1University of Georgia, Griffin Campus, Griffin, GA 30223
2Calgene, LLC, Leesburg, GA 31763
Virulent Leptosphaeria maculans isolates were first detected on autumn planted canola in the southeastern USA, in Georgia late in the 1992-1993 season. Extensive damage resulting from plants dying or lodging prior to maturity was observed in a few fields during the 1993-1994 season. Several isolates from fields with extensive damage have been identified as pathogenicity groups 3 and 4. Canola stubble with lesions, collected from different areas of Georgia was scattered over disease nursery sites after planting each year since then. Stubble infested with the fungus from each nursery site was collected and distributed over three new sites the next year. Two of the three nursery sites were irrigated early in the season, if necessary, to promote early infection by L. maculans. Disease incidence was very high in two nurseries each of the last two seasons. The percentage of plants dead or dying from blackleg prior to normal maturity varied from zero to 100% in replicated plots. Cultivars were rated on a scale of zero to 9, where 9 indicated all plants were lodged or dead. Examples of the mean disease ratings of selected cultivars in these four nurseries were: Westar, 8.7; GA188-20B, 8.4; Cyclone, 7.1; Impulse, 6.9; Jewel, 5.6; Brigade, 5.6; Crusher, 5.5; Defender, 5.4; Magnum, 5.2; Ebony, 4.8; Oscar, 2.1; RPX 06-5-1, 1.4; and Flint, 0.4. These reactions indicate that there are a few canola cultivars with high levels of resistance to the pathogen populations present in the southeastern USA and that cultivars intended for production in this region of the USA need very high levels of resistance to L. maculans.
KEYWORDS blackleg, disease, disease resistance, pathogenicity group
Canola (Brassica napus) production in the southeastern USA began about ten years ago in Georgia. Current production in the region is in the states of Alabama, Florida, Georgia and South Carolina. Most of the production in the region is spring-type canola grown during the winter season, in a production system similar to that in Australia.
Virulent isolates of L. maculans were first found late in the 1992-1993 season at several different locations in southern Georgia. During the 1993-1994 season, extensive lodging caused by blackleg was observed in a few fields and moderate disease levels were observed in many additional fields. In subsequent seasons control of blackleg has been accomplished by moving production to avoid known infested fields and very limited use of resistant cultivars.
Three reasons why resistant cultivars have been used infrequently are: 1) much of the production has been contract production of specialty oil cultivars with no resistance to blackleg; 2) cultivars developed in Canada and Europe that are well adapted in the region often are not highly resistant to the biotypes of L. maculans present in the southeastern USA and 3) cultivars developed in Australia with good blackleg resistance are often not well adapted in much of the production region.
A program to evaluate canola cultivars for blackleg resistance was established in conjunction with a program to evaluate cultivars for overall performance in the southeastern USA. The first blackleg evaluation nursery in the region was established near Midville, Georgia during the 1993-1994 season and disease evaluation trials have been conducted each season since. This is a report of the results of trials at two locations over the past two seasons.
Pathogenicity Group testing
Isolates of L. maculans from diseased canola were tested to determine their pathogenicity group (PG). Inoculation of B. napus cultivars Westar, Quinta and Glacier and pathogenicity evaluations were done as described by Mengistu et al (1991).
Field disease evaluations
To insure that all important biotypes of the pathogen were present in the first nursery, diseased stubble from all geographic areas of the state was scattered over the nursery site immediately after planting. The following year stubble from that nursery was scattered over three new sites. In addition, stubble from new geographic areas where the disease was found the previous season was scattered over each nursery. This process has been repeated each year to insure that each trial site contains a representative mixture of pathogen biotypes from all over the production region.
To reduce volunteer plants, each year the disease evaluation nurseries were located at sites that had not produced canola the previous year but were near fields with blackleg the previous season. Each cultivar was planted in two rows 18 cm apart and 3 m long. The two row plots were spaced approximately 0.75 m apart. Diseased stubble was scattered over the site within two weeks after planting. The trials were planted mid- to late-October each year. If rainfall was below normal and irrigation was available, the trials were irrigated during the first six weeks after planting. Thereafter, trials were irrigated only during extended dry periods.
Stand counts were made in early December each year. Trials were examined biweekly for disease development, cold injury and overall condition. Just prior to maturity each plot was rated for blackleg severity. The blackleg severity was rated on a 0 to 9 scale as follows: 0=no lodged or dead plants; 1=1 to 10% lodged or dead plants; 2=11 to 20%; 3=21 to 30%; 4=31 to 40%; 5=41 to 50%; 6=51 to 60%; 7=61 to 70%; 8=71 to 99%; 9=all plants lodged or dead. This rating considered only those plants still visible at rating time. If significant cold injury was not observed, the disease rating was adjusted for differences between the December stand count and the final stand. If the final stand was less than 33% below the December stand, no adjustment was made; if the final count was 33 to 66% below the December count, the blackleg severity rating was increased by 1; if the final count was more than 66% below the December count, the rating was increased by 2. If significant plant losses from cold injury were observed during the season, no adjustment was made.
Isolates of L. maculans from eight locations in southern Georgia in 1993 were used to inoculate cotyledons of Westar, Glacier, and Quinta. Isolates from five locations were highly virulent on all three, indicating PG4. Both PG4 and PG3 were isolated at the sixth location. Two other isolates from this location, one from one year-old residue and one from a weed host were avirulent on all three, indicating PG1. An isolate from the seventh location was highly virulent only on Westar, indicating PG2. An isolate from the eighth location was not classified as any of the PG types because it was highly virulent on Westar and Quinta but gave a weak and mixed reaction on Glacier. Two isolates from northern Georgia in 1990 were avirulent on these three cultivars. Isolates from seven different locations in Georgia in 1994 were all highly virulent on the three differentials, indicating PG4.
Pongam et al (1999) tested five isolates from canola stubble collected in Georgia in 1993. They classified three as PG2 and considered the other two as different from any of the pathogenicity groups previously defined by these three differentials. Thus, 13 of 25 isolates collected in Georgia from 1990 to 1994 were PG4, one was PG3, four were PG2, four were PG1 and three gave reactions atypical of any defined PG group. Our practice of scattering stubble from different locations over each nursery site, each year was designed to assure that canola lines being tested were exposed to this diverse population of biotypes of L. maculans.
Cultivar disease reactions
The disease nurseries each contained several hundred breeding lines in addition to the lines or cultivars presented in Table 1. The selected cultivars presented in Table 1 include all of the named cultivars planted in the nursery plus several promising breeding lines. The disease reactions are presented to allow comparisons of disease intensity among these four nurseries as well as to allow comparisons of the reaction of named cultivars to their reaction in other countries.
Several lines or cultivars have high numbers of replications because they were planted repeatedly throughout the nursery to determine uniformity of disease intensity. Flint, G96200E, Dunkeld and Oscar were included as resistant cultivars, Cyclone as a moderately resistant cultivar and Westar as a highly susceptible cultivar. Because Westar matured very early in these nurseries, G188-20B was included as a later maturing, highly susceptible line. Glossy Bingo, a mutant of the cultivar Bingo with glossy leaves and stems, was included repeatedly as a locational marker that could be identified at any time of the season. This cultivar was invaluable as an indicator that the plots were properly planted, was an excellent reference marker during note-taking and was very susceptible to blackleg.
The cultivar Westar was extensively damaged in all four nurseries. However, the reactions of other cultivars indicated that the disease severity at the Arlington location in 1996-1997 and at the Griffin location in 1997-1998 was slightly below that in the other two nurseries. Dry weather early in the season may have reduced early infection rates at these two sites.
The disease reaction of most of the named cultivars tested in these nurseries indicated that they were moderately to highly susceptible to the biotypes of L. maculans present in southeastern region of the USA. Three notable exceptions were Oscar and Dunkeld, developed under high blackleg pressure in Australia and Flint, released in 1998 by the University of Georgia. Flint has been extensively tested for blackleg resistance in the southeastern USA as well as in nurseries at Mt. Barker, WA and Lake Bolac, VIC, Australia. The reactions of Oscar, Dunkeld, Flint and several other lines from the University of Georgia breeding program indicate that lines selected for high blackleg resistance in either the southeastern USA or in Australia are resistant when tested in both countries.
Canola production in the southeastern USA is expected to increase in the future because of the high yield potential in this region. Increased production will soon eliminate the possibility of moving production to disease-free areas and increase the reliance on cultivar resistance. When this happens, cultivars with blackleg resistance levels equal to or higher than cultivar Oscar will be needed.
Table 1. Blackleg reactions of selected canola lines or cultivars in two Georgia screening nurseries in 1996-97 and 1997-1998.
Griffin Arlington Griffin Arlington
Line or Disease No. of Disease No. of Disease No. of Disease No.of
Cultivar Rating Reps Rating Reps Rating Reps Rating Reps
Flint 0.5 6 0.7 6 0.3 46 0.3 22
G96200E — — 0.4 15 0.7 13
G96200C — — 1.2 6 0.8 6
CL100 2.0 4 2.6 8 — —
Dunkeld 2.5 40 1.9 34 1.3 3 —
RPX 06/5/1 2.7 9 0.9 7 1.0 3 1.3 3
CL106/6 2.7 6 1.6 9 — —
CL104/11 3.5 2 2.5 6 — —
Oscar 3.7 6 2.1 15 1.2 30 1.9 39
G96202 — — 1.5 6 0.3 6
CL87/24 3.7 3 1.7 6 — —
Jewel 3.7 3 5.0 3 6.8 6 8.3 6
SC95/04 4.0 3 4.7 3 — —
HN 9466 4.7 6 2.5 6 — —
Dynamite — — 3.0 3 5.3 3
Ebony 5.7 3 3.0 3 4.3 3 5.3 3
PR 3169 — — 5.7 3 8.3 3
PR 4108 — — 6.0 3 6.3 3
Narendra 5.7 3 2.3 3 — —
Crusher 5.8 5 5.3 6 2.7 3 8.0 6
Norseman 6.0 2 8.0 3 5.7 3 8.0 3
Impulse 6.7 6 7.2 6 6.3 3 8.7 3
Brigade 6.8 6 5.5 6 3.0 3 7.6 6
Legacy 7.0 3 5.7 3 — —
Glossy Bingo 7.0 39 6.1 75 7.7 53 8.5 57
Sponsor 7.0 3 6.3 3 — —
LG 3310 7.0 3 6.3 3 3.7 3 8.7 3
Magnum 7.0 3 6.7 3 5.3 6 6.2 6
Defender 7.4 7 5.6 8 3.0 3 8.7 3
Tobin 8.0 1 8.2 5 — —
G188-20B 8.5 24 8.3 18 7.8 25 9.0 17
Legend 8.7 6 7.0 6 — —
Cyclone 9.0 6 6.0 11 5.2 21 8.0 28
Hyola 401 9.0 1 8.0 3 — —
Westar 9.0 4 8.8 18 8.8 48 8.9 37
Hysyn 11 9.0 1 9.0 3 — —
Rating 0 to 9, 0 = no disease to 9 = all plants dead or lodged.
1. Mengistu, A., Rimmer, S. R., Koch, E., and Williams, P. H. 1991. Pathogenicity grouping of isolates of Leptosphaeria maculans on Brassica napus cultivars and their disease reaction profiles on rapid-cycling brassicas. Plant Disease 75:1279-1282.
2. Pongam, P., Osborn, T. C., and Williams, P. H. 1999. Assessment of genetic variation among Leptosphaeria maculans isolates using pathogenicity data and AFLP analysis. Plant Disease 83:149-154.