Sequential Post-Heading Applications for Controlling Wheat Blast: A 9-Year Summary of Fungicide Performance in Brazil.

Wheat blast, caused by Pyricularia oryzae Triticum lineage, is a major constraint to wheat production, mainly in the tropics of Brazil, where severe epidemics have been more frequent. We analyzed disease and wheat yield data from 42 uniform field trials conducted over 9 years (2012 to 2020) to assess whether the percent control and yield response were influenced by fungicide type, region (tropical or subtropical), and year. Six treatments were selected, all evaluated in at least 19 trials. Two fungicides were applied as solo active ingredients (MANCozeb, and TEBUconazole), and four were premixes (AZOXystrobin plus TEBU, TriFLoXystrobin plus PROThioconazole, TFLX plus TEBU, and PYRAclostrobin plus EPOXiconazole). Percent control, calculated from back-transforming estimates by a meta-analysis network model fitted to the log of the means, ranged from 43 to 58%, with all but PYRA plus EPOX showing efficacy >52% on average, not differing among them. The variation in both efficacy and yield response was explained by region, and all but TEBU performed better in the subtropics than in the tropics. Yield response from using three sequential sprays was approximately two times greater in the subtropics (319 to 532 kg/ha) than in the tropics (149 to 241.3 kg/ha). No significant decline in fungicide efficacy or yield response was observed in 9 years of study for any of the fungicides. These results reinforce the need to improve control by adopting an integrated management approach in the tropics given poorer performance and lower profitability, especially for the premixes, than in the subtropics.

Resistance to QoI Fungicides Is Widespread in Brazilian Populations of the Wheat Blast Pathogen Magnaporthe oryzae.

Wheat blast, caused by Magnaporthe oryzae, is an important disease across central and southern Brazil. Control has relied mainly on strobilurin fungicides (quinone-outside inhibitors [QoIs]). Here, we report the widespread distribution of QoI resistance in M. oryzae populations sampled from wheat fields and poaceous hosts across central and southern Brazil and the evolution of the cytochrome b (cyt b) gene. Sequence analysis of the cyt b gene distinguished nine haplotypes, with four haplotypes carrying the G143A mutation associated with QoI resistance and two haplotypes shared between isolates sampled from wheat and other poaceous hosts. The frequency of the G143A mutation in the wheat-infecting population increased from 36% in 2005 to 90% in 2012. The G143A mutation was found in many different nuclear genetic backgrounds of M. oryzae. Our findings indicate an urgent need to reexamine the use of strobilurins to manage fungal wheat diseases in Brazil.