ABSTRACT

The fungicides difenoconazole, flusilazole + carbendazim, prochloraz + carbendazim and tebuconazole applied as two or four spray treatments at full or half dose gave good control of phoma leaf spot on canker-susceptible winter oilseed rape cv. Rocket. Treatments were applied when 100% plants had phoma leaf spot and only four sprays of difenoconazole or flusilazole + carbendazim at full or half dose gave good (>65%) control of canker. These fungicides appear to limited curative activity and two spray treatments equivalent to commercial practice gave poor control. Monitoring of crops on farms suggested that an increase in the use of autumn fungicides from 34% crops in 1995 to 64% in 1998 together with better spray timing is improving canker control. Further work is needed to predict the onset of the autumn leaf spotting which is very variable and to optimise use of fungicides.

INTRODUCTION

Canker caused by Leptosphaeria maculans remains a major cause of yield loss in winter oilseed rape in the UK in spite of an increase in fungicide use (Fitt et al., 1997). The most seriously affected area is in eastern England, where crops may have >75% plants with moderate or severe cankers and suffer a yield loss of up to 1 t/ha. Good control of canker had been achieved with intensive (monthly) programmes of a fungicide mixture containing prochloraz + iprodione + thiophanate-methyl (Sansford et al., 1996), but little information is available to define economic optima for number of treatments and fungicide dose on commercial crops. The results of an experiment designed to determine the effectiveness of programmes at full and half dose in relation to epidemic development are presented in this paper together with a review of current commercial practice.

MATERIALS AND METHODS

In 1994/95, fungicides were evaluated on winter oilseed rape cv. Rocket sown on 23 August 1994 at a high disease site cropped continuously with winter oilseed rape since 1988 at ADAS Boxworth, Cambridge. The fungicides difenoconazole (125 and 62.5 g a.i./ha), flusilazole + carbendazim (200 + 100 and 100 + 50 g a.i./ha), prochloraz + carbendazim (494 + 185 and 247 + 93 g a.i./ha) and tebuconazole (250 + 125 g a.i./ha) were applied as either four-spray programmes (target: 4-6 week interval between treatments) on 11 and 29 November, 2 February and 13 March or two-spray programmes (11 November and 13 March), at both full label dose or half label dose in a randomised block design with three replicates. Disease incidence and severity (% leaf area affected or stem disease index (0 - healthy, 4 - dead)) were assessed at each spray date on 10 plants/plot, four weeks after the last spray and on 25 plants/plot pre-harvest. Canker severity was expressed on a 0-100 scale where 100 indicates all plants were dead. Plot yields were taken with a plot combine and adjusted to 90% dry matter.

Monitoring of farm crops of winter oilseed rape in England and Wales has been undertaken by ADAS and CSL since 1986 on a regional basis stratified according to the area grown. Disease assessments have been done on 25 plants per crop in autumn at mid leaf production (GS 1,5-1,9) (Sylvester-Bradley and Makepeace, 1985), spring at early stem extension (GS 3,1-3,3) and pod ripening (GS 6,3-6,5) in July. The incidence of canker was recorded in July and examined in relation to fungicide use on 94-114 crops per year for 1995-1998.

RESULTS

At Boxworth, phoma leaf spot incidence increased from 17% plants affected on 17 October 1994 to 100% plants affected on 11 November (when the first sprays were applied) and leaf spotting remained at a high incidence until flowering in April (Fig. 1a). Cankers started to appear in April and affected 100% untreated plants by late June (GS 6,4). Only difenoconazole and flusilazole + carbendazim applied four times at either full or half dose provided good (91% and 81% at full dose; 65% and 72% at half dose, respectively) control of canker, which was severe at harvest (untreated index 3.1) (Fig. 1b). Treatments generally protected plants from phoma leaf spotting for 6-8 weeks and on 19 January (7 weeks after the second spray in the four spray programme and 10 weeks after the first spray in two spray treatments) all treatments except single sprays of tebuconazole or prochloraz + carbendazim at half doses, gave good control of phoma leaf spot (Fig. 1c). Treatments gave yield increases of up to 0.77 t/ha more than untreated yield of 3.16 t/ha. Yield was negatively correlated with canker index (adjusted r²= 39.9%, P<0.001).

Mean canker incidence has declined since 1995, when severe attacks were particularlty widespread in the main production areas of east and south east England (Table 1). The development of phoma leaf spot was evident in October in 1994 and 1995, but was delayed until December or later by a dry autumn in 1996. In autumn 1997, symptoms appeared during November. Fungicide use in autumn has increased since 1995 from 34% to 64%, but shown little change in spring or at flowering (Table 1). During 1995 and 1996, canker incidence was higher in fungicide treated crops than in untreated crops. It therefore appears that the more severely affected crops received fungicides and that untreated crops with only low (12-25%) canker were appropriately selected (Table 1). There was no indication that fungicides had contributed to canker control in 1995 and 1996. In 1997, the phoma leaf spot epidemic developed late because of a dry autumn and canker was much less severe than in the previous two years. For the first time, however, canker had a lower incidence in treated than in untreated crops though there was no difference between single and two spray treatments (Table 2).

Table 1 Mean incidence (%) of canker in England and Wales and fungicide use, 1995-1998.

Figure 1 a) Development of phoma leaf spot and canker in winter oilseed rape cv. Rocket at ADAS Boxworth, 1994-1995, b) Canker severity in relation to fungicide dose and number of treatments, ADAS Boxworth, 1994-1995, c) Severity (% area) of phoma leaf spot on 19 January 1995 in relation to fungicide dose and number of treatments, ADAS Boxworth.

Table 2 Incidence of canker in relation timing of fungicides, 1995-1998.

More detailed examination of the first spray treatment revealed slight benefits from using sprays later than 60 days after sowing (i.e. early November onwards) compared with 40-60 days after sowing (Table 3). The later timings were associated with progressively lower canker incidence in 1997 whilst the earliest timing appeared most satisfactory in 1998. In both 1997 and 1998, selected timings gave lower canker than the untreated crops (Table 3).

Table 3 Incidence of canker in relation to date of autumn spray, 1995-1998.

* Insufficient samples for analysis

CONCLUSIONS

The fungicide experiment demonstrated that current products have limited curative activity once phoma leaf spot has appeared in the crop. The timing of the first spray is critical and subsequent experiments have confirmed that fungicides are most effective when used as protectants with at least two applications (P Gladders, unpublished data). It appeared that difenoconazole and flusilazole + carbendazim had greater curative activity than prochloraz + carbendazim and tebuconazole. Indeed, 80% of plants treated with four full doses of difenoconazole had no canker at harvest. This suggests a maximum ‘kick back’ activity from the first spray on 11 November to about 17 October when 17% plants had leaf spotting. Other fungicides and reduced rates appeared to have shorter ‘kick back’ activity and application of sprays when phoma leaf spotting was already present may well explain the rather variable canker control reported in previous experiments (Evans et al., 1984). These results are consistent with the suggestion that fungicides controlled the pathogen within the leaf or as it grew within the petiole (Hammond et al., 1985), but not after it had reached the stem base. The failure to apply fungicides in the autumn at the onset of the phoma leaf spot epidemic appears to be the main reason for poor control on farms despite repeated applications of fungicides (Gladders et al., 1998). In the field experiment, ‘commercial’ programmes based on half dose applications in autumn and spring gave poor control of canker when phoma leaf spot was present on all plants.

Examination of annual disease survey data in relation to fungicide use suggests that fungicides nationally were not being used effectively, because first treatments not being applied when phoma leaf spot appeared in October in autumns of 1994 and 1995 (Gladders et al., 1998). In 1997, early sprays are likely to have been less effective because the epidemic was delayed until December by dry weather and this would explain the strong trend for lower canker incidence with the later sprays (Table 3). In 1998, phoma leaf spot developed in November and earlier sprays appeared to result in marginally lower canker incidence.

Disease monitoring has provided early indications that canker control may be improving on farms and researchers now need to develop forecasting schemes which can provide crop specific guidance on the development of epidemics, the risk of yield loss and the optimum combination of fungicide treatments.

ACKNOWLEDGEMENTS

Funding for this work from the Ministry of Agriculture, Fisheries and Food is gratefully acknowledged.

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