Research: avenues being explored for better control of rapeseed diseases and pests

• The characterization of Leptopshaeria maculansThis study, presented by Marie-Hélène Balesdent (INRAE), was carried out to characterise 832 isolates of phoma between 2019 and 2020: most isolates were avirulent in the face of the disease. The latest study conducted in France, presented by Marie-Hélène Balesdent (INRAE), characterised 832 phoma isolates between 2019-2020: most isolates were avirulent to the resistance genes Rlm6, Rlm11 and RlmS, suggesting the effectiveness of these genes in controlling the disease.
• INRAE Bioger (Talbi et al.) has also undertaken work with other research teams to better understand the epistatic interactions between Avr genes in the fungus. Indeed, AvrLm4-7 suppresses the recognition of AvrLm3 and AvrLm5-9 by the R proteins Rlm3 and Rlm9. This analysis is a first step towards understanding broad-spectrum resistances that may allow the management of multi-pathogenic diseases.
• Several works have reported on methods to advance resistance in oilseed rape. The INRAE Bioger Paris-Saclay team (Rouxel et al.) established a method to express late effectors of the fungus in oilseed rape cotyledons and to search for quantitative sources of resistance at early stages. The results were promising with the identification of genotypes that appeared resistant. The identification of a late gene-for-gene interaction in oilseed rape stems opens up new avenues for building oilseed rape varieties with durable resistance. The INRAE IGPEPP team at Le Rheu (Chèvre et al.) has proposed strategies to facilitate the introgression of an Rlm 10 resistance gene into oilseed rape from mustard. Finally, a Polish team (Niemann et al.) has developed markers associated with resistance genes to assist in oilseed rape breeding.

• Knowledge of the pathogen responsible for the hernia, Plasmodiophora brassicae : in Germany (Namani-Noor et al.), pathotypes were compared with 86 plant species to identify host and non-host plants. The results suggest a higher host diversity than is known.
• Biocontrol to try to reduce the impact of clubroot, with the use of Pseudomonas protegens or Acremonium alternatum (Auer et al., Yamen et al.), whose results seem to depend on the pathotypes. Several teams (Wang et al., Schwelm et al.) have studied soil-hernia interactions. In particular, the importance of bacterial communities in promoting or not the stimulation of hernia spore germination has been suggested.
• Genetic resistance in oilseed rape: the identification of resistance factors in cabbage, specific or not to the isolates tested (INRAE IGEPP, Delourme et al.), can be introgressed into oilseed rape. These sources of resistance are essential to improve the resistance of oilseed rape with the evolution of the virulence of the pathotypes. For Arabidopsis thaliana, the work of INRAE (petitpas et al.) underlines that the epigenetic architecture of resistance can be modulated by environmental conditions. Finally, GEVES presented the progress of the Opitplasm project, which aims to improve the registration tests of rapeseed varieties in France for the hernia resistance criterion.

Morphological and pathogenicity characteristics of 57 sclerotinia isolates collected in 2020-2021 in Germany (Zamani-Noor) were evaluated. Variability was observed for all criteria, including aggressiveness. These findings have already been observed in other countries. The German team (brand et al.) tried to update the environmental data affecting the SkleroPro model, which helps to predict the optimal stage for applying fungicides in Germany. But this weather-based model does not take into account certain factors, including the amount of initial inoculum.

The team of Laurent Ruck, head of insecticides and biocontrol at Terres Inovia, presented the current status of pyrethroid resistance in rapeseed pests in France, in particular winter flea beetle and terminal bud weevil.

To limit the unintended effects of spraying, another team (Hausmann et al.) tested the interest of spraying into the canopy rather than over it with a sprayer equipped with hangers. This technique reduces residues in nectar and pollen as a result of flowering treatments. Parasitism rates of the main parasitoid of the meligethes are also higher with this technique.

The value of interfering RNA (iRNA) technology is discussed by other researchers (Willow and Veromann). These RNAi are able to bind to and degrade the messenger RNA (mRNA) of a key gene for a given pest species. Thus, the translation of the mRNA into protein no longer takes place and leads to the death of the insect. The advantage of this technology is that it can be very specific and avoid unintended effects on other species. The authors presented the complexity, the unknowns and the obstacles to progress in this sector, such as the lack of knowledge on pest physiology, or the necessary steps in the fields of bioinformatics, laboratory studies, semi-controlled conditions or field studies.

Entomopathogenic fungi, bacteria and fatty acids against winter flea beetles have also been tested in the laboratory (Horau et al.), by direct contact or ingestion. Entomopathogenic fungi and fatty acids showed promising results.

The number of presentations on this topic is gradually increasing from one IOBC-ICOC congress to the next. With 5 presentations and many posters, this session has become one of the most important.

Arnaud Van Boxsom, Head of Variety Evaluation at Terres Inovia, took advantage of this conference to present the methods and results of recent years on the evaluation of varieties for their vigour and tolerance to autumn insects.

In order to develop new management strategies for meligesta, another team (Austel and colleagues) studied the natural chemical variation of different brassica species and identified plant compounds that reduce feeding damage by meligesta. They first identified candidate glucosinolates, flavonoids and saponins and tested them individually. Saponins and glucobarbarin (glucosinolate) found only in B. vulgaris have deterred the beetles from feeding. Thus, honey beetles are well adapted to the defence compounds of a wide range of crucifer species, but not to the specific compounds of B. vulgaris.

INRAE IGEPP and Innolea (Bellec et al.) are also working on meligethes and in particular on contact metabolites that have an impact on feeding behaviour, whether they are stimulants or deterrents. Previous work has shown that white mustard (Sinapis alba) is less well accepted than rapeseed (Brassica napus) and that there is variation between mustard populations. The results obtained in this study confirm that feeding behaviour is partly related to the effect of several compounds present in the perianth having an additive or synergistic effect.

Finally, an Indian team (Panwar et al.) is working on the development of varieties resistant to the turnip aphid, an important pest in India. By introgressing genes from Brassica fruticulosa à B. juncea, they obtained promising genotypes for reducing nymph survival and aphid adult fecundity and longevity.

A better understanding of the biology and ecology of pests is a key step in devising new management strategies.

Thus, forecasting crop colonisation and spatial distribution of pests is essential for effective management. Tixeront and his team worked on the winter flea beetle. The results showed that more individuals entered the crop than left it, and that the beetles moved rapidly towards the centre of the field. The captures were also found to be higher during the day than at night. The sex ratio of the captured individuals was both balanced and stable over the study period and sexual maturity occurred during the experiment. Finally, integration of the sampling data with local meteorological data showed that catches are dependent on temperature, humidity and wind speed.

Lundin and his team presented work on modelling the population dynamics of the oilseed rape flea beetle (Psylliodes chrysocephalus). They have shown that there are regular cycles in the abundance of the species in Sweden over several years. The causes of the fluctuations are not yet known. However, these observations pave the way for a possible modulation of the control according to the expected population levels, if they could be predicted with sufficient accuracy.

In the UK, as in France, the reduction of management options (withdrawal of insecticides and the emergence of pyrethroid-resistant populations) has resulted in significant production losses. Work is therefore underway to find new management strategies. Ortega-Ramos and his colleagues have sought to better understand the ecology of the winter flea beetle and the role of parasitoid micro-hymenoptera. Sowing date, field size and the interaction between temperature and rainfall in September, October and November and the average temperature in December and January had a significant effect on the number of flea beetle larvae. In addition, by using molecular biology methods ("barecoding"), this team was able to study the spatio-temporal distribution and biocontrol potential of Microctonus brassicae, the main parasitoid attacking flea beetle adults. M. brassicae is widespread throughout the UK with parasitism rates ranging from 0 to 36%.

Céline Robert, in charge of crop pests and auxiliary fauna studies at Terres Inovia, presented these decision rules and the associated tools for estimating the risk related to winter flea beetle larvae and terminal bud weevil. In addition to an indicator of pest pressure, the risk assessment integrates different criteria to evaluate the growth dynamics of the rapeseed in autumn and spring.

At the opening of this session, the R2D2 (Ecophyto EXPE) project, led by Nicolas Cerrutti of Terres Inovia, was presented. The principle: ten farmers from the same territory are supported to design and implement a step-by-step territory project mobilising all the levers of integrated protection, in particular those favourable to auxiliary fauna. A focus was made on one of the strategies implemented in the autumn of 2021, which consists of planting attractive crucifers in intercropping plots to divert winter flea beetles from rape plots.

The Hiltpold team presented results on the impact of faba beans combined with oilseed rape on spring pests. In these trials, the beans did not freeze during the winter. In addition to an effect on flea beetle larvae, it showed an impact on oilseed rape stem weevil egg laying and associated damage as well as a reduction in the populations of honey beetles. The impact on yield was also positive.

The installation of flower strips is also an important lever for encouraging auxiliary fauna. Gardarin and his team (INRAE Grignon) have shown a significant increase in the rate of parasitism up to 20 metres away on cabbage stem weevil, flea beetle and bean leaf beetle. The researchers found an increasing relationship between the proportion of nectar-accessible species and the rate of parasitism in the fields. The morphology of the flowers of the nectariferous species thus plays a role in the accessibility of the parasitoid species to nectar. It is therefore important to design mixtures of species whose flowers will allow physical access of parasitoid insects to the trophic resource. This team (Pigot et al) also questioned the role of these strips as an estivation site for flea beetles. Even if estivating flea beetles are observed in flowered strips or in former rape fields, they are more numerous in wooded areas.

Finally, Sulg and his team showed that the abundance of honey beetles and their natural enemies was lower more than 500 metres from a former oilseed rape field. Heimbach and his team demonstrated a lower level of attack of several oilseed rape pests, especially flea beetle larvae, in areas without oilseed rape in previous years. Thus, the choice of location of rapeseed plots can be a lever of choice for the implementation of territorial strategies.

In ecology, cameras are mainly used to study large animals such as mammals, but recently more attention has been paid to invertebrates. These cameras allow, for example, the identification of predators of pests with less bias than conventional methods. Seimandi-Corda presented a study where these cameras are used to monitor the predation of larvae of meligethes and silique midges placed on the ground. Multiple predation events were recorded, showing that this method can produce useful and reliable results. This technique opens new perspectives as it was possible to estimate the time of day when predators were most active and to identify the time window when larvae are most likely to be predated.

Bick and his team proposed a study in which near-infrared insect sensors, in addition to more conventional traps (dips, sticky traps), were used to detect flights and dispersal of honey beetles in oilseed rape. This work provides evidence of pollinator beetle aggregation in an oilseed rape field, in the middle or at the edge of the field, before the oilseed rape flowers. This work also confirms that the insects head into the wind. The sampling strategy must therefore be considered in relation to these two elements. Finally, the sensors implemented allowed for early detection and could be used to set economic thresholds for a new management strategy.