The power of citizen science to contribute to both science and society is gaining increased recognition, particularly in physics and biology. Although there is a long history of public engagement in agriculture and food science, the term ‘citizen science’ has rarely been applied to these efforts. Similarly, in the emerging field of citizen science, most new citizen science projects do not focus on food or agriculture. Here, we convened thought leaders from a broad range of fields related to citizen science, agriculture, and food science to highlight key opportunities for bridging these overlapping yet disconnected communities/fields and identify ways to leverage their respective strengths. Specifically, we show that (i) citizen science projects are addressing many grand challenges facing our food systems, as outlined by the United States National Institute of Food and Agriculture, as well as broader Sustainable Development Goals set by the United Nations Development Programme, (ii) there exist emerging opportunities and unique challenges for citizen science in agriculture/food research, and (iii) the greatest opportunities for the development of citizen science projects in agriculture and food science will be gained by using the existing infrastructure and tools of Extension programmes and through the engagement of urban communities. Further, we argue there is no better time to foster greater collaboration between these fields given the trend of shrinking Extension programmes, the increasing need to apply innovative solutions to address rising demands on agricultural systems, and the exponential growth of the field of citizen science.

Citizen science, broadly defined as public participation in scientific research and knowledge production, is becoming an increasingly well developed and valued approach with global reach and used in a wide range of scientific domains [1][2][3] . Much of this growth is driven by the availability of information technology infrastructures such as mobile phones and low-cost sensors for gathering and reporting data, the internet for sharing data, and cloud storage for hosting and storing data 4,5 . Growing literacy levels and educational attainment in many parts of the world also make it possible for many more people to contribute to knowledge creation in a meaningful way 6,7 .Citizen science initiatives involve the public in the research process to generate genuine scientific outcomes [8][9][10][11] . These outcomes include discoveries, such as in astrophysics 12 and archaeology projects 13 ; new insights, such as in epidemiology 14 and socio-linguistics projects 15 ; evidence-based policymaking, such as in pollution-monitoring initiatives [16][17][18] ; interventions such as in public health research 19 ; and environmental governance, including in ecology and biodiversity monitoring initiatives [20][21][22] . Citizen science research can fill important data gaps across both time and space 23 , which might not otherwise be possible without the contribution of many participants, including people with local and lay knowledge 24,25 or Indigenous knowledge 26,27 .

Globalization has increased the frequency of inadvertent introductions of plant pathogens. Many catastrophic invasions of both natural and agricultural systems have been initiated through anthropogenic dissemination pathways. Phytophthora species are a group of invasive plant pathogens causing many of the most important plant disease epidemics. A review of Phytophthora species descriptions published following the publication of the first DNA-based Phytophthora phylogeny was conducted to highlight patterns of recent introductions and to provide insights for early pathogen detection initiatives. Seventy-two publications from 2001-2016 describing 98 Phytophthora species were evaluated. Of the 91 species with geographic location isolation data, 22% of species were described from type specimens isolated from urban environments, 33% from agricultural environments and 45% from natural environments. Within the urban environment, ornamental plant trading nurseries were the most important sources. Specifically, for Phytophthora ramorum, a species causing multiple epidemics globally, the largest proportion of first report publications were from urban environments, including nurseries. Nearly a quarter of the species descriptions were based on isolates from the urban environment, including ornamental nurseries, and the majority of the first reports for P. ramorum were also from this environment. We therefore suggest that detection programs for invasive plant pathogens within the urban environment would be valuable. In this regard, specialized monitoring and citizen science projects that target urban areas where live plant-trading industries are concentrated would be particularly effective to both promote early detection and to facilitate a rapid response to new species invasions.