Oilseed rape: Disease development, forecasting and yield loss relationships

Abstract

A series of fourteen replicated plot scale experiments was established between 1991 and 1994 to determine the effect of disease on the yield of winter oilseed rape and the economic threshold and optimum fungicide timings for disease control. The cultivar Envol was selected for use throughout the experiment as it was a popular cultivar and was moderately susceptible to both canker and light leaf spot.

A mixture of fungicides effective against all major and most minor fungal diseases of oilseed rape (iprodione and thiophanate-methyl (@ 167g/l each plus prochloraz @ 450g/l at 1.5 and 0.55 l/ha respectively) was applied to plots of oilseed rape at four-weekly intervals from the beginning of October. The experiment design was planned to allow the differential development of a series of epidemics to occur by the sequential application of multiple sprays of the fungicide mixture to the plots. The first series of sprays all began in the autumn and finished progressively later, continuing until harvest; the second series of sprays all finished at harvest and started progressively earlier.

The range of diseases that developed across the experiment sites was extensive, with all of the major, and most of the minor diseases detectable at some or all of the locations. This report is based upon the three major oilseed rape diseases which generated the most data and which appeared to have the greatest effect on yield.

Leaf spot and canker caused by Leptosphaeria maculans developed to damaging levels at ten sites. A strong relationship was found between yield and the incidence of canker at pod ripening at seven of the sites such that for every 1 per cent of stems affected by canker at pod ripening 0.01 t/ha yield loss occurred. Yield losses related to canker were mainly associated with sites where the mean leaf area affected by the foliar phase of the disease in untreated plots was > 1.0 per cent before or during January. The disease was controlled by sprays applied between November and February. Economic disease control was associated with sites where foliar severity exceeded 1.0 per cent before or during January with 34.0 per cent of stems affected by canker at pod ripening.

Light leaf spot caused by Pyrenopeziza brassicae became damaging at nine sites and appeared to cause twice as much yield loss as canker. For every 1 per cent of stems affected by light leaf spot at pod ripening, 0.019 t/ha yield loss occurred. Yield losses related to light leaf spot infection of the stem were mainly associated with sites where the mean leaf area affected in untreated plots during the season on at least one assessment date was > 13.0 per cent. No relationship existed between pod disease and yield. As with canker, sprays applied between November and February gave good disease control. Economic disease control was associated with sites where foliar disease severity exceeded 13.0 per cent during the season with more than 17.0 per cent of stems affected by light leaf spot at pod ripening.

Sclerotinia stem rot caused by Sclerotinia sclerotiorum was only damaging at two sites. At one site the disease was particularly damaging causing a high yield loss of 0.016 and 0.032 t/ha respectively for every 1 per cent of stems or plants with racemes affected at pod ripening. At this site the disease was well controlled on the stems and the racemes by sprays applied at late flowering (GS 4.7 and 4.8). At the second site the disease was less severe and caused a yield loss of 0.01 t/ha for every 1 per cent of stems or plants with racemes affected at pod ripening. Symptoms on the main stem were not well controlled at this site, but significant reductions in the incidence of plants with disease on the racemes occurred as a result of the application of post-flowering sprays in June. Of particular interest was the significant increase in disease on the main stems which resulted from treatments that received their final fungicide applications in early spring, just prior to flowering. This was thought to be related to the possible destruction of beneficial organisms pre-flowering, which may otherwise have helped to prevent infection by S. sclerotiorum.

This detailed 3-year study into the effect of disease on yield and the economics of disease control has shown that light leaf spot, canker, and sclerotinia stem rot are all damaging diseases reducing seed yield on average by 0.010, 0.019, and 0.016 t/ha respectively for every 1 per cent of stems affected at pod ripening. The optimum timing and number of fungicide sprays to control both light leaf spot and canker has been shown to be between early November and late February with the application of one or two sprays. Although few of the sites were affected by sclerotinia stem rot the data obtained indicated that the disease can be successfully controlled by the application of late or post-flowering sprays. Sclerotinia can infect the oilseed rape crop at any time during the flowering period, hence the traditional early to mid-flowering treatment. Judgement on spray timings for this disease should in practice be based on an assessment of risk derived from information on cropping and disease history together with weather data and petal culturing to detect ascospore inoculum. Hence, this unusual result has shown that it is possible to delay treatment for Sclerotinia until a late stage in the development of the crop, but this approach should not be normally adopted unless more information is available to support it.

Prior to this study, little or no data were available on the effect of disease on double-low cultivars. With respect to light leaf spot there was no evidence that light leaf spot infection of the stem had any effect on yield, but it is now clear that stem infection can be extremely damaging. Data on canker and yield were inconsistent and were only available for single-low cultivars such as Jet Neuf, as few disease epidemics had occurred in the UK since double-low cultivars were introduced. This study has produced substantial data on both diseases. The disease-yield loss relationships calculated in this experiment may however be specific to the cultivar Envol, or similar cultivars, since oilseed rape cultivars with different NIAB ratings for disease resistance may suffer greater or less loss in yield for the same amount of disease.

It is now clear that the application of fungicides to winter oilseed rape in the autumn can be beneficial for the control of light leaf spot and canker. However, it was clear that there is insufficient information on which to base a judgement with respect to the threshold level of either disease at the time when the autumn spray should be applied. It is also clear that where a spring treatment is required, the timing should be brought forward from stem extension, which often occurs in March, to a spray applied to the crop by the end of February.

Further work is required to determine more specifically the exact timing, dose and frequency of fungicide application required within the November to February period for the control of light leaf spot and canker, or the flowering period in relation to the development stage of the crop for sclerotinia stem rot, and to forecasting potential disease development and yield loss, in relation to early levels of disease symptoms, or the detection of infection by non-visual means such as diagnostic kits. In conjunction with meteorological data this information will allow specific judgements to be made for disease control in the winter oilseed rape crop.