| | Abstract
Fusarium head blight (FHB) and spot blotch are the most prevalent disease of barley in the eastern Canadian prairies. The diseases also occur in western regions, but usually are of lesser importance and concern. Both can infect developing kernels to cause discoloration and shriveling, and in the case of FHB, contamination of grain by the mycotoxin DON. The pathogens involved may compete on barley spikes for infection sites and substrate utilization. Antagonism has been detected in Cochliobolus sativus (spot blotch pathogen) against Fusarium graminearum (FHB pathogen) and demonstrated in the laboratory and in barley plants. However, a role for C. sativus as a practical bio-control agent against Fusarium / FHB could not be verified in the field. Partial resistance is available and already present in some western Canadian barley cultivars, but resistance must be enhanced and more widely applied to mitigate the effects of these destructive diseases.
Discussion
Fusarium head blight (FHB) continues to be the most troublesome disease of barley (Hordeum vulgare L.) in the eastern Canadian prairies, and threatens regions further west. Since 1994, when first documented as widespread in Manitoba, FHB has affected barley crops annually, resulting in loss of yield, and most importantly in reduced grain quality, due to contamination with mycotoxin(s); this has compromised the grain’s end-use options (Tekauz et al. 2000). For health and safety reasons, and livestock productivity, barley must be tested for mycotoxin (deoxynivalenol or DON) content prior to use as feed, malting and brewing, or other food uses. The low or nil tolerances for DON have resulted in barley being selected preferentially from regions further west, particularly for malting use. In western Saskatchewan and in Alberta, the disease is still relatively rare and less severe (Pearse et al. 2003, Turkington et al. 2003).
FHB is caused by Fusarium spp. that infect the spike after heading. Discoloration and some shriveling of kernels result, and as a by-product of infection the grain may become contaminated with DON. Fusarium graminearum Schwabe is the main species isolated from infected kernels, and the principal recognized producer of DON; however, other species also are involved in the disease syndrome. The relative frequency of Fusarium spp. varies, and the proportion of species other than F. graminearum has recently risen, suggesting a cyclical pattern likely influenced by the environment (Table 1).
Accurate visual estimation of FHB in a standing barley crop or in harvested grain is difficult. Unlike the situation in wheat, in which affected kernels are lighter (bleached) in colour compared to healthy ones (i.e., ‘tombstone’ kernels in hard ‘red’ spring wheat), in barley Fusarium infection usually darkens kernel colour. Darkening or staining of barley kernels can also be caused by abiotic factors and other fungi, such as Cochliobolus sativus (Ito & Kurib.) Drechs. ex Dastur and Pyrenophora teres (Died.) Drechsl. The latter are barley pathogens that cause spot blotch and net blotch, respectively, but readily spread to spikes to infect developing kernels. As such, the cause of discolored barley kernels cannot be attributed with certainty, unless as is sometimes the case with FHB, this is accompanied by characteristic orange-coloured Fusarium sporodochia (spore masses) on glume surfaces. To be certain of the causal agent(s) involved, kernels must be placed on artificial growth medium (e.g., potato dextrose agar or PDA) to promote colony growth and sporulation, and subsequent fungal identification.
Table 1. Frequency of Isolation of Fusarium spp. from Kernels of Barley from Surveyed Farm Fields in Manitoba.
| Year | Fusarium species |
 | F. g.a
% | F. p.
% | F. s.
% | F. a.
% | F. c.
% | F. e.
% |
|
| 2002 | 45 | 21 | 26 | 4 | 1 | 4 |
| 2001 | 73 | 19 | 3 | 4 | 0 | 1 |
| 2000 | 93 | 4 | 1 | 2 | 0 | 1 |
| 1999 | 90 | 6 | 2 | 2 | 1 | 0 |
| 1997 | 67 | 23 | 8 | 1 | 1 | 1 |
a g. (graminearum); p. (poae); s. (sporotrichioides); a. (avenaceum); c. (culmorum); e. (equiseti)
The identification and estimation of levels of Fusarium spp. on barley kernels is done routinely as a first reliable indicator of the presence, severity, and potential impact of FHB in any crop year, based on field samples collected during annual disease surveys (Pearse et al. 2003, Tekauz et al. 2002b), or to gage the results of field trials investigating the disease. At this time the presence of other pathogenic fungi also is recorded; C. sativus is usually the most common of these and its opportunistic levels on seed can be high (Table 2).
Table 2. Frequency of Isolation of Fusarium spp. and Cochliobolus sativus and Resulting Deoxynivalenol (DON) Levels in Kernels of Barley Grown at East Selkirk, Manitoba in 2000.
| Barley cv. | F. g.a | F. p. | F. s. | F. a. | F. c. | F. e. | Total Fus. | C. s.b | DON |
| % | % | % | % | % | % | % | % | ppm |
| 2-row (n=5) | | | | | | | | | |
| CDC Dolly | 22 | 2 | 1 | 2 | 0 | 0 | 27 | 62 | 1.7 |
| AC Metcalfe | 8 | 1 | 1 | 0 | 0 | 0 | 10 | 58 | 0.9 |
| Phoenix | 13 | 1 | 0 | 1 | 0 | 1 | 16 | 36 | 0.7 |
| Seebe | 4 | 3 | 0 | 1 | 0 | 0 | 8 | 79 | 1.1 |
| Tercel | 12 | 3 | 2 | 0 | 0 | 0 | 17 | 61 | 2.0 |
| 6-row (n=5) | | | | | | | | | |
| Bronco | 51 | 1 | 2 | 2 | 0 | 0 | 56 | 13 | 10.8 |
| AC Harper | 48 | 1 | 0 | 1 | 0 | 0 | 50 | 29 | 4.8 |
| Kasota | 36 | 1 | 2 | 3 | 0 | 0 | 42 | 19 | 7.4 |
| Trochu | 41 | 4 | 1 | 4 | 0 | 2 | 52 | 8 | 3.4 |
| Vivar | 61 | 3 | 2 | 1 | 0 | 0 | 67 | 23 | 9.4 |
a g. (graminearum); p. (poae); s. (sporotrichioides); a. (avenaceum); c. (culmorum); e. (equiseti)
b Cochliobolus sativus
Recently, the importance of spot blotch as a component of the leaf spot complex on barley in southern Manitoba has increased markedly. Spot blotch is now the predominant foliar disease of barley in the region (Table 3) and responsible for much of the damage and yield loss observed (Tekauz et al., 2003, 2002a, 2001).
Table 3. Frequency of Isolation of Leaf Spot Pathogens from Affected Barley Leaves from Surveyed Farm Fields in Manitoba.
| Year | Pathogen |
| Pyrenophora teres | Cochliobolus sativus | Septoria passerrinii |
| Fields
% | Lesions
% | Fields
% | Lesions
% | Fields
% | Lesions
% |
|
| 2002 | 56 | 23 | 100 | 73 | 12 | 2 |
| 2001 | 62 | 28 | 89 | 69 | 7 | 2 |
| 2000 | 85 | 52 | 77 | 40 | 26 | 5 |
| 1999 | 92 | 52 | 92 | 43 | 23 | 5 |
Barley cultivar responses to C. sativus vary, and current Canadian genotypes demonstrate both susceptible and partially resistant infection phenotypes. Seedling and adult plant responses for a selected group of genotypes are shown in Table 4. Infection phenotypes are usually more resistant in adult than seedling plants, and within adult plants, more resistant in 6- than 2-rowed lines. The data suggest that pathotypes of C. sativus exist in Manitoba (western Canada), as differential reaction (based on infection responses of 1 - 4 as the resistant phenotype, and those of 6-9, susceptible) occurred among the genotypes and isolates tested. The presence of pathotypes, if confirmed, would be novel information for Canada; their number and virulence would need to be considered in screening and breeding for effective and durable resistance. The presence of three pathotypes of C. sativus has previously been reported in North Dakota (Valjavec-Gratian & Steffenson 1997).
Table 4. Barley Seedling and Adult-plant Infection Responses to Isolates of Cochliobolus sativus
(spot blotch) from Manitoba.
| Seedling plants | | Adult plants |
| Barley Line | Isolate | Mean | Isolate | Mean |
| 1908 | 1909 | 1910 | 1911 | (3.3-6.8) | 1909 | 1910 | (2.0-7.5) |
| 2-row (n=8) | | | | | | | | |
| CDC Bold | 6 | 7 | 6 | 8 | 6.8 | 8 | 7 | 7.5 |
| Bowman | 4a | 8 | 5 | 5 | 5.5 | 6 | 4 | 5.0 |
| Harrington | 7 | 4 | 6 | 7 | 6.0 | 3 | 7 | 5.0 |
| CDC McGwire | 4 | 3 | 4 | 4 | 3.8 | 3 | 4 | 3.5 |
| Newdale | 5 | 4 | 5 | 8 | 5.5 | 2 | 3 | 2.5 |
| CDC Stratus | 7 | 3 | 6 | 7 | 5.8 | 2 | 6 | 4.0 |
| TR 251 | 3 | 3 | 3 | 4 | 3.3 | 2 | 3 | 2.5 |
| TR 360 | 5 | 3 | 4 | 4 | 4.0 | 2 | 3 | 2.5 |
| 6-row (n=5) | | | | | | | | |
| Argyle | 5 | 3 | 3 | 6 | 4.3 | 2 | 3 | 2.5 |
| Excel | 6 | 5 | 5 | 7 | 5.8 | 2 | 3 | 2.5 |
| Robust | 4 | 4 | 6 | 4 | 4.5 | 2 | 2 | 2.0 |
| CDC Sisler | 4 | 4 | 4 | 4 | 4.0 | 3 | 3 | 3.0 |
| Stander | 6 | 5 | 5 | 7 | 5.8 | 2 | 2 | 2.0 |
| | | | | | | | |
| Isolate mean (2r) | 5.1 | 4.4 | 4.9 | 5.9 | | 3.5 | 4.6 | |
| Isolate mean (6r) | 5.0 | 4.2 | 4.6 | 5.6 | | 2.2 | 2.6 | |
| Isolate mean (all) | 5.1 | 4.3 | 4.8 | 5.8 | | 3.0 | 3.8 | |
a Infection responses in bold type indicative of differential reactions to C. sativus
The routine presence of both Fusarium and C. sativus on spikes and kernels of barley suggests these species may compete here for infection sites and nutrients. Any competitive advantage by one or the other would help its exploitation of this nutrient source. During routine plating of barley seed on PDA medium for fungal species identification, it was noted that the growth of Fusarium was sometimes affected by near-by colonies of C. sativus. Suppression of Fusarium colony expansion was evident, suggesting antagonism was occurring. This led to further investigation of this phenomenon to examine its potential as a bio-control strategy against FHB.
Pairing of single C. sativus and F. graminearum culture plugs at opposite ends of the same PDA plate demonstrated the ability of the former to curtail the growth of Fusarium, as measured by reduced colony diameter, compared to checks, after several days incubation. This suggested that a metabolite(s) was being produced by C. sativus, which was released into the agar medium and subsequently curtailed the growth of Fusarium. Isolates of C. sativus appeared to have differential abilities to produce the putative metabolite(s), based on the width of the zones of growth inhibition. To test whether this phenomenon also occurred in vivo, barley plants were grown in pots in the greenhouse and inoculated with F. graminearum and C. sativus, singly and in timed combinations. Results of this successful bio-control experiment are shown in Table 5.
Table 5. Fusarium Head Blight Levels in Barley Challenged with Fusarium graminearum (F.g.) and Cochliobolus sativus (C.s.).
| Treatment | Barley cv. |
| AC Metcalfe (2r) | Stander (6r) |
| FHB-Ia
% | F.g. recovery
% | DON
ppm | FHB-I
% | F.g. recovery
% | DON
ppm |
|
| F.g. | 71 | 100 | 220 | 71 | 94 | 140 |
| F.g. + C.s.b | 68 | 95 | 227 | 69 | 94 | 155 |
| F.g. / C.s.b | 66 | 93 | 168 | 82 | 94 | 142 |
| C.s. + F.g. | 1 | 24 | 4 | 2 | 69 | 20 |
a FHB Index (visual disease severity)
b a + b (‘a’ followed 72 h later by ‘b’); a / b (‘a’ and ‘b’ applied together)
A similar trial was conducted at the CRC Field Station, Glenlea MB, in 2002, to evaluate its potential under natural conditions. The results were not promising, and an early application of C. sativus to spikes had no effect in reducing FHB levels. This result is not unusual for bio-control strategies against FHB, i.e., promising data in the laboratory are not necessarily reproducible in the field (Hershman and Milus 2002, Meyer et al. 2002). The trials are being repeated in 2003, with revised protocols to increase the quantity of C. sativus applied, as single and multiple events.
The deliberate application of one pathogen to control another is not likely to be a viable commercial option. While C. sativus does not produce DON or other mycotoxins, it can discolour seed, reducing its quality and/or grade. Furthermore, its spray-application to spikes would likely result in spot blotch lesions on flag and other leaves. The loss of foliar photosynthetic area would result of reduced filling of grain, resulting in shriveling and yield loss. Neither stained or shriveled grain are desirable, particularly for malting and brewing purposes. As an alternative, the use of a weak or non-pathogenic strain of C. sativus, or of related Cochliobolus species, that produce the same, or a similarly-effective metabolite, could be tenable. The most elegant and practical strategy would be to purify and identify the putative anti-fungal metabolite produced by C. sativus, and then formulate this into a product that could be applied to plants in the field for effective control of FHB without compromise. Research on these aspects is underway.
Both FHB and spot blotch are prevalent and problematic in the eastern prairies. Their spread and intensification westward, particularly into the major barley producing regions of Alberta and Saskatchewan should be monitored, as much of the barley acreage is planted to susceptible cultivars. The presence of the spot blotch pathogen on spikes and developing kernels may reduce levels of FHB in barley, but the role of C. sativus as an antagonist of Fusarium, and its status as a bio-control agent, needs further validation. The availability of partial resistance to these pathogens holds promise, but better and alternative sources are needed and the resistance needs to be more widely adopted. Screening nurseries for the diseases must be established and utilized in breeding programs to expedite the development and subsequent availability of resistant, adapted barley cultivars for western Canada.
References
Hershman, D.E. & Milus, E.A. 2002. Proc. 2002 National FHB Forum, Erlanger KY. Pp. 82-87.
Meyer, S. et al. 2002. Proc. 2002 National FHB Forum, Erlanger KY. Pp. 104-105.
Pearse, P. et al. 2003. Can. Plant Dis. Surv. 83 (in press).
Tekauz, A. et al. 2003. Can. Plant Dis. Surv. 83 (in press).
Tekauz, A. et al. 2002a. Can. Plant Dis. Surv. 82: 55-56.
Tekauz, A. et al. 2002b. Can. Plant Dis. Surv. 82: 57-58.
Tekauz, A. et al. 2001. Can. Plant Dis. Surv. 81: 63-64.
Tekauz, A. McCallum, B.D. and Gilbert, J. 2000. Can. J. Plant Pathol. 22: 9-16.
Turkington, K. et al. 2003. Can. Plant Dis. Surv. 83 (in press).
Valjavec-Gratian, M.& Steffenson, B.J. 1997. Plant Dis. 81:1275-1278.
A. Tekauz
Cereal Research Centre, Agriculture and Agri-Food Canada, Winnipeg MB, R3T 2M9
Presented at the 3rd Canadian Barley Symposium, June 19-20, 2003 |
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