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Wheat stripe mosaic virus (WhSMV) is the causal agent of soilborne wheat mosaic disease (SBWMD) in Brazil, which is a serious threat to wheat production in the southern part of the country. WhSMV also occurs in Paraguay and South Africa. The virus is soilborne, transmitted by Polymyxa graminis, and management strategies are mainly based on genetic resistance. Variation in the reaction (type and severity of symptoms) of cultivars has been reported depending on the region and/or year of cultivation, leading wheat breeding programmes to test their materials in several locations, which increases costs and is time‐consuming. One hypothesis is that this variation in symptoms is a result of the genetic variability of WhSMV. We assessed the genetic variability and population structure of WhSMV infecting wheat in Brazil and South Africa. In field experiments conducted in different locations in southern Brazil, a consistent reproducibility of symptoms was observed in a set of cultivars, and molecular analysis showed a low degree of genetic variability of the Brazilian viral population. The hypothesis that the variation in SBWMD symptoms in Brazil is due to genetic variability of the virus was thus rejected. Comparison of the Brazilian and South African WhSMV isolates indicated that they comprise distinct subpopulations and that the Brazilian subpopulation is more variable than the South African subpopulation. The lower genetic variability of the South African subpopulation suggests genetic stability or a recent emergence of WhSMV in that region. [ABSTRACT FROM AUTHOR]

Rice stripe necrosis virus (RSNV) is the causal agent of the disease ‘rice crinkling’ and is transmitted by the protozoan Polymyxa graminis. Although genetic resistance has been explored, no resistant commercial cultivars are currently available. Oryza glaberrima has been identified as a promising source of resistance. However, it remains unclear whether this resistance is effective against the virus, the vector, or both, as well as whether it can be transferred to Oryza sativa cultivars. Disease‐resistant genotypes are primarily selected through visual observations of symptom expression. The absence of a severity scale for RSNV makes this process difficult, and relying solely on visual assessments can introduce subjectivity. We developed a severity scale and a reverse transcripiton‐quantitative PCR (RT‐qPCR) assay for screening resistance levels in rice genotypes against RSNV and its vector and to analyse the genetic variability of RSNV isolates. To achieve absolute quantification, experiments were conducted using O. glaberrima and three O. sativa cultivars. Inoculation occurred naturally using soil from an area with a history of the disease. Visual symptoms were recorded and disease intensity was evaluated. Subsequently, total nucleic acid extraction was performed on the samples and viral and vector loads were quantified through RT‐qPCR and qPCR, respectively. To characterize the virus variability, symptomatic rice samples were collected in the 2021/2022 crop season. RT‐PCR was conducted to amplify the coat protein gene of RSNV, and molecular variability descriptors were analysed. [ABSTRACT FROM AUTHOR]