Australasian Association of Nematologists
Evaluation of non-toxigenic, nematode-vectored Clavibacter spp. for potential biocontrol of Clavibacter toxicus, the bacterium responsible for Annual Ryegrass Toxicity
Ian Riley (The University of Adelaide) and George Yan (Agriculture Western Australia)
Introduction
Clavibacter toxicus produces corynetoxins in annual ryegrass (Lolium rigidum) and is responsible for the livestock poisoning know as annual ryegrass toxicity (ARGT). Other related bacterial species (Table 1), like C. toxicus, also colonise poaceous hosts using nematode vectors (Anguina spp., seed gall nematodes) but do not produce toxin. Some strains of these faster-growing, non-toxigenic species will adhere to Anguina funesta, the vector in ryegrass. It is postulated that these species could colonise ryegrass to displace C. toxicus and offer potential for novel biocontrol.
Table 1. Non-toxigenic, Anguina-vectored bacteria
Bacterium |
Host |
Origin |
"Corynebacterium agropyri" |
Wheat grass |
USA |
Clavibacter iranicus |
Wheat |
Iran |
Clavibacter rathayi |
Cocksfoot |
UK, NZ |
Clavibacter tritici |
Wheat, barely |
Egypt, India, Iraq |
Materials and methods
Container-grown ryegrass was inoculated with A. funesta galls and C. toxicus-colonised galls at sowing. The treatments, suspensions of one of 17 non-toxigenic strains (6 reps and 18 uninoculated controls), were sprayed on the soil surface three times at fortnightly intervals from a month after sowing. The experiment was repeated in two seasons in an approved quarantine facility. At maturity, nematode and bacterially colonised galls were counted in individual seedheads and whole pots. Toxicity of galls was assayed by a bacterial inhibition and isolates from galls were identified serologically.
Results
Eight strains gave a proportion of non-toxic galls (Table 2); three "C. agropyri" strains (20-100%), two C. rathayi strains (6-30%) and three C. tritici strains (10-55%). Inoculation with some strains also reduced overall gall levels in the 1997 experiment. Re-isolates from non-toxic galls were confirmed to be the species applied and C. toxicus from toxic galls.
Conclusions
The work has demonstrated that:
- non-toxigenic Clavibacter spp. can colonise ryegrass with Anguina funesta as a vector, and
- some strains could potentially displace C. toxicus, thereby reducing the risk of ARGT to livestock.
Further studies may be needed before field-testing is approved.
Table 2. Effect of inoculation of ryegrass with non-toxigenic, Anguina-vectored bacteria on nematode gall production (Anguina funesta), bacterial colonisation of galls and toxin production
Bacterium |
Strain |
1996 |
1997 |
||||||
Total galls |
Bacterial galls |
Per cent bacterial |
Per cent non-toxic |
Total galls |
Bacterial galls |
Per cent bacterial |
Per cent non-toxic |
||
"C. agropyri" |
CS35 |
592 |
273 |
46 |
20 |
592* |
62 |
11 |
100 |
CS105 |
816 |
302 |
37 |
0 |
1513 |
13 |
01 |
- |
|
CS106 |
403 |
180 |
45 |
91 |
666* |
67 |
10 |
90 |
|
CS107 |
349 |
122 |
35 |
41 |
537* |
176 |
33* |
39 |
|
C. iranicus |
CS13 |
642 |
356 |
56 |
0 |
1537 |
4 |
0 |
- |
C. rathayi |
CS5 |
952 |
459 |
48 |
30 |
1185 |
1* |
0* |
- |
CS7 |
664 |
232 |
35 |
0 |
1287 |
2* |
0* |
- |
|
CS18 |
442 |
159 |
36 |
6 |
1124 |
6 |
1 |
- |
|
CS26 |
613 |
193 |
31 |
0 |
1530 |
3 |
0* |
- |
|
C. tritici |
CS4 |
734 |
278 |
38 |
0 |
1131 |
9 |
1 |
- |
CS11 |
686 |
346 |
50 |
0 |
1434 |
4 |
0 |
- |
|
CS12 |
536 |
246 |
46 |
0 |
1006 |
3 |
0 |
- |
|
CS16 |
514 |
168 |
33 |
25 |
1088 |
28 |
3 |
- |
|
CS21 |
494 |
179 |
36 |
10 |
954* |
55 |
6 |
33 |
|
CS22 |
616 |
163 |
26 |
0 |
1399 |
1* |
0* |
- |
|
CS101 |
745 |
182 |
24 |
54 |
814* |
9 |
1 |
19 |
|
CS103 |
436 |
119 |
27 |
0 |
1408 |
7 |
1 |
- |
|
Control |
686 |
256 |
37 |
0 |
1399 |
28 |
2 |
- |
|
Strains in bold produced non-toxic bacterial galls indicating biocontrol potential. *Data statistically different from control (P<0.05). Galls counts analysed transformed (log(x+1)) and presented back transformed. Per cent bacterial galls logit transformed for analysis. - indicates toxin not assayed.
