Effects of fungicides used to control Rhynchosporium secalis where benzimidazole resistance is present

Crop Pror~~ron Vol. 17, No. 9, pp. 7277734. 199X 0 1998 Elsevier Science Ltd. All rights reserved Prmted in Great Britam 026-2l’J4/98,‘$ - see front matter

PII: SO261-2194(98)00081-7 ELSEVIER

Effects of fungicides used to control Rhynchosporium secalis where benzimidazole resistance is present P.J. Taggart*§, L.R. Cooke *t, P.C. Mercer*+ and M.W. Shaw* *Department of Applied Plant Science, The Queen’s University of Belfast, Newforge Lane, Belfast, Northern Ireland BT9 5PX, UK tApplied Plant Science Division, Department of Agriculture for Northern Ireland, Newforge Lane, Belfast, Northern Ireland BT9 5PX, UK *Department of Agricultural Botany, The University of Reading, 2 Eariey Gate, Whiteknights, Reading RG6 6AU, UK

The effects of fungicides used to control Rhynchosporium secalis in winter barley were investigated in five field trials in Co. Down, Northern Ireland. Benzimidazole resistance was present in the R. secalis population of each site. Fungicide performance was evaluated in terms of disease control, maintenance of green leaf area, grain yield and grain quality. Samples of R. secalis isolates taken before and after treatment in each field were assessed for sensitivity to carbendazim in the laboratory. Carbendazim contributed little to disease control, the maintenance of green leaf area, grain yield or grain quality. Carbendazim was associated with a significant increase in disease severity in some plots. Treatments containing carbendazim, either alone or in mixture, caused an increase in the proportion of carbendazim-resistant isolates within R. se&is populations. Propiconazole-containing treatments performed well in terms of disease control, maintenance of green leaf area and grain quality. They caused no change in carbendazim resistance frequency. 0 1998 Elsevier Science Ltd. All rights reserved

Keywords: Rhynchosporium

secalis; carbendazim;

Introduction Fungicide formulations containing both a demethylation inhibitor (DMI) fungicide and the benzimidazole carbendazim are commonly used to control leaf blotch, caused by Rhynchospotium secalis (Oudem) J. J. Davis, in barley (Garthwaite et al., 1995). These mixtures have sometimes been associated with superior disease control and grain yields compared to formulations containing only DMIs and may also help to counteract selection for reduced DMI sensitivity (Jones, 1990; Margot et al., 1990; Kendall et al., 1993). However, in the early 199Os, benzimidazole resistance was detected in the R. secalis populations of the UK (Locke and Phillips, 1995; Taggart et al., 1994). In surveys in 1993, 17% of isolates collected in England and Wales were carbendazim-resistant (Locke and Phillips, 199.5), whereas 39% of Northern Ireland isolates were resistant (Taggart, 1996). Thus, benzimidazole resistance appears to be particularly Ireland R. se&is common in the Northern population. Torresponding

author.

fungicide

resistance

Research was undertaken at sites where carbendazim-resistant R. secufis strains were plentiful to investigate the effects of various fungicides on disease severity, grain yield and the proportion of carbendazim-resistant strains.

Materials and methods Location of trial sites Field trials were carried out in commercially grown crops of winter barley, cv. Fighter, in Co. Down. There were five trials: two in 1993 and three in 1994. In 1993, one trial was near Portaferry and the other near Strangford. In 1994, one trial was on Mahee Island near Comber, another near Newtownards and the other near Strangford. Layouts and treatments Each site was laid out as a randomised block (4 blocks x 5 treatments). Each plot measured 5 x 10 m and blocks were separated by 2 m buffers. The same treatments (Table I) were imposed at all sites. Fungicide treatments were applied twice, at growth stages

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Effects of fungicides used to control Rhynchosporium secalis: P.J. Taggart et al. Table 1. Field trial treatments

Active ingredients None Carbendazim Propiconazole Propiconazole + carbendazim Propiconazole + fenpropidin

Application rate (g a.i. ha -I)

Proprietary name

Manufacturer

Control Derosal WDG Tilt 250 EC Hispor 45WP

AgrEvo Novartis Novartis

250 125 125 + 100

Legend EC

Zeneca

125 + 562.5

(GS) 31-33 and 51-59 (Zadoks et al., 1974) using a hand-held sprayer (Killaspray with a variable jet nozzle) and an application volume of 200 1 ha -I.

Disease and green leaf area assessments Leaf blotch was the main disease at all sites. Ten tillers per plot were sampled by walking the length of the plot twice, 1 m in from each side, and taking a tiller every two steps. The percentage area affected by leaf blotch was estimated on the top two leaves of each tiller at GS 65-75 (20-26 d after the second spray), using the key described by James (1971) and the mean of the 10 samples of each leaf was recorded. In 1994 only, 10 tillers per plot were similarly sampled and the percentage area remaining green on the top two leaves was estimated at GS 83 (17-24 d after the second disease assessment) and the mean estimate for each leaf layer was recorded. Grain yield and quality assessments Shortly before harvest, the numbers of heads per square metre were estimated, by randomly placing two 0.25 m2 quadrats within each plot. The tillers contained within each quadrat were cut and the heads were counted. Total grain yields were estimated from a central 10 x 2 m swath combined in each plot. The moisture contents of cleaned sub-samples were determined after drying at 80°C so that yields could be expressed at a constant moisture content. Oven-dried (80°C) thousand grain sub-samples from each plot were weighed and specific weight estimates made on oven-dried sub-samples from each plot.

streptomycin sulphate 200 mg, distilled water 1 1) in Petri dishes and incubated under white light at 18°C. R. secalis conidia or mycelia were picked from the leaf surfaces using a sterile needle 6 d later and transferred to fresh antibiotic YMA plates. Approximately 2 ml of sterile distilled water were added to each plate as an inoculum-dispersal medium. After incubation under white light at 18°C for 14 d, the contents of each plate were used to prepare a conidial suspension (lo6 conidia ml -‘) which was tested for sensitivity to carbendazim. Isolates were assayed for carbendazim sensitivity by inoculation of YMA amended with technical grade carbendazim (BASF, UK) at 10 mg 1-I. Carbendazim-amended and control plates (3 replicates/ isolate) were incubated under white light at 18°C and assessed after 10 d. Isolates were classified as resistant if any growth was observed on the carbendazim-containing plates. Data analysis For each trial, all data, with the exception of those relating to shifts in sensitivity to carbendazim, were subjected to analysis of variance (Snedecor and Cochran, 1967). Percentage data were arcsin transformed before analysis. Trial means of green leaf area data, grain yield and grain quality data were additionally analyzed together, treating sites as blocks. Carbendazim sensitivity data were analyzed by comparing the change in resistance after spraying across treatments. The resistance level in the population was measured as the logarithm of the ratio of resistant to sensitive isolates, equivalent to the logit of the resistance frequency. The analysis was done using the generalised linear modelling facilities in GENSTAT (NAG, Oxford, UK). The logit of the change in resistance frequency was modelled as a sum of factors determined by site differences (factor SITE) and fungicide treatments (factor FUNG), and their interaction. The change in resistance frequency appeared in the software commands as the effect of a third factor TIME, fitted first; and the SITE and FUNG factors appeared as interactions with TIME. The residual deviance from the full model was significant against the x2 distribution, so the effects of factors were tested against the F-distribution (Baker and Nelder, 1978).

Sensitivity to carbendazim Approximately 100 leaf samples showing leaf blotch symptoms were collected from each plot shortly before the first spray and again 20-30 days after the second spray. On the second occasion only the two uppermost leaves were sampled. Up to 10 R. secalis isolates (each from a separate leaf) were obtained from both the pre-treatment and the post-treatment leaf samples collected from each plot and were subsequently assayed for sensitivity to carbendazim. R. seculis was isolated from lesions on surfacesterilised leaf sections that were placed on antibiotic yeast malt agar (YMA, yeast extract 10 g, Oxoid Agar No. 3, malt extract 10 g, with chloramphenicol 50 mg,

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Results Disease and green leaf area assessments The severity of R. se&is colonization varied among sites and years (Table 2). Leaf blotch was most severe in Strangford in 1993 and least severe in Mahee Island in 1994. Generally, carbendazim used alone was ineffective at controlling leaf blotch. At Strangford in 1993, use of carbendazim, both alone and with propiconazole, was associated with increased disease severity relative to severity in the control and propiconazole-alone plots, respectively. The three propico-

Effects

of fungicides used to control Rhynchosporium

Table 2. Effect of fungicide treatments on severity of Rhynchosporium secalis colonization (arcsin transformed percentage data, presented as degrees of angle) in field trials (1993-1994)

secalis: P.J. Taggart et al.

nazole-containing formulations of leaf blotch, control, at sites. Green maintained by carbendazim-alone the three propiconazole-containing

the untreated was not treatment, but

1).

Rhynchospotium secalis colonization

Grain yield Treatments

Leaf 1

Leaf 2

24a lb 6b 9b 4b 10

25a lob lob lob 5b

32a 30a 8b 15b 7b 11

20bc 34a 9d 25ab 1Ocd 8

Portafeiiy 1993 Control Carbendazim Propiconazole Propiconazole +carbendazim Propiconazole + fenpropidin L.S.D. (PGO.05) StranMpordI993 Control Carbendazim Propiconazole Propiconazole fcarbendazim Propiconazole + fenpropidin L.S.D. (PGO.05) Mahee Island 1994 Control Carbendazim Propiconazole Propiconazole + carbendazim Propiconazole + fenpropidin L.S.D. (PGO.05) Newtownards 1994 Control Carbendazim Propiconazole Propiconazole + carbendazim Propiconazole + fenpropidin L.S.D. (PGO.05) Strangford 1994 Control Carbendazim Propiconazole Propiconazole + carbendazim Propiconazole + fenpropidin L.S.D. (PGO.05)

2.8a I.la 0.8a 0.5a 0.5a 2.6

12.7a 9.2b 6Sbc 5.7c 8.0bc 2.9

4.5a 4.la 2.lb 1.7b o.oc 1.7

17.3a 16.4a 11.4b 6.7~ 4.7c 4.4

9.5a 3.9bc 4.6b 0.6~ 1.8bc 3.6

24.8a 24.3a 15.7b 12.5bc 11.9c 3.5

Numbers relating to the same trial in the same column followed by the same letter do not differ significantly (P > 0.05).

80.0

contml Figure 1, Effect of fungicide treatments prop+carb = propiconazole+carbendazim,

carbendazim

propiconazole

prop+carb

prop+fen

on green leaf area (% arcsin) of leaf 1 at GS 83 in 1994 field trials (mean of three prop+fen = propiconazoletfenpropidin). The vertical bar shows the L.S.D. (PCO.05)

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1998 Volume 17 Number 9

Effects of fungicides used to control Rhynchosporium secalis: P.J. Taggarl et al. Table 3. Grain yields in field trials (1993-l 994) Grain yield (16% moisture content) (t ha -‘) 1994

1993 Treatments

Portaferry

Strangford

Mahee Island

Newtownards

Strangford

Mean of all sites

5.9a 5.8a 5.8a 6.la 6.9a 1.0

4.la 3.8a 5.0a 5.4a 5.3a 1.9

6.0a 5.4a 6.la 6.la 6.4a 1.4

5.8a 5.8a 7.0a 6.5a 6.4a 1.0

6.3b 6.6b 7.4a 7.4a 7.6a 0.7

5.6b 5.5b 6.3a 6.3a 6.5a 0.4

Control Carbendazim Carbendazim + propiconazole Propiconazole Propiconazole + fenpropimorph L.S.D.(P < 0.05)

Numbers in the same column followed by the same letter do not differ significantly (P > 0.05).

need to be examined. Changes in frequency attributable to selection are represented by the TIME.FUNG interaction: a change in frequency that depends on spray treatment. This did not differ detectably between sites, as indicated by the non-significant SITE.(TIME.FUNG) interaction. The FUNG.TIME interaction is very significant, showing shifts in resistance attributable to fungicide treatment. The background shift at each site, averaged over spray treatments, differed as shown by the significant SITE.TIME interaction. The parameter estimates from the best fitting model, containing SITE, TIME, FUNG and their 2-way interactions, are given in Table 7. The frequency of resistance changed between the two sampling times in the unsprayed plots (Table 7, line 10, P
ments containing carbendazim (Table 7, lines 15-16, P-C0.001). This indicates that carbendazim selected for resistance causing a 9-fold increase in the ratio of resistant to sensitive isolates after two sprays (Table 7, line 15). The increase was somewhat smaller when the fungicide was used in a mixture with propiconazole, but not significantly so (Table 7, line 16). The change in resistance level in fungicide treatments without carbendazim was not significantly different from that in unsprayed plots (Table 7, lines 17-18).

Discussion Carbendazim, when used alone, failed to control R. seculis infections in most trials and contributed little to disease control with propiconazole. The failure of carbendazim to control disease was reflected in advanced senescence at GS 83, low grain yields and poor grain quality. Since carbendazim has contributed

Table 4. Thousand grain weights in field trials (1993-l 994) Thousand grain weight (oven-dry) (g) 1993 Treatments

1994

Portaferry

Strangford

Mahee Island

Newtownards

Strangford

Mean of all sites

30.4c 31.6bc 33.lab 32.7b 34.4a 2.0

29.3~ 31.5b 35.5a 34.9a 36.5a 2.1

34.5a 33.0a 34.7a 34.9a 36.2a 2.1

35.6~ 36.7bc 38.6a 37.9ab 37.4ab 1.8

38.5c 39.7b 40.lb 41.6a 41.5a 0.9

33.7b 34Sb 36.4a 36.4a 37.2a 1.4

Strangford

Mean of all sites

64.5~ 64.9bc 65.7ab 65.9ab 66.la 1.0

58.0b 58.4b 60.6a 59.9a 60.9a 1.4

Control Carbendazim Carbendazim + propiconazole Propiconazole Propiconazole + fenpropimorph L.S.D. (P < 0.05)

Numbers in the same column followed by the same letter do not differ significantly (P > 0.05).

Table 5. Specific weights in field trials (1993-1994) Specific weight (oven-dly) (kg hl _ ‘) 1993 Treatments

1994

Portaferry

Strangford

Mahee Island

56.5b 56.3b 57.8ab 57.4ab 59.la 1.9

52.2d 55.7c 58.2ab 57.lbc 59.5a 2.3

53.4a 52.0a 56.7a 54.5a 54.9a 3.9

Control Carbendazim Carbendazim + propiconazole Propiconazole Propiconazole + fenpropimorph L.S.D.(P $0.05)

Numbers in the same column followed by the same letter do not differ significantly (P > 0.05).

730

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1998 Volume

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Newtownards 63.2b 63.21, 64.7a 64.7a 64.8a 1.4

Effects of fungicides

significantly to control of R. secalis in the past, where benzimidazole resistance was not detected (Jones, 1990; Margot et al., 1990) it is probable that its failure in the trials here reported was due to the presence of carbendazim-resistant strains, which were plentiful at all sites. The propiconazole-containing treatments controlled R. secalis effectively, helped to maintain green leaf area and improved both grain yield and quality. Although the performance of the propiconazole plus fenpropidin formulation was not dramatically better than that achieved by propiconazole alone, the use of similar formulations, or tank mixtures, may be advisable because of reduced sensitivity to DMIs and the associated possible decline in field performance (Jones, 1990; Kendall et al., 1993). R. secalis infection was associated with reduced grain weight and density. It is not possible to state whether the pathogen also caused reductions in the

used to control Rhynchosporium

secalis: P.J. Taggat-l et al.

number of heads per square metre, or the number of grain per head, because of the large errors associated with the estimation of these two variables. However, the fact that carbendazim was associated with a complete failure to control disease or increase grain yield, while significantly increasing thousand grain weight in two trials, suggests that the pathogen may also be responsible for a decline in the number of grain per unit area. Thus, in carbendazim-treated plots, where infection was severe, there may have been fewer heads per unit area, or fewer grains per head, but grains may have been heavier, because of less inter-grain competition for light and nutrients. Alternatively, carbendazim may have controlled unidentified sub-clinical pathogens which impeded grain filling, or the heavier grain may be explicable in terms of cytokinin-like behaviour bv carbendazim, as has been _ previously reported for benzimidazoles (Thomas, 1974).

is before

Portafeny, 1993

n after

control

carbendazim

propiconazde

prop+carb

prop+fen

propiconazole

propecarb

prop+fen

. ..

.. .

control

carbendazim

Figure 2. Proportions of carbendazim-resistant Rhynchosporium se&is (prop + carb = propiconazole + carbendazim, prop + fen = propiconazole

isolates collected +fenpropidin).

before and after fungicide

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treatments,

1993 trials

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731

Effects of fungicides used to control Rhynchosporium secalis: P.J. Taggart et al.

control

carbendazim

propiconazole

proP+mdJ

Propcfen

Newto~ards, 1994

.

control

carbendazim

popiconazole

v-w+-*

pmp+fen

prapiamazob

wP+=*

proWen

Strangford, 1994

control

carbendazim

Figure 3. Proportions of carbendazim-resistant Rhynchosporium secalis isolates collected before and after fungicide treatments, 1994 trials (prop + carb = propiconazole + carbendazim, prop +fen = propiconazole +fenpropidin).

732

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Effects of fungicides

used to control Rhynchosporium

secalis: P.J. Taggat-t et al.

Table 6. Changes in sensitivity to carbendazim over the season: changes in deviance as factors are sequentially dropped from linear logistic model. Since the residual from the model, tested against x2, was significant, model terms are tested against F. Only tests relevant to the experimental aims are shown; the ‘nuisance’ main effects of SITE and FUNG were fitted first Factors in model

Degrees of freedom

Before vs after spray (TIME) FUNG.TIME SITE. TIME SITE. TIME. FUNG Residual

1 4 4 16 150

Change in deviance

Mean deviance

Deviance ratio

Significance of ratio

16.8 58.9 40.7 23.6 202.6

16.78 14.73 10.18 1.47 1.35

12.4 10.9 7.5 I.1

*** *** 2.*i ns ** (vs XL)

ns=P>O.OS: ** =P
Line

Factor

Parameter estimate

in various categories,

logistic

on a

Standard error

Significance

1.84

0.34

***

-4.30 - 1.52 - 2.67 -2.56

0.40 0.32 0.33 0.34

*** x;* *** ***

~ 0.04 PO.34 0.09 0.15

0.33 0.32 0.32 0.31

ns ns ns ns

- 1.60

0.43

**

1.79 1.43 1.95 2.19

0.53 0.43 0.43 0.44

*** ** *** 1;**

15 16

In unsprayed, Strangford 93 Differences in unsprayed at other sites: Portaferry 93 Strangford 94 Newtownards 94 Mahee Island 94 Differences when sprayed with: Carbendazim Carbendazim + propiconazole

2.15 1.50

0.45 0.41

*** **

17 18

Propiconazole Propiconazole + fenpropidin

PO.33 PO.39

0.42 0.42

Base levels:

1 2 3 4 5

6 7 8 9 10 11 12 13 14

ns=Pz0.05;

Strangford 93, no fungicide, actual level Differences in initial levels (no fungicide) at Portaferry 93 Strangford 94 Newtownards 94 Mahee Island 94 Average differences before spraying between no spray plots and plots to be sprayed with: Carbendazim Carbendazim + propiconazole Propiconazole Propiconazole + fenpropidin Change gfter treatment:

*- =/‘~n.nl:

***

=P<

tlS

ns

>O.OOl.

Not only did carbendazim fail to control R. secalis, but it was associated with an increase in disease severity in one trial. This was surprising, since even in a population with a high proportion of resistant strains, control of the sensitive strains might be expected to result in a smaller pathogen population overall. Nevertheless, similar effects have been observed with Drechslera teres in barley (ToubiaRahme et al., 1995). There are several possible explanations. For example, carbendazim may stimulate carbendazim-resistant strains of R. secalis, perhaps by increasing sporulation, as was observed in Drechsleru teres (Toubia-Rahme et al., 1995). Alternatively, carbendazim may affect the phylloplane microflora, favouring the proliferation of carbendazim-resistant strains of R. secalis. Increased disease severity after spraying with benomyl was associated with a reduction in the population density of the natural saprophytic mycoflora in field experiments with Cochliobolus sativus on rye by Fokkema et al. (1975). Carbendazim selected for carbendazim resistance in R. secalis in the trials reported here. This effect has been reported in many crops. In cereals, the effect has been measured in field studies of Pseudocercosporella herpotrichoides in wheat treated with carbendazim (Bateman, 1994). In the present study,

the increase was somewhat smaller when the fungicide was used in a mixture with propiconazole. Mixtures might therefore have slightly retarded the evolution of resistance to carbendazim, as predicted by some theories (Skylakakis, 1984; Shaw, 1989, 1993; Birch and Shaw, 1997). Propiconazole did not select against carbendazim resistance. In one field experiment the resistance level tended to decline in the absence of fungicide, but not in the other five. These two observations do not support a useful role of short-term alternations in controlling resistance to carbendazim. Considering that carbendazim failed to control disease satisfactorily, it should not be recommended as a fungicide against R. secalis in Northern Ireland at least. Mixtures of DMIs with other fungicides (morpholines or the newer strobilurin or anilinopyrimidine fungicides) should come to assume a more prominent role.

Acknowledgements

The authors are grateful to the Home-Grown Cereals Authority for funding this work. We thank Dr S. Watson, The Queen’s University of Belfast for her

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1998 Volume 17 Number 9

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Effects of fungicides used to control Rhynchosporium secalis: P.J. Taggart et al.

help in analyzing and interpreting the data and the cereal growers, Mr H. Chambers, Mr R. McFerran, Dr J. Orr and Mr R. Wilson, for permitting field trials on their land. References

Margot, P., Kirk, W. W. and Taylor, A. C. (1990) Sensitivity to propiconazole of Rhynchospotium seculis populations from individual barley fields in north east Scotland and East Anglia. In Proceedings of the 1990 Brighton Crop Protection Conference Pests and Diseases 3, pp. 1141-1146

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Shaw, M. W. (1989) Independent action of fungicides and its consequences for strategies to retard the evolution of fungicide resistance. Crop Protection 8, 405-411

Manual. Royal Statistical Society, London

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Bateman, G. L. (1994) Selection in populations of the eyespot fungus in continuous wheat by repeated applications of carbendazim and prochloraz. In Fungicide Resistance, eds Heaney, S., Slawson, D., Hollomon, D. W., Smith, M., Russell, P. E., Parry, D. W., BCPC Monograph No. 60. BCPC, Farnham, pp. 219-224

Skylakakis, G. (1984) Quantitative evaluation of strategies to delay fungicide resistance. In Proceedings of the 1984 British Crop Protection Conference - Pests and Diseases 2, pp. 565-572

Baker, R. J. and Nelder, J. A. (1978) The GLZM System, Release 3:

Birch, C. P. D. and Shaw, M. W. (1997) When can reduced doses and pesticide mixtures delay the build-up of pesticide resistance? A mathematical model. Journal ofApplied Ecology 34, 1032-1040 Fokkema, N. J., van de Laar, J. A. J., Nelis-Blomberg, A. L. and Schippers, B. (1975) The buffering capacity of the natural mycoflora of rye leaves to infection by Cochliobolus sativus, and its susceptibility to benomyl. Netherlands Journal of Plant Pathology

Snedecor, G. W. and Cochran, W. G. (1967) Statistical Methods. The Iowa State University Press, Iowa, pp. 299-307 Taggart, P. J. (1996) Benzimidazole resistance in Rhynchosporium Ireland. M. Phil. thesis, The Queen’s University of Belfast, Belfast, UK

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D. G., Thomas, M. R. and Hart, M. (1995) Pesticide Usage Survey Report 127. Arable Farm Crops in England and Wales, 1994. MAFF Publications, London, 97pp

Taggart, P. J., Cooke, L. R. and Mercer, P. C. (1994) Benzimidazole resistance in Rhynchosporium secalis in Northern Ireland and its implications for disease control. In Fungicide Resistance, eds Heaney, S., Slawson, D., Hollomon, D. W., Smith, M., Russell, P. E., Parry, D. W. BCPC Monograph No. 60. BCPC, Farnham, pp. 243-246

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Jones, D. R. (1990) Sensitivity of Rhynchosporium se&is to DMI fungicides. In Proceedings of the 1990 Brighton Crop Protection Conference -Pests and Diseases 3, pp. 1135-1140

Toubia-Rahme, H., Ali-Haimoud, D.-E., Barrault, G. and Albertini, L. (1995) Effect of four fungicides on barley net blotch caused by Drechslera teres. Journal of Phytopathologv 143, 335-339

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Kendall, S. J., Hollomon, D. W., Cooke, L. R. and Jones, D. R. (1993) Changes in sensitivity to DMI fungicides in Rhynchosporium secalis. Crop Protection 12,357-362

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Zadoks, J. C., Chang, T. T. and Konzak, F. C. (1974) A decimal code for the growth stages of cereals. Weed Research 14,415-421 Received 9 March 1998 Revised 21 September 1998 Accepted 23 September 1998