ABSTRACT

Stem rot in rapeseed, caused by Sclerotinia sclerotiorum, is a major cause of yield losses in China. Three double low hybrid varieties have been used in a study to develop cultural control methods as a part of a project to produce strategies for integrated control of the disease. The field experiments show that: disease incidence was significantly different between the varieties; disease incidence was greater in earlier sown plots than later sown ones, there were few effects of plant density because of wet weather. The results of an experiment with a rotational regression combination design suggest that balanced fertilizer may be an important factor in control of the disease. The use of hybrid varieties into which the tolerant lines have been incorporated through a GMS system in integrated strategies of controlling the disease are discussed.

INTRODUCTION

Stem rot in rapeseed, caused by Sclerotinia sclerotiorum (Lib.) de Bary, is a major factor of yield losses in China and many other countries. The disease causes yield losses of up to 50% (average 10-20%) in China (Liu, 1991). The problem has become even more serious since the double-low (low erucic acid and low glucosinolates) varieties have been extended in China. The variability tolerance to stem rot among varieties suggests that it may be possible to introduce tolerance to decrease the severity of the disease (Liu, 1991 Hu, 1995). It would be sensible to incorporate tolerant lines into a variety to reduce infection and expansion of the pathogen. The emergence and wide cultivation of hybrid varieties make the incorporation possible because different tolerant lines can be introduced into a hybrid variety easier through a cytoplasmic male sterility (CMS) or genetic male sterility (GMS) systems than into a pedigree variety. The effectiveness of hybrid tolerant varieties would be enhanced cultural control methodologies to reduce the impact of disease were developed as part of a strategy for integrated control of the disease. This paper describes performance of our hybrid varieties with respect to disease and the effect of cultural control on Sclerotinia stem rot.

MATERIALS AND METHODS

Three double low hybrid varieties were chosen for this study: C022 and 5C21, based on recessive GMS system into which the tolerant lines have been incorporated; and Huaza No.3, based on CMS system.

A series of field experiments were conducted at Hefei, Anhui and 8 other sites in east of China during the 1997-98 growing season. Experiment 1 was a national adaptability test. Ten varieties tolerant to S. sclerotiorum (C022 and 9 others) were used (Table 1). They had all been recently developed in China, except for 821 which has been cultivated for more than 10 years. The same complete random design with three replications was used at the 9 sites. Experiment 2 was a randomized block design with twelve treatments (2 varieties, 5C21 and Huaza No 3; 3 sowing dates and 2 sowing densities) and three replications. Experiment 3 was a rotational regression combination design with five factors (sowing date, plant density, N, K and P fertilizer) and five levels per factor. The plots of all experiments were exposed to natural inoculum and were assessed for disease prior to harvest in late spring 1998.

RESULTS

Experiment 1: The results of the national adaptability test are summarized in Table 1. Because of abnormal wet weather during growing season, disease incidence (% plants infected) and disease index were higher than expected at most sites. But the incidence and index of individual varieties had similar trends at the different sites. C022 tended to have lower disease incidence and disease index than most other varieties, 821 and the two other double high hybrid varieties. The average yield of C022 was also the highest in the trials.

Table 1. The disease incidence and disease index in national adaptability test

DL: double low, SL: single low, DH: double high

Experiment 2: Disease incidence and index and grain yield are summarized in Table 2. Although the results have not been fully analyzed, they suggest that disease incidence and index were significantly different for varieties 5C21 and Huaza No. 3. Disease incidence also appeared to be greater in the earlier sown plots. There was little difference between different plant densities, probably because of particularly wet weather during the growing season that favored disease development. But the results suggest that earlier sown plots had higher yields than later sown ones, even although disease incidence was higher at earlier sown plots.

Table 2. The effect of different culture methods on the disease incidence and index in treatments

Experiment 3: The results of this experiment cannot be fully analyzed with only one year's data. However, the results of the first year suggest that balanced fertilizer may be an important factor in limiting the effects of the disease. It is possible that plants receiving balanced nutrition, in which all required elements are supplied in appropriate amounts, may be better able to protecting themselves from infection. The disease incidence and index at different sowing dates in this experiment are given in Table 3. As is the other field experiment earlier sown plots tended to have greater disease incidence and index than later sown plots. But, as in experiment 2, earlier sown plot also had higher yields than later sown.

Table 3. The disease incidence and index at different sowing dates

DISCUSSION

The value of resistance in controlling plant disease was recognized early this century. But sources of genes for resistance to S. sclerotiorum have not yet been discovered. Sclerotinia stem rot on rapeseed has become even more serious after the expansion in the growth of double low varieties in China which has taken place since the late 1980`s. It has been necessary to look for tolerance genes in varieties growing in areas where the disease is severe, and to incorporate them into new varieties. The finding of pol CMS in Brassica napus in China made it possible to introduce tolerant lines into hybrid varieties. The newly released double low hybrid varieties in China have shown similar or even better tolerance to the disease than existing double high varieties. It may be more efficient to incorporate tolerant lines into hybrid varieties through a recessive genetic male sterility system (for example hybrids 5C21 and C022). Our experience suggests that in this system it is easier to transfer tolerant lines and easier to find restores which showed disease tolerance than the CMS system (Chen, 1998) The use the tolerant hybrid varieties that are bred through the system should be one of the effective strategy for integrated control of the disease.

In this study the observation that earlier sown plots had greater disease incidence and index agreed with our the results of a survey of commercial farmer's fields and with the observation that in China disease in early maturing varieties is generally more serious than that in later maturing ones. It would appear that later sowing may be a good way to decrease disease incidence, but our results also show that the yield of later sown crops, especially later than certain date, decline sharply. Further work on the interaction between cultural practices and disease incidence and effects will help us develop integrated strategies for better control of stem rot in rapeseed..

ACKNOWLEDGEMENT

This study is part of an INCO-DC project funded by the EU (Contract No.: ERBIC18CT970173). The authors would like to thank for EU financial support

REFERENCES

1. Liu,C.Q., et al. (1991). Study on tolerance to Sclerotinia sclerotiorum and the inheritance properties in B. napus. Scientia Agricultura Sinica, 24 (1): 41-4

2. B.C.Hu, et al.(1995). Stem rot (Sclerotinia sclerotiorum) tolerance and disease avoidance in CMS line of Brassica napus, Proceedings of the 9th International Rapeseed Congress. 4: 1211-1213

3. Chen Fengxiang, et al. (1998). Genetic studies on GMS in Brassica napus L. I. Inheritance of recessive GMS line 9012A. Acta Agronomica Sinica, 24 (4): 431-438