Despite its focus on other, higher-priority criteria, the soybean breeding programme in Switzerland has yielded varieties that are tolerant of soybean mosaic virus (SMV) – the most serious of the viral diseases.
Soybean production in Europe – where, among others, seeds of Swiss origin are used – is booming: the surface area devoted to this crop has increased by 10% in recent years, reaching 975,000 hectares in 2018. In Switzerland, it is chiefly the percentage of organic soybean grown that is rising steadily.
Within the overall European context of soybean production, the threat of increased transmission of soybean mosaic virus by seed will continue to exist as long as the seed certification process does not mandate checks for the virus.
SMV, a quick-propagating virus
Soybean mosaic virus (SMV) is the most common as well as the most serious soybean viral disease. SMV can cause highly variable yield losses (up to 80% in certain varieties). The spread of the disease throughout all production zones worldwide is favoured by the exchange of seed. At plot level, SMV transmission from one plant to another can be facilitated by over 30 species of aphid, whilst the speed with which they acquire and transmit the disease makes the application of insecticide ineffective. Moreover, the primary infection by the sting of the vector produces few symptoms and often passes unnoticed, which renders the roguing of the diseased plants in the field less than effective.
Means of prophylactic control
The means of control are therefore strictly prophylactic, based on the use of healthy seeds and reliance on varieties with little or no susceptibility to viral infection.
And what about the Swiss varieties, bred by Agroscope? The soybean breeding programme in Switzerland focuses on agronomic and qualitative criteria: precocity, cold tolerance, adaptation to the climatic conditions specific to Switzerland, and lastly, taste and suitability for processing in the case of soya intended for human consumption. As is often the case in neighbouring countries, the highly complex process of breeding for resistance to SMV is not a matter of priority.
Given the current context, the behaviour of the Swiss varieties and the prevalence of SMV in Switzerland need to be better characterised. Agroscope researchers studied the prevalence of the virus and its impact over the course of 2015 and 2016 on different varieties bred on the Changins site – the varieties Tourmaline, Gallec and Opaline, which in 2015 accounted for the bulk of the Swiss market.
Strong presence of SMV at Changins site
It turns out that the varieties developed by Agroscope are bred under constant viral constraint, i.e. in an environment where the virus has been present on a massive scale for many years (in weeds and in the crop itself). This breeding under constraint allows early elimination of the symptomatic lines that accumulate the virus within their seed. Only those genotypes little marked by the presence of the virus – the tolerant genotypes – are retained from generation to generation. This strategy produces varieties whose yields are not affected by the virus-pressure conditions observed in Switzerland. Nevertheless, these symptomless varieties are not resistant, since they can have high SMV infection rates.
- In the soybean-growing zones in Switzerland, the infection rate in the field depends mainly on environmental factors.
- Although the sanitary quality of the seed should not be ignored, it seems to play a marginal role in the spread of SMV.
- The varieties bred by Agroscope are tolerant of SMV, and well adapted to the virus-pressure conditions of the region. Even a high level of infection does not lead to a drop in yield in these varieties, which do not exhibit any typical symptoms of infection.
- This selection under virus pressure allows us to focus on researching agronomic parameters of interest for the local market, such as precocity or seed quality, without specifically targeting resistance. It dispenses with genetic-marker-assisted selection, which is particularly complex in the case of resistance to SMV.