Management of septoria tritici blotch using cultivar mixtures

Septoria tritici blotch (STB) is among the most devastating diseases in European wheat production. In recent years, there has been increased interest in using cultivar mixtures as part of an integrated control strategy against diseases. This study investigated different cultivar mixtures for their ability to control STB across three years and at seven trial sites in Denmark with a range of fungicide strategies, yielding a total of 194 individual cultivar mixture combinations. The mixtures were composed of two, three or four cultivars which were either similar or contrasting in their susceptibility to STB. Across all trials, the cultivar mixtures reduced disease severity significantly, by 14% compared to the component cultivars grown in monoculture. The reductions were larger when the disease pressure was high and when the mixtures included more cultivars. Mixtures composed of four cultivars reduced disease severity significantly, by 24%. Across all trials, cultivar mixtures significantly increased yield by 2% compared to the component cultivars grown in monoculture. The yield increase was significant for plots treated with one or two fungicide applications, and cultivar mixtures increased yield significantly, by 4.4% in untreated plots. The yield increase was smaller for mixtures with a high proportion of resistant cultivars. Based on the results from this study, cultivar mixtures can contribute positively to an IPM strategy, by reducing disease severity for STB and increasing yield. The most pronounced benefits from cultivar mixtures were found in fields with moderate to low fungicide input, under conditions with high disease pressure, when combining four cultivars with varying susceptibilities.

Functional–Structural Plant Modeling Highlights How Diversity in Leaf Dimensions and Tillering Capability Could Promote the Efficiency of Wheat Cultivar Mixtures

Increasing the cultivated diversity has been identified as a major leverage for the agroecological transition as it can help improve the resilience of low input cropping systems. For wheat, which is the most cultivated crop worldwide in terms of harvested area, the use of cultivar mixtures is spreading in several countries, but studies have seldom focused on establishing mixing rules based on plant architecture. Yet, the aerial architecture of plants and the overall canopy structure are critical for field performance as they greatly influence light interception, plant interactions and yield. The very high number of trait combinations in wheat mixtures makes it difficult to conduct experimentations on this issue, which is why a modeling approach appears to be an appropriate solution. In this study, we used WALTer, a functional structural plant model (FSPM), to simulate wheat cultivar mixtures and try to better understand how differences between cultivars in key traits of the aerial architecture influence mixture performance. We simulated balanced binary mixtures of cultivars differing for different critical plant traits: final height, leaf dimensions, leaf insertion angle and tillering capability. Our study highlights the impact of the leaf dimensions and the tillering capability on the performance of the simulated mixtures, which suggests that traits impacting the plants' leaf area index (LAI) have more influence on the performance of the stand than traits impacting the arrangement of the leaves. Our results show that the performance of mixtures is very variable depending on the values of the explored architectural traits. In particular, the best performances were achieved by mixing cultivars with different leaf dimensions and different tillering capability, which is in agreement with numerous studies linking the diversity of functional traits in plant communities to their productivity. However, some of the worst performances were also achieved by mixing varieties differing in their aerial architecture, which suggests that diversity is not a sufficient criterion to design efficient mixtures. Overall, these results highlight the importance of simulation-based explorations for establishing assembly rules to design efficient mixtures.

Effects of Wheat Cultivar Mixtures on Population Genetic Structure of Puccinia striiformis f. sp. tritici

Wheat stripe rust, caused by Puccinia striiformis f. sp. tritici (Pst), is one of the most destructive wheat diseases worldwide. Understanding the effects of cultivar mixture on disease control and population genetic structure of Pst could promote the effective application of cultivar mixtures. This study designed nine cultivar mixtures that composed of different component cultivars with field experiments in Zitong county, Sichuan province in two seasons from 2016 to 2018. Diseased leaves were collected in each plot of treatment in the spring, and a total of 835 and 568 isolates were obtained and genotyped with ten pairs of SSR markers in the two years, respectively. The observed and theoretical values of the area under the disease progress curve (AUDPC), wheat yield, Pst genotypic diversity and the frequency of Pst dominant genetic groups were compared among cultivar mixtures. With the two years’ experiments, there were three and two cultivar mixtures showing significant effect on the relative reduction of AUDPC, respectively; there were no significant effects on the relative yield increase in all mixtures in the two years. There were four and five cultivar mixtures showing positive effects on the relative increase of Pst genotypic diversity in the respective two years. Moreover, all cultivar mixtures could decrease the frequencies of Pst dominant genetic groups on highly susceptible component cultivar compared with their theoretical frequencies.

Annual dynamics of Zymoseptoria tritici populations in wheat cultivar mixtures: a compromise between the efficiency and durability of a recently broken-down resistance gene?

Cultivar mixtures slow polycyclic epidemics but may also modify the evolution of pathogen populations by diversifying the selection pressures exerted by their plant hosts at field scale. We compared the dynamics of natural populations of the fungal pathogen Zymoseptoria tritici in pure stands and in three binary mixtures of wheat cultivars (one susceptible cultivar and one cultivar carrying the recently broken-down Stb16q gene) over two annual field epidemics. We combined analyses of population ‘size’ based on disease severity, and of population ‘composition’ based on assessments of changes in the frequency of virulence against Stb16q in seedling assays with more than 3000 strains. In the field, disease levels were lower in mixtures, with each cultivar providing the other with reciprocal protection. The three cultivar proportions in the mixtures (0.25, 0.5 and 0.75) modulated the decrease in (i) the size of the pathogen population relative to the two pure stands, (ii) the size of the virulent subpopulation, and (iii) the frequency of virulence relative to the pure stand of the cultivar carrying Stb16q. Our findings suggest that optimal proportions may differ slightly between the three indicators considered. We identified potential trade-offs that should be taken into account when deploying a resistance gene in cultivar mixtures: between the dual objectives ‘efficacy’ and ‘durability’, and between the ‘size’ and ‘frequency’ of the virulent subpopulation. Based on current knowledge, it remains unclear whether virulent subpopulation size or frequency has the largest influence on interepidemic virulence transmission.

Crop Diversification to Control Powdery Mildew in Pea

Pea is a temperate grain legume cultivated worldwide that can be severely constrained by powdery mildew infection. Control by fungicides and the use of resistant cultivars is possible, but there is a growing interest in alternative control methods such as crop diversification, particularly in low input agriculture. The aim of this work was to assess the potential of intercropping pea with other crops and of pea cultivar mixtures for powdery mildew management on pea crop. Results show a reduction of powdery mildew on pea when intercropped by replacement at a 50:50 ratio with barley or with faba bean, but not when intercropped with wheat. A barrier effect seems to explain a major part of this decrease, especially in the pea/barley intercrop. This hypothesis was further supported by inoculated seedlings under controlled conditions, where powdery mildew infection on pea decreased with the distance to the inoculation point, this decrease being larger in the intercrop with barley than in the intercrop with wheat and in the pea monocrop. Powdery mildew was also reduced in the mixture of resistant and susceptible cultivars, with infection decreasing with the increasing proportions of the resistant one. Overall, this work shows that crop diversification may be a good strategy to reduce powdery mildew in pea.