Control of rice blast drives yield benefits from variety mixture in rice: A meta-analysis

Context: Rice blast, caused by the fungus Magnaporthe oryzae, is the most important disease of rice globally. Variety mixture can greatly reduce the incidence of rice blast and increase yield. However, the effectiveness of variety mixture as a disease control method varies across studies, and no overarching synthesis of the available data has been made to date. Objective: Here, we synthesized the available experimental data on the effect of variety mixture on rice blast and yield by quantifying the disease reduction and yield gain in mixtures of glutinous and hybrid rice cultivars in relation to nitrogen (N) fertilization and the use of fungicides. Methods: We searched the global literature for field studies on disease control and yield benefits from variety mixture in rice and synthesized 1255 observations from 38 publications using meta-analysis and meta-regression. Results: The disease odds were reduced by 68 % on average in variety mixtures as compared to pure stands, and the reduction was substantially greater in blast-susceptible glutinous rice varieties (80 %) than in the more blast-resistant hybrid rice varieties (34 %). Variety mixture provided a similar level of disease control in glutinous rice as the use of fungicides. Blast-resistant hybrid rice varieties became susceptible to blast at high fertilizer N input but the disease controlling effect of mixture was unaffected by fertilizer N input. The relative yield total in mixture relative to pure stands increased with disease pressure; from 1.08 to 1.36 if the disease incidence in the pure stands increased from 0 % to 100 %. At the same time, the relative yield increased from 0.92 to 1.06 for hybrid rice (land share 0.84), and from 0.16 to 0.30 for glutinous rice (land share 0.16). Conclusions: Variety mixture significantly decreases blast incidence while increasing grain yield of rice as compared with pure stands. The level of blast control by mixture is not significantly impacted by the quantity of N fertilizer, the use of fungicides, or the phase of the epidemic. The control effectiveness of variety mixture on susceptible cultivars is similar to that of fungicides. The relative increase in yield due to variety mixture is greater at high than low disease incidence in the pure stands. Implications: Variety mixture offers a sustainable and environmental friendly solution for promoting the cultivation of indigenous rice varieties with high market and cultural value but low resistance to blast.