ABSTRACT

In field experimental studies where increasing amounts of Plasmodiophora brassicae soil inoculum were applied to the soil and yield loss was quantified, strong relationships were found between disease incidence and yield (R²= 0.90), and between natural soil inoculum and yield (R²= 0.94) of spring oilseed rape (Brassica napus L.). A clear relationship (R²= 0.87) was also found between disease incidence and degree of soil infestation as tested in a bioassay. In another serie of field experiments testing effects of partly resistant lines of summer oilseed turnip rape (B. campestris), multiplication of clubroot was moderate, 2.1-10 % plants were infected after harvest, when initial soil inoculum levels were high: 47 to 72 % of the test plants infected in the bioassay. The yield of resistant lines was 5-10 % higher than the yield of the susceptible cultivar SW Kulta. In a third serie of field experiments placed on naturally infested soils, yield reduction in summer oil seed rape was 1.62 t ha^-1 (50 %) at the highest level of infection (91 % plants infected at harvest), while at < 20 % plants infected, yield reductions were estimated to about 10 %. We conclude that on fields with low levels of soil infestation, < 20 % plants infected in the bioassay, the risk of heavy losses from club-root could be avoided by including a partly resistant cultivar in the crop rotation.

INTRODUCTION

Clubroot, caused by Plasmodiophora brassicae, has become an important disease of cruciferous crops in certain areas of Sweden, where it also limits successful oilseed production. Following the introduction of oilseed rape in the early 1940´s, this crop has become a valuable component in the often cereal-dominated crop rotations. Thus, during the 1970´s rotations with an oilseed rape crop every fourth year were encouraged. Probably, as a consequence of this, severe attacks of clubroot were increasingly observed to decrease yields in the early 1980´s. In a survey carried out in a spring oilseed-growing area in 1986 clubroot was detected in 78 % out of 190 fields assayed for soil-borne infestation (Wallenhammar, 1996).

These observations well agree to Buczacki (1983) who states that the impact of this pathogen has never been greater than it is today due to the wide variety of crucifers grown. However, among the numerous reports on damages caused by P. brassicae in cabbage crops, only scarce information can be found on the effect of clubroot on the yield of oilseed rape (Davies 1986). The longevity of the spores also makes control of this disease difficult (Wallenhammar 1996), and no chemical treatments are available for field-scale use in oilseed crops. Breeding for resistance also involves complications due to the fact that field populations of P. brassicae contain a mixture of pathotypes (Jones et al 1982). Thus, yield loss due to clubroot may presently be minimized mainly by testing soils for the level of P. brassicae infestation and by using the test values avoid cropping on heavily infested fields (Wallenhammar 1996). We here show Swedish field experimental data estimating yield losses in oilseed crops at

¹ Correspondance: Örebro läns hushållningssällskap. P.O. Box 271, 701 45 Örebro

different clubroot infestations, and field experimental evidence that spring oilseed turnip rape cultivars

partly resistant to clubroot may be another mean to minimize both yield loss risks and adverse effects on narrow crop rotations.

MATERIAL AND METHODS

Establishment of a soil infestation gradient in spring oilseed rape in a field experiment with various infestation levels

A biennial field experiment, in which infested soil was added to the soil to give various levels of soil inoculum, was carried out in 1993-94. Increasing amounts of infested soil, collected from a heavily infested field, were distributed prior to planting. The experiment was arranged in a randomized block design with four replications. Both years the cultivar Sv Paroll was planted in all plots.

The plots were sampled in June each year and the infection capacity of the soil was measured by assessing the percentage of bait plants infected. A final evaluation of clubroot infection was done after harvest in the stubble. Plants were assessed as healthy or infected, and the yield was measured each year.

Assessment of yield and clubroot infection in spring oilseed rape in a field experiment with natural soil-borne inoculum

In a field experiment carried out in 1995 in spring oilseed rape (cv. Sv Sponsor), and located east of Västerås (59° 5´ N), severe attacks of clubroot were observed in parts of the experiment. Assessments of disease severity were performend in the stubble after harvesting and a correlation between disease incidence in the stubble and the yield was obtained.

Assessment of agronomic properties of spring oilseed turnip rape lines partly resistant to clubroot

Four field experiments comparing effects of non-resistant and partly resistant cultivars were carried out in 1997, on fields that were severly infested with clubroot the prevoius year. Two experiments were placed in the area of Örebro (59° N), one in the aera of Lidköping (58° 5´ N) and one west of Västerås (59° 5´ N). The treatments consisted of three different lines of summer oilseed turnip rape; SW A3155, SW 9433808 and SW 9433827 kindly supplied by Svalöf Weibull AB and one susceptible cultivar, SW Kulta. Three experiments were harvested with a specially designed combiner. Experimental site 4 was not experimentally harvested due to late maturity.

The plots were soil sampled twice, after planting and after harvest, and the level of P. brassicae infestation was measured by assessing the percentage of bait plants infected. Infected plants were scored according to a modified disease index based on (Toxopeous et al 1986), 0= no galls, 1= enlarged lateral roots, 2= enlarged tap root, 3= enlarged napiform tap root, 4= enlarged napiform taproot, lateral roots healthy, 5= enlarged napiform tap root, lateral roots infected, and as percentage diseased plants. DSI= (Class no x plants in each class) x 100/ (Total no plants x no of classes-1). Assessment of field disease severity was done in the stubble following harvest. The roots were scored according to a field disease severity index (FDSI) where 0= no galls, 1= slight galls on lateral roots, 2= moderate galls ( < 50 % of the main root system), 3= severe galls (> 50 % of main root system galled). FSDI was calculated as DSI desrcibed above.

RESULTS

Yield and disease assessments in a field experiment with added soil inoculum

In the first year, 1993, an increasing percentage of infected plants, was found in concordance with the increasing amounts of soil inoculum applied (Wallenhammar, 1998). The percentage of infected plants ranged between 0, without inoculum applied, and 16.1 in the treatment with the highest amount of inoculum applied. As compared to the control, yield was reduced by 263 kg ha^-1 (9 %) in this treatment.

In the second year, 1994, the classification of infected plants showed that clubroot had been efficiently distributed from infested plots to plots where no disease was expected. Thus, in the control treatment 53.8 % of the plants were infected. A good relationship between yield (y) and disease incidence (x) y = 2956- 31.6x, R² = 0.90 (Fig. 1a) was, however, obtained and the relationship between yield (y) and degree of infestation (x) according to the bioassay performed was y = 3009- 27x , R² = 0.94 (Fig. 1b). The relationship between disease incidence (y) and degree of infestation according to the bioassay (x) was y= 3.86+ 1.12x, R² = 0.87 (Fig. 1c).

Figure 1. Results from a field experiment with applied soil inoculum (after Wallenhammar, 1998).

A/ Relationship between yield (y-axis) and disease incidence (x-axis)

B/ Relationship between yield (y-axis) and degree of infestation according to bioassay (x-axis)

C/ Relationship between disease incidence (y-axis) and degree of infestation according to bioassay (x-axis).

Yield and disease assessment in a field experiment with natural soil-borne inoculum

A strong gradient in soil infestation was found in this experiment (Wallenhammar 1998). At a low disease incidence (< 20 % plants infected) the yield reduction was about 300 kg ha^-1 (fig 2), while yield was decreased by 1.63 t ha^-1 (50 %) at the highest disease incidence (91 % plants infected). A high correlation between disease incidence (x) and yield (y) was found y= 2981-14.3x, R² = 0.79.

2. Relationship between yield (y-axis) and disease incidence (x-axis) in a field experiment with natural soil inoculum (after Wallenhammar, 1998).

Yield and disease assessment in field experiments with partly resistant lines of summer oilseed turnip rape

The relationship between disease severity index (DSI) after planting (x) and field disease severity index (FSDI) as read in the field after harvest (y) was y= 1.32x- 17.53, R² = 0.66 for the susceptible cultivar SW Kulta. Disease severity indices for the partly resistant lines on experimental sites 1-3 were lower than for the control cultivar, SW Kulta, ranging between 1.5 and 9.1. On experimental site 4, field disease severity indices after harvest for the partly resistant lines were higher, with an average DSI of 60. Average DSI after planting was estimated to 64 for these treatments.The yield of partly resistant lines was 5-10 % higher compared to the susceptible cultivar SW Kulta which yielded on average 1.73 t ha ^–1. Average DSI after harvest in these trials (site 1-3) was calculated to 15.3 for SW Kulta and ranged between 3.6 and 4.4 for the partly resistant lines. Corresponding disease incidences (% infected plants) were 22, 4.6 and 5.9 respectively.

CONCLUSIONS

From our general observations and experience we regard the dissemination of clubroot in many oilseed rape growing districts in Sweden to be underestimated and it is often not observed by the growers. The economic losses are often substantial, but may not either be always evident as they, in addition to the main factors, time of infection and soil inoculum density, are dependent also on other factors influencing disease development (e.g. soil moisture content, soil temperature, soil pH and soil type).

In using the relationships presented here, disease incidence and yield loss may be predicted from the outcome of a soil bioassay test. However, such predictions are limited to the closest crop since for each crop of oilseed rape, the soil inoculum is multiplied and it is also easliy dispersed by farm equipment (Wallenhammar 1998). The partly resistant summer oilseed turnip rape lines included in our field experiments showed a high, although not complete, resistance with an average FDSI of 4.3 after harvest when average DSI after planting was 37. These levels are in concordance with the results presented by Happstadius et al (1998) where partly resistant lines of summer oilseed turnip rape, including the ones investigated in this study, were tested on naturally infested soil. Although not complete, such resistance may be a valuable mean both for decreasing loss risks and for avoiding high inoculum build up. According to the results obtained here, it is possible to integrate a partly resistant cultivar in the crop rotation and avoid yield losses in fields where the soil infestation level gives a DSI of < 10, or where < 20 % of the bait plants are infected in the bioassay. However, if in common use, the resistance properties in these cultivars has to be investigated continously in order to detect changes in virulence of the pathogen.

Acknowledgements

The work was supported by the Foundation of the Swedish Oilplant Research and by the Swedish Farmers Foundation for Agricultural Research. The authors are grateful to Mr C. Persson, Svalöf Weibull AB for providing seeds and to Mr V. Carbaillo assisting in the bioassaying of soil samples.

Literature

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2. Davies, J. M. L., 1986. Diseases of Oilseed rape. Pages 195-236 in: Oilseed Rape eds. D.H. Scarisbrick and R. W. Daniels. Collins London, 309 pp.

3. Jones, D.R., Ingram, D.S. and Dixon, G. R., 1982. Factors affecting test for differential pathogenicity in populations of Plasmodiophora brassicae. Plant Pathology, 31, 229-38.

4. Happstadius, I., Jonsson, R. and Persson, C., 1998. Breeding for clubroot resistance in oilseed rape. NJF seminar 302: Resistance biology of agricultural crops. 5.

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6. Wallenhammar, A-C., 1996. Prevalence of Plasmodiophora brassicae in a spring oilseed rape growing area in central Sweden and factors influencing soil infestation levels. Plant Pathology, 45. 710-719.

7. Wallenhammar, A-C., 1998. Observations on yield loss from Plasmodiophora brassicae infections in spring oilseed rape. Zeitschrift fur Pflanzenkrankheiten und Pflanzenschutz. 105, 1-7.