The use of cultivar mixtures to control foliar fungal diseases is well documented for windborne diseases, but remains controversial for splash-dispersed diseases. To try to improve this strategy, a cultivar mixture was designed consisting of two wheat cultivars with contrasted resistance to Mycosphaerella graminicola, responsible for the rainborne disease septoria tritici blotch (STB), in a 1:3 susceptible:resistant ratio rather than the 1:1 ratio commonly used in previous studies. The impact of natural STB epidemics in this cultivar mixture was studied in field experiments over 4 years. Weekly assessments of the number of sporulating lesions, pycnidial leaf area and green leaf area were carried out on the susceptible cultivar. In years with sufficient STB pressure, disease impacts on the susceptible cultivar in the mixture were always significantly lower than in the pure stand (e.g. 42% reduction of pycnidial leaf area for the three upper leaves in 2008 and 41% in 2009). In years with low STB pressure (2010 and 2011), a reduction of disease impacts was also shown but was not always significant. After major rainfall events, the number of sporulating lesions observed on the susceptible cultivar after one latent period was reduced on average by 45% in the mixture compared to the pure stand. All the measurements showed that a susceptible cultivar was consistently protected, at least moderately, in a mixture under low to moderate STB pressure. Therefore, the results prove that the design of an efficient cultivar mixture can include the control of STB, among other foliar diseases.

The efficiency of biological control agents is often affected by local conditions such as low relative humidity (r.h.). This is especially true for mites of the family Phytoseiidae, and particularly for their egg stage. A key issue in biological control is thus to assess the drought tolerance of phytoseiid species that are (or that will be) commercialized. In this study, we evaluated the egg hatching response to a humidity range for six species of Phytoseiidae, by assessing the relative air humidity and vapour pressure deficit (VPD) at which 50% of the eggs hatch: r.h.50 and VPD50. Typhlodromus athiasae Porath & Swirskii and Phytoseiulus longipes Evans (Argentinean strain) were the most drought‐tolerant, with a r.h.50 of 43% (VPD50 = 18.2 hPa) and 43% (18.1 hPa), respectively. The most drought‐sensitive species were Amblyseius swirskii (Athias‐Henriot) (63%, 11.4 hPa), Phytoseiulus macropilis (Banks) (69%, 9.8 hPa), and Phytoseiulus persimilis Athias‐Henriot (70%, 9.4 hPa). Neoseiulus idaeus Denmark & Muma and P. longipes (Chilean strain) showed intermediate responses with 49% (16.1 hPa) and 50% (15.9 hPa), respectively. These results were similar to what had been obtained for the same species by other authors. However, unexpected inter‐ and intraspecific differences uncorrelated to climatic conditions of the area of origin were observed. The possibility to localize species and/or populations that could be efficient as biological control agents under dry conditions is discussed.

The results show agreement with previous data obtained using experimental approaches. They demonstrate that in order to maximize the potential mixture efficiency against a splash-dispersed pathogen, optimal susceptible/resistant cultivar proportions (ranging from 1/9 to 5/5) have to be established based on host resistance levels. The results also show that taking into account dispersal processes in explicit 3-D plant canopies can be a key tool for investigating disease progression in heterogeneous canopies such as cultivar mixtures.

These results suggest that mixture effects on spore dispersal could be modulated by an adequate choice of architectural characteristics of cultivars. In particular, even small differences in plant height could reduce spore dispersal.