effective way to control phoma stem canker (blackleg) caused by Leptosphaeria maculans in oilseed rape (Brassica napus) is through the breeding of resistant cultivars. Race specific major genes that mediate resistance from the seedling stage have been identified in B. napus or have been introgressed from related species. Many race specific major genes have been described and some of them are probably identical in B. napus (allotetraploid AACC) and the parental species B. rapa (diploid AA). More work is needed using a set of well-characterised isolates to determine the number of different major resistance genes available. In some B. napus cultivars, there is resistance which is polygenic (mediated by Quantitative Trait Loci) and postulated to be race non-specific. Many of these major genes and Quantitative Trait Loci for resistance to L. maculans have been located on B. napus genetic maps. Genes involved in race specific and polygenic resistance are generally distinct.

Environmental effects on seed composition of Victorian canola

Data from advanced breeding experiments between 1985 and 1994 were used to determine the effects of region, year and environment on the quality of canola grown across Victoria. Estimates from these unbalanced data were made using residual maximum likelihood. Environmental effects were large relative to cultivar effects for oil and protein content, while the reverse occurred for glucosinolate content. High oil contents (and low seed protein contents) were correlated with cooler spring temperatures and higher spring rainfall. Oil contents were lowest, on average, in canola grown in dry years, or from the hotter regions, such as the Mallee, and were highest in canola from the cooler, wetter regions, such as south-western and north-eastern Victoria. Fatty acid composition varied with year and region. Means for saturated fatty acid content averaged 6.4 ± 0.1%. The oleic acid content averaged 60.3 ± 0.4% and was higher in canola grown in central Victoria and the Wimmera, and in most years, in north-eastern Victoria compared with other regions. Low temperatures and low rainfall reduced oleic acid content. Linoleic acid content averaged 19.7 ± 0.3% and linolenic acid averaged 10.4 ± 0.3%, with the content of these fatty acids negatively correlated with the content of oleic acid. Erucic acid levels were below 0.6% in all regions.

Blackleg disease on oilseed Brassica in Australia: a review

Blackleg, caused by the fungus Leptosphaeria maculans, is the major disease of canola (Brassica napus) in Australia. The development of blackleg-resistant B. napus varieties has been a major factor in the resurgence of the industry nationwide. The main sources of resistance used in Australian public breeding programs are Japanese spring varieties and French winter varieties. In these programs, all early generation material is screened in field blackleg nurseries sown on, or adjacent to, infested canola stubble from the previous season. Little is known about the genetic control of resistance, and the mechanisms responsible for generating pathogenic variability of L. maculans isolates in Australia is largely uncharacterised. Australian B. napus varieties are the most blackleg-resistant spring varieties in the world. Apart from growing blackleg-resistant varieties, other strategies that minimise infection and delay any breakdown in varietal resistance include growing canola on the same area only once every 3 years, destroying stubble, and eradicating volunteer plants between cropping seasons. Additionally, strategic use of chemicals can provide effective control to supplement varietal resistance in areas prone to severe blackleg infestation.

Assessment of genetic diversity in selected breeding lines and cultivars of canola quality Brassica juncea and their implications for canola breeding

AFLP markers were used to assess the genetic diversity of 77 breeding lines from three of the worlds major canola qualityBrassica juncea breeding programs from Canada (Agriculture and Agri-Food Canada and Saskatchewan Wheat Pool) and Australia (Agriculture Victoria). The objectives of the paper were to assess the genetic diversity within and between these three breeding programs and to assess genetic diversity of the canola quality germplasm as compared to mustard quality B. juncea. Fifteen lines of mustard quality B. juncea from India, China, Russia and Australia were also included in the investigation. Ten EcoR1/Mse1 based primer pairs generated 751 scorable fragments with an average of 26 polymorphic bands per primer pair (35%). Similarity coefficients were calculated using the Simple Matching coefficient and adendrogram was developed using the UPGMA procedure, resulting in germplasm being partitioned into five main groups. Line specific markers were discovered that have potential in enhancing the efficiency of individual breeding programs using breeding techniques like accelerated backcrossing. Further understanding the genetic diversity within and between programs has implications for future breeding and collaboration within and between the three programs.

Crop isolation, not extended rotation length, reduces blackleg (Leptosphaeria maculans) severity of canola (Brassica napus) in south-eastern Australia

Due to the large increase of canola production in Australia, current blackleg cultural control recommendations (extended rotation length and isolation distance from canola stubble) are not adhered to by farmers in many canola-producing regions. Canola crops are increasingly being sown in short rotation and, in many instances, adjacent to paddocks containing canola stubble. In this study, the level of disease in commercial canola crops was determined for different rotations and distances from canola stubble. There was a strong relationship between the presence of canola stubble from the previous year (6-month-old stubble) and distance to current canola crops, but no relationship between the presence of older (18–42 month old) stubble and distance to current canola crops. Blackleg severity was highest where canola crops had been sown adjacent to 6-month-old canola stubble, with the level of blackleg severity decreasing markedly in the first 100 m. Disease severity then generally declined up to 500 m. Plants 500–1000 m from 6-month-old stubble had similar levels of blackleg infection. Blackleg severity was similar between canola crops sown into 18-month-old canola stubble (short rotation) and crops sown into paddocks that had no history of canola for at least the previous 3 years (long rotation). Based on these findings, we recommend that canola crops should be sown at distances greater than 100 m and preferably 500 m from last seasons canola stubble, rather than extending rotation length between crops.

Reaction of Brassica juncea (Indian mustard) lines to Australian isolates of Leptosphaeria maculans under glasshouse and field conditions

Brassica juncea (Indian mustard) lines from diverse geographical locations around the world and from Australian breeding programs were screened for resistance to the blackleg fungus, Leptosphaeria maculans, in both glasshouse and field trials. The five Australian L. maculans isolates used in glasshouse trials could be classified into two groups; those that attacked all B. juncea lines, and those that attacked none. All these isolates caused lesions on cotyledons of B. napus cultivars including Westar, Glacier and Quinta, suggesting that they are in Pathogenicity Group 4 as described by Koch et al. (1991). The two isolates that attacked B. juncea also attacked B. napus lines to a similar extent, but did not attack the two B. carinata lines tested. Brassica lines were sown in a blackleg disease nursery at Lake Bolac, Victoria, Australia, and five indicators of blackleg disease were measured (survival rate, disease rating, disease incidence, external and internal lesion length). All 92 B. juncea lines developed blackleg symptoms. Although they displayed a high disease incidence in the field, almost all of the B. juncea lines were more blackleg-resistant than a B. napus cultivar, Dunkeld, which is amongst the most resistant cultivars in commercial production in Australia. Four B. carinata lines were more resistant than any of the B. juncea lines, suggesting that this species may be a useful source of blackleg resistance in B. napus breeding programs.

Seedling and adult plant evaluation of race variability in Leptosphaeria maculans on Brassica species in Australia

Seedling and adult plant studies were used to identify the race structure of Leptosphaeria maculans (the cause of blackleg) on oilseed Brassica species in Australia. Host-pathogen interactions using a set of 12 differential host lines identified 14 seedling relationship, however, between seedling and adult plant reactions to individual isolates, indicating that seedling and adult plant resistance are under different genetic control. While non-specific adult plant resistance was observed in the B. napus line Jet Neuf, host-pathogen interactions confirmed the existence of race-specific adult plant resistances in other differential lines.

Identifying resistance genes to Leptosphaeria maculans in Australian Brassica napus cultivars based on reactions to isolates with known avirulence genotypes

Abstract. Blackleg disease, caused by the fungus Leptosphaeria maculans, is the major disease of canola (Brassica napus) worldwide. A set of 12 Australian L. maculans isolates was developed and used to characterise seedling resistance in 127 Australian cultivars and advanced breeding lines. Plant mortality data used to assess the effectiveness of seedling resistance in canola growing regions of Australia showed that Rlm3 and Rlm4 resistance genes were less effective than other seedling resistance genes. This finding was consistent with regional surveys of the pathogen, which showed the frequency of Rlm4-attacking isolates was >70% in fungal populations over a 10-year period. Differences in adult plant resistance were identified in a subset of Australian cultivars, indicating that some adult gene resistance is isolate-specific.

Comparative analysis of fatty acids in pollen and seed of rapeseed

Abstract Lipids accounted for 31.7% of the dry wt of mature, viable pollen of rapeseed ( Brassica napus ). External lipids of the pollen coat, specified by the diploid parental genome, and internal cytoplasmic lipids, presumably specified by the haploid pollen genome, were separated by differential extraction. External lipids and paraffins represented 9.8 % of pollen dry wt, and contained predominantly linolenic (18:3) with significant amounts of palmitic (16:0), stearic (18:0) and myristic (14:0) acids, as well as paraffins. Internal lipids accounted for 21.9% of pollen dry wt and comprised predominantly 18:3 and 16:0. Mature seeds of B. napus comprised 45.0 % lipid, with a fatty acid composition different from that of either pollen fraction. Decanoic (10:0), lauric (12:0) and hexadecatrienoic (16:3) acids were detected in pollen but not in seeds. These data are discussed in relation to pollen biochemistry and the potential for pollen selection in rapeseed breeding.

Factors affecting production of inoculum of the blackleg fungus (Leptosphaeria maculans) in south-eastern Australia

The production of windborne ascospore inoculum of the blackleg fungus (Leptosphaeria maculans) was determined during 2000 and 2001 in 3 environments (Birchip, low rainfall; Wonwondah, medium rainfall; Lake Bolac, high rainfall) in Victoria. The weight of canola stubble (kg/ha) remaining on the soil surface in paddocks was estimated 6, 18, 30 and 42 months after harvest of the original canola crop. In all 3 environments only small amounts of stubble were present 18 months after harvest. Eighty percent of the 6-month-old stubble comprised stems and branches, with the remaining 20% being root material, while 42-month-old stubble consisted only of root material. Paddocks subjected to raking and burning contained only half the weight of stubble compared with paddocks that were harrowed. Where canola was harvested in January, even when no management strategy was used, 80% of subsequent stubble was no longer on the soil surface by July of that year. Pseudothecia from 6-month-old stubble from the high rainfall environment discharged significantly more ascospores than stubble of the same age from the medium rainfall environment, which in turn discharged more than stubble from the low rainfall environment. In all environments, paddocks containing 6-month-old canola stubble discharged 30-fold as many ascospores per hectare as older stubble paddocks.