Differential Spatial Gradients of Wheat Streak Mosaic Virus into Winter Wheat from a Central Mite-Virus Source

Pest categorisation of High Plains wheat mosaic virus

The EFSA Panel on Plant Health conducted a pest categorisation of High Plains wheat mosaic virus (HPWMoV) for the EU territory. The identity of HPWMoV, a member of the genus Emaravirus (family Fimoviridae), is well established and reliable identification methods are available. The pathogen is not included in the EU Commission Implementing Regulation 2019/2072. HPWMoV has been reported from Argentina, Australia, Canada, Ukraine and USA, and it is not known to be present in the EU. HPWMoV infects plant species of the family Poaceae (i.e. wheat, maize and several other cultivated or wild Poaceae species). It is the causal agent of High Plains disease of wheat and maize, inducing symptoms ranging from mild to severe mosaic, chlorosis and necrosis in wheat, and chlorotic streaks in maize plants. The virus is transmitted by the wheat curl mite Aceria tosichella, which is present in the EU. HPWMoV transmission via seeds was reported to occur in sweet corn. Sweet corn seeds for sowing were identified as the most relevant pathway for entry of HPWMoV into the EU. Seeds from other hosts and viruliferous wheat curl mites were identified as entry pathways associated with uncertainties. Machinery not appropriately cleaned may move infected seeds and/or parts of cereals infested by viruliferous mites. Cultivated and wild hosts of HPWMoV are distributed across the EU. Would the pest enter and establish in the EU territory, economic impact on the production of cultivated hosts is expected. Phytosanitary measures are available to prevent entry and spread of the virus in the EU. HPWMoV fulfils the criteria that are within the remit of EFSA to assess for it to be regarded as a potential Union quarantine pest.

Resistance to the wheat curl mite and mite-transmitted viruses: challenges and future directions

Wheat curl mite (WCM) is the only known arthropod vector of four wheat viruses, the most important of which is Wheat streak mosaic virus (WSMV). Host resistance to WCM and WSMV is limited to a small number of loci, most of which are introgressed from wild relatives and are often associated with linkage drag and temperature sensitivity. Reports of virulent WCM populations and potential resistance-breaking WSMV isolates highlight the need for more diverse sources of resistance. Genome sequencing will be critical to fully characterize the genetic diversity in WCM and WSMV populations to better understand the incidence of WCM-transmitted viruses and to evaluate the potential stability of resistance genes. Characterizing host resistance genes will help build a mechanistic understanding of wheat-WCM-WSMV interactions and inform strategies to identify and engineer more durable resistance sources.

The Epidemiology of Plant Virus Disease: Towards a New Synthesis

Epidemiology is the science of how disease develops in populations, with applications in human, animal and plant diseases. For plant diseases, epidemiology has developed as a quantitative science with the aims of describing, understanding and predicting epidemics, and intervening to mitigate their consequences in plant populations. Although the central focus of epidemiology is at the population level, it is often necessary to recognise the system hierarchies present by scaling down to the individual plant/cellular level and scaling up to the community/landscape level. This is particularly important for diseases caused by plant viruses, which in most cases are transmitted by arthropod vectors. This leads to range of virus-plant, virus-vector and vector-plant interactions giving a distinctive character to plant virus epidemiology (whilst recognising that some fungal, oomycete and bacterial pathogens are also vector-borne). These interactions have epidemiological, ecological and evolutionary consequences with implications for agronomic practices, pest and disease management, host resistance deployment, and the health of wild plant communities. Over the last two decades, there have been attempts to bring together these differing standpoints into a new synthesis, although this is more apparent for evolutionary and ecological approaches, perhaps reflecting the greater emphasis on shorter often annual time scales in epidemiological studies. It is argued here that incorporating an epidemiological perspective, specifically quantitative, into this developing synthesis will lead to new directions in plant virus research and disease management. This synthesis can serve to further consolidate and transform epidemiology as a key element in plant virus research.