ALTERNATIVE SOURCES OF RESISTANCE TO AUSTRALIAN BLACKLEG
S.J. Marcroft1, T.D. Potter3, N. Wratten4, R. Khangura5, P.A. Salisbury 1,2,
and W.A. Burton1
2Institute of Land and Food Resources, The University of Melbourne, Parkville Victoria 3052 Australia
3South Australian Research and Development Institute, PO Box 618, Naracoorte, South Australia 5271 Australia
4Wagga Wagga Agricultural Institute, PMB, Wagga Wagga, NSW 2650 Australia
5Agriculture Western Australia, Baron-Hay Court, South Perth, WA 6151 Australia
When rapeseed was first grown commercially in Australia using imported varieties, blackleg (Leptosphaeria maculans) destroyed the industry. Overseas germplasm proved to have insufficient blackleg resistance for Australian conditions. As a result of intensive breeding and selection, most current Australian bred varieties are highly resistant to blackleg, however this resistance is not complete. As more canola is grown and rotation times shortened, the disease pressure increases with the potential for the fungus to overcome the present resistance mechanisms. Consequently, alternative sources of resistance are being sought, especially those with both enhanced seedling and adult plant resistance. During 1998, a number of lines were screened in blackleg nurseries in Victoria, New South Wales, South Australia and Western Australia. Several lines with significantly higher blackleg resistance than current varieties have been identified and these sources of resistance are being incorporated into a canola background. These sources include European winter types, Brassica juncea, Brassica nigra and Brassica carinata. Overseas spring canola does not have sufficient resistance, meaning that new traits such as herbicide resistance must be incorporated into an Australian canola background before commercialisation.
KEYWORDS: Leptosphaeria maculans, seedling resistance, plant survival, stem canker, disease nursery.
Leptosphaeria maculans (blackleg) is the major disease of canola in Australia, being responsible for the crop’s demise in the early 1970’s (Bokor et al. 1975). The impact of the fungus has been minimised by breeding resistant varieties; most current Australian bred varieties have adequate adult plant resistance to blackleg. However, there is potential for the severity of the disease to increase to a point where it may again pose a serious threat to the industry. Pressure from blackleg is increasing due to a number of factors:
· The rapidly increasing area of canola under cultivation provides the blackleg fungus with an increasing amount of stubble on which to survive;
· The relatively robust nature of canola stubble, enabling the fungus to survive for a number of years, especially in low rainfall areas and under minimum tillage systems;
· Strategies such as not growing canola adjacent to canola stubble paddocks from the previous year, are no longer feasible due to the large number of paddocks in production;
· Reduced use of stubble management practices, such as stubble burning;
· In many areas canola is now being grown in tighter rotations, (eg. every second or third year in the same paddock in some cases);
· In some low rainfall areas the most adapted varieties have lower levels of blackleg resistance, (eg. Karoo in Western Australia);
As a result of increased blackleg pressure, some farmers are beginning to experience economic loss, especially during the seedling stage. There is also the risk that the blackleg fungus may adapt and overcome the present resistance mechanism. Current blackleg resistance is mainly from French winter and Japanese spring types (Salisbury et al. 1995). Any resistance breakdown would jeopardise most current commercial varieties, having a major effect on the industry. In order to minimise the possibility of a wide-scale breakdown in resistance, it is crucial that alternative sources of resistance to blackleg are identified and exploited.
Canola is particularly vulnerable to blackleg at the early seedling stage as germination can coincide with the release of blackleg ascospores. Ballinger and Salisbury (unpublished data) have shown that current varieties are susceptible to the majority of seedling blackleg races. For the industry to continue to grow with limited yield loss from blackleg, it is crucial that blackleg resistance (especially seedling resistance) is improved to cope with increasing blackleg pressure.
This study was initiated to determine the levels of blackleg resistance in a range of alternative cruciferous species, and to incorporate this variation into Australian canola cultivars.
Lines were screened in blackleg nurseries, planted on canola stubble. Susceptible and resistant controls were used to determine disease pressure. Nurseries with insufficient disease pressure (less than 75% death of the susceptible control), were not included in the results. All lines were sown in rows up to 10 m long with four replicates per line.
Thirty four lines were screened in blackleg nurseries in five sites across Australia; at Lake Bolac, Blackrange (Vic), Wagga Wagga (NSW), Struan (SA) and Mt Barker (WA). The sources that were assessed included; spring type Brassica napus, winter type B napus, B. carinata, B. juncea, B. nigra, Sinapis alba, and B. napus containing B. juncea resistance genes.
Plant counts were recorded throughout the season, to determine seedling and adult plant survival. Counts were conducted at 6, 8, 10 and 12 weeks after sowing to calculate seedling survival and at 28 weeks after sowing (plant maturity) to calculate adult plant survival.
40 plants from each line were cut at ground level (at plant maturity) and the cross section of their stems examined. This procedure was used to determine if they had suffered from the stem canker phase of the disease (many plants have internal infection with no visible sign of external cankering). The data was analysed for across-site variability and the resistance of all lines were ranked relative to the resistant control cultivar, Dunkeld.
The Mt Barker nursery (Western Australia) was not included in the results due to very high survival (43%) of the susceptible control. Plants at all other sites of the susceptible control suffered 100% plant death. There was wide variation in the level of blackleg resistance in the germplasm tested; plant survival ranged from 100% to 0%. Differences in seedling and adult plant resistance were also identified. Some lines expressed exceptional seedling survival but had poor adult survival e.g. B. napus with B. juncea resistance, while other lines had poor seedling but good adult survival, e.g. experimental B. napus spring types. Salisbury and Ballinger (1993, 1995) showed similar results in laboratory screening of spring type canolas. A number of lines displayed both good seedling and adult survival, e.g. B. carinata.
The best adult plant resistance was displayed by the B. carinata lines; they suffered little or no adult plant death or stem canker (Figure 1). B. juncea had good plant survival, however, they did exhibit considerable internal stem canker. All of the European winter types displayed excellent seedling survival. However, some of these cultivars did not posses good adult plant survival. Plant survival in the B. nigra lines was better than the controls but not as good as the B. carinata lines. In addition the B. nigra lines had very little stem canker. S. alba had slightly less adult plant survival than the B. nigra but suffered badly from stem canker.
At Lake Bolac, Figure 2 shows seedling survival of all of the different sources of resistance. The data presented is of the lines with the most and the least survival from each source; if a line does not have a “least” score it means that there was only one line from that particular source tested. This data shows that four out of the ten groups tested, obtained scores of 100% seedling survival, compared to the susceptible control, which had complete plant death. The resistant control, Dunkeld, suffered 56% seedling death and the best B. napus cultivar suffered 22% seedling death. There were two groups B. nigra and the B. napus with B. juncea resistance, which did not suffer any seedling death. B. carinata had four out of five lines which had 100% seedling survival.
Overseas spring type lines evaluated at Lake Bolac, averaged approximately 50 % seedling survival except for one cultivar, AC Elect which had 73% seedling survival. Unfortunately, none of the overseas spring type lines had any adult plant resistance to Australian blackleg races (they all had final plant scores of 0%).
Across-site analysis of the trials showed very good correlation for Victoria, New South Wales and South Australia (Table 2). This result indicates that blackleg races affected the lines tested in a very similar way across these sites; meaning that any new sources of resistance should be robust in all canola growing areas across Australia. This result supports studies of blackleg races (Salisbury et al. 1989) where a similar mixture of blackleg seedling races were identified from five different sites.
Figure 1. Plant Survival for each Resistance Source Compared to the Resistant Control, cultivar Dunkeld. The results are a summary of data from all sites.
(B. carinata 7.19 means that B. carinata is 7.19 times more likely to survive than Dunkeld)
Figure 2. Seedling Survival for each Resistance Source Compared to the Resistant Control, cultivar Dunkeld. Results are from Lake Bolac (highest disease pressure early in the season).
Table 2. Correlation Between Sites
Victoria – New South Wales
Victoria – South Australia
New South Wales – South Australia
There were substantial differences in the blackleg resistance between all of the lines tested. Most of the lines were more resistant, than the resistant control, cultivar Dunkeld. Both good adult resistance and seedling resistance was identified, this resistance is very robust across sites but must be incorporated into a spring type B. napus background before commercialisation. Overseas spring type material has some useful seedling resistance but no adult plant resistance.
We thank Jack Kay, Ian Ludwig, David Robson and Leisa White for their assistance with trial management and data collection and Patick Lim for data analysis. Laboratory screening of lines was performed by Dr Barbara Howlett and Agus Purwantara.
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Salisbury, P.A. and Ballinger, D.J. Wratten, N., Plummer, K.M. and Howlett, B.J. (1995). Blackleg disease on oilseed Brassica in Australia: a review. Australian Journal of Experimental Agriculture, 35, pp 665-72.