Federal Centre for Breeding Research on Cultivated Plants, Institute for Epidemiology and Resistance, Theodor-Roemer-Weg 4, D-06449 Aschersleben
During the last years great differences were detectable in the attack of oilseed rape fields by turnip yellows luteovirus (TuYV, syn. beet western yellows luteovirus, BWYV) in different regions of Germany.
In the 1995/96 growing season 147 fields of 152 fields tested were infested by TuYV, with an average of 71,5% of the plants being infected in the northern, western and eastern part of Germany. On the other hand, a mean value of the infection level of only 10,7% could be found in samples from 46 fields of the southern part of Germany. No virus infections were detected in the most fields in the 1996/97 season. During the 1997/98 growing season TuYV infections were found in 161 of 178 observed fields from the middle and southern part of Germany. The infection degree ranged from 2% until to 100%, with an average of 29,8% of infected plants. In contrast to the results of the 1995/96 season a low infection degree could be found in the eastern and northern growing regions.
The infestation of oilseed rape by TuYV in the different growing regions appears to be related to a dynamic flight activity of aphids in autumn 1995 and 1997. In 1995 we found a high flight activity of aphids, but meanly cereal aphids like Rhopalosiphum padi, in a relatively warm autumn. The aphid flight of 1997 was characterised by a relatively high part of Myzus persicae in the aphid population during summer and autumn.
In a three-year experiment, plots of the cvs. Falcon and Zeus with 90% to 100% TuYV infections yielded 12% to 34% fewer seeds than nearly virus-free plots.
In relation to the observed high attack of oilseed rape fields by TuYV it can be concluded that it is necessary to breed new virus resistant cultivars.
KEYWORDS: Virus, aphids, flight activity, yield losses
Increasing levels of infection of winter oil seed rape by turnip yellows virus (TuYV, syn. beet western yellows virus, BWYV) were detected in Great Britain, France, the USA, the Czech Republic and Germany during the last decade (Smith and Hinkes, 1985; Hardwick et al. 1994; Kerlan, 1991; Thomas et al., 1993; Polak and Majkowa, 1992; Schröder, 1996).
In Great Britain, yield trial plots of two cultivars with high levels of TuYV infection resulted in 10% and 26% reduction in seed yield (Smith and Hinkes, 1985; Jay and Smith, 1995).
The investigations to determine the virus situation in oilseed rape fields of growing regions in the middle, eastern and northern part of Germany were started at Aschersleben in the growing season 1990/1991.
The flying activity of aphids during the growing periods we determined in further studies. In field trials with the cvs. Falcon and Zeus the yield reduction by TuYV infection were investigated at Aschersleben.
In the growing season 1990/1991 we began to test plant samples of oilseed rape fields for infection by the TuYV. Since the growing season 1995/1996 the virus situation was determined in cooperation with the plant protection service of some federal countries. In order to assess the situation of infestation in oilseed rape fields in late autumn or in spring leaf samples were picked up, mostly 50 leaves of each field, and tested in DAS-ELISA using a polyclonal TuYV antiserum, produced by F. Rabenstein (unpublished data) Aschersleben, for virus infection. The dimension of fields tested ranged from 2 ha until to 173 ha.
Aphids were trapped by a 40 feet suction trap (type Rothamsted) (Schliephake and Karl, 1995) at the institute of Aschersleben. The trap works continuously during the season from the end of April to the middle of November. All of the aphids are separated from other trapped insects and the species were determined.
For the determination of the economic importance of the virus plots of the cvs. Falcon and Zeus were inoculated with TuYV by artificial colonisation with viruliferous Myzus persicae (Sulz.), propagated on TuYV infected oilseed rape plants in the end of September/beginning of October. At the same time the control plots were sprayed with a selective aphicide (pirimicarb, 0,05%). In the first two years we used for the experiments a virus isolate from oilseed rape at Aschersleben (isolate BN 5 ASL). In the third year was used a second isolate from Lippstadt (isolate LP 3/5). Each plot measured 2 m x 5 m in 1992 and 1993 and 3 m x 5 m in 1994, and was drilled on 26.8.1992, 19.08.1993 and 23.8.1994. Seed yield was estimated after harvesting each plot with a combine harvester.
Incidence of TuYV
Great differences in the degree of TuYV infections could be detected in investigation period in Germany. High infestation degrees were found in 1991/1992, 1994/1995 and 1995/1996 (Figure 1). During the 1995/1996 growing season the degree of TuYV infection in samples from 181 fields of 198 fields tested from different parts of Germany ranged from 2% until 100%. A very high infection degree was present especially in the northern, middle, western and eastern part of Germany. The samples from 147 fields of 152 fields tested of these regions were infected by the TuYV with an average of 71,5% of the plants being infected, among them samples of 86 crops 76% until 100% TuYV infections. In contrast of them an average of infection degree from 13% and 16% of 34 fields of 46 fields tested could be found in the southern federal countries.
Figure 1. Infestation of oilseed rape fields by TuYV determined since the 1990/1991 growing season in Germany
No infection was detectable in samples from 93 fields of 121 fields tested in 1996/1997. The highest portions of TuYV infections (15% to 23%) could be found in samples of three fields only from the northern part of Germany.
Altogether samples of 262 rape seed fields were tested from all growing region in the 1997/1998 growing season. The degree of TuYV infections in samples from 58 fields of 132 fields tested from the middle and southern part of Germany ranged from 25% until 100%, with an average of 59,8% of the plants being infected. In contrast of them and to the results of the 1995/1996 growing season a low infection degree could be found in the samples of 130 fields from the eastern and northern part of Germany.
The epidemic infestation of oilseed rape in the growing regions of different parts of Germany is only partly related to a dynamic flight activity of aphids. Higher flight activities in autumn of 1990, 1995 or 1997 are connected with higher virus infections in the spring of the following years (Figure 2). But higher infestation degrees in the field appear also after a relatively low flight activity like 1994/1995.
Figure 2. Number of aphids detected in autumn since the growing season 1990/1991 by the suction trap at Aschersleben (1996 no data available)
The results of yield assessment of the cultivars Falcon and Zeus infected by TuYV carried out for three years showed significant reductions in seed yield. Whereas in 1993 both cvs. yielded 12% less seed, seed yields of Falcon and Zeus were reduced by 19% and 34%, respectively in 1994 when compared to virus-free plots (Figure 3). In the third year the yield was reduced by 18% and 22%,
Figure 3.Yield losses of oilseed rape induced by TuYV infections in plot trials, *α<0.05, **α<0.01
respectively, by the isolate BN 5 and by 22% and 28%, respectively, by the isolate LP 3/5. The average reduction in seed yield of the three years was 20,4% what is about equivalent of 8 dt per hectare. The determination of the seed quality indicated that TuYV infection did not cause a decrease in the oil and protein contents. On the other hand in 1994 seeds from TuYV infected plants of both cvs. and in 1995 of the cv. Zeus had increased contents of glucosinolate (Tillmann, personal communication).
Our studies to the occurrence of TuYV on winter oilseed rape revealed two trends:
1. From year to year great differences appear in the infection rates.
2. Great differences were detected between the growing regions.
Probably, the cause of the high degree of TuYV infection in oilseed rape in several years is its wide host range and the great number of aphid species which are able to transmit TuYV to oilseed rape plants. In current host range studies, one TuYV isolate from oilseed rape was able to infect 65 out of a total of 130 species, among them many common weed and several cultural crops (Graichen and Rabenstein, 1996, Graichen unpublished data). Under experimental conditions 17 of 24 tested aphid species are able to transmit the TuYV (Schliephake et al., 1999)
High infestation rates by TuYV in winter oilseed rape is closely related to the flight activity of aphid vectors and its propagation in the fields. Suction traps represent the distance flight of the aphids and give a good overview to the infection pressure for the primary infections. The dominant species trapped in the suction trap at Aschersleben are the cereal aphids and Brevicoryne brassicae. But M. persicae is also one of the 10 most common aphid species.
How important the cereal aphids species with their low transmission rates are for the primary infections of rape fields is not clear as yet.
Also very important for the virus infestation degree is the spread of the virus inside of the fields (secondary infections). This infections are made by the apterous individuals during the growth of the population. Mild and warm autumns and winters allowed for long time or completely the survival of the aphids in the field (anholocyclic overwintering). Observations in oilseed rape crops in 1995 and 1997 showed that in autumn single plants are settled up by 100 to 150 aphid individuals.
Until now therefore it is difficult to decide, what are the main factors for high virus incidence, the flight activity of the virus vectors, the climatic factors, that influence the population development of aphids and/or the presence of virus reservoirs.
Similar like the experiments by Smith and Hinkes (1985) and Jay and Smith (1995) our yield trials gave a clear reduction of the seed yield by virus infection.
On the basis of the high attack of oilseed rape fields in some growing regions of Germany and the distinct yield losses determined in field experiments it can be concluded that it is necessary to control the infestation of oilseed rape by TuYV.
In Great Britain applications of insecticide to control the virus vectors lead to unsatisfactory results (Hill et al., 1989, Walsh et al., 1989). In the studies of Read and Hewson (1988) were repeated treatments necessary for effective control of vectors and decreasing of virus infestation in winter oilseed rape crops. But repeated insecticide treatments of the fields to control the virus vectors are unacceptable for oilseed rape producers. For economical and ecological reasons chemical treatments should be not in the foreground. The best way to prevent yield losses on oilseed rape caused by virus infection is the use of new cultivars resistant to TuYV.
Only one resynthesized rape form of 650 summer and winter oilseed rape cultivars, actual breeding lines and resynthesized rape forms screened for resistance to TuYV showed any degree of resistance. Crosses with current cultivars and breeding lines to transfer this resistance resulted in progenies with different levels of TuYV resistance (Graichen and Peterka, 1995).
The successful transfer of TuYV resistance in susceptible oilseed rape cultivars and breeding lines showed that it will be possible to prevent virus induced yield losses by virus resistant cultivars in future.
The authors thank Dr. F. Rabenstein, Institute for Resistance Research and Pathogen Diagnostics Aschersleben for supplying polyclonal TuYV antiserum used in the experiments for virus detection. We thank M. Knauft, G. Brantin, U. Fuhrmann and S. Gropp for excellent technical assistance.
The works were supported by grants from the Ministry of Food, Agriculture and Forestry Germany.
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