Although healthy plants are vital to human and animal health, plant health is often overlooked in the One Health literature. Plants provide over 80% of the food consumed by humans and are the primary source of nutrition for livestock. However, plant diseases and pests often threaten the availability and safety of plants for human and animal consumption. Global yield losses of important staple crops can range up to 30% and hundreds of billions of dollars in lost food production. To demonstrate the complex interrelationships between plants and public health, we present four case studies on plant health issues directly tied to food safety and/or security, and how a One Health approach influences the perception and mitigation of these issues. Plant pathogens affect food availability and consequently food security through reductions in yield and plant mortality as shown through the first case study of banana Xanthomonas wilt in East and Central Africa. Case studies 2, 3 and 4 highlight ways in which the safety of plant-based foods can also be compromised. Case study 2 describes the role of mycotoxin-producing plant-colonizing fungi in human and animal disease and examines lessons learned from outbreaks of aflatoxicosis in Kenya. Plants may also serve as vectors of human pathogens as seen in case study 3, with an example of Escherichia coli (E. coli) contamination of lettuce in North America. Finally, case study 4 focuses on the use of pesticides in Suriname, a complex issue intimately tied to food security though protection of crops from diseases and pests, while also a food safety issue through misuse. These cases from around the world in low to high income countries point to the need for interdisciplinary teams to solve complex plant health problems. Through these case studies, we examine challenges and opportunities moving forward for mitigating negative public health consequences and ensuring health equity. Advances in surveillance technology and functional and streamlined workflow, from data collection, analyses, risk assessment, reporting, and information sharing are needed to improve the response to emergence and spread of plant-related pathogens and pests. Our case studies point to the importance of collaboration in responses to plant health issues that may become public health emergencies and the value of the One Health approach in ensuring food safety and food security for the global population.

Emerging, re-emerging and endemic plant pathogens continue to challege our ability to safeguard plant health worldwide. Further, globalization, climate change, increased human mobility, and pathogen and vector evolution have combined to increase the spread of invasive plant pathogens. Early and accurate diagnoses and pathogen surveillance on local, regional, and global scales are necessary to predict outbreaks and allow time for development and application of mitigation strategies. Plant disease diagnostic networks have developed worldwide to address the problems of efficient and effective disease diagnosis and pathogen detection, engendering cooperation of institutions and experts within countries and across national borders. Networking maximizes impact in the face of shrinking government investments in agriculture and diminishing human resource capacity in diagnostics and applied pathology. New technologies promise to improve the speed and accuracy of disease diagnostics and pathogen detection. Widespread adoption of standard operating procedures and diagnostic laboratory accreditation serve to build trust and confidence among institutions. Case studies of national, regional, and international diagnostic networks are presented.

Bactericides, fungicides, and other pesticides play an important role in the management of plant diseases. However, their use can result in residues on plants and in the environment, with potentially detrimental consequences. The use of streptomycin, oxytetracycline, copper-based products, and some fungicides is correlated with increased resistance among plant pathogens to these agents. Likewise, the recent rise in the incidence of environmental triazole fungicide-resistant Aspergillus fumigatus, the cause of aspergillosis in humans, has caused concern, particularly in Europe. Through horizontal gene transfer, genes can be exchanged among a variety of bacteria in the plant production environment, including phytopathogens, soil bacteria, and zoonotic bacteria that are occasionally present in that environment and in the food chain. Through mechanisms of horizontal gene transfer, co-resistance, cross-resistance, and gene up-regulation, resistance to one compound may confer resistance and multi-drug resistance to other similar, or even very dissimilar, compounds. Given the global rise in antimicrobial-resistant (AMR) organisms, and their effects on plant, animal, and human health, the prudent use of pesticides is required to maintain their effectiveness for food security and sustainable production, and to minimize the emergence and transmission of AMR organisms from horticultural sources.

Field trials were conducted over 2 years to assess the effects of compost amendments on disease development in organic and conventional processing tomato (Lycopersicon esculentum L.) production systems. The incidence of anthracnose fruit rot was reduced in organic tomato plots amended with a high rate of composted cannery wastes compared with the incidence in nonamended control plots in 1998 when disease incidence was high. Marketable yield was increased by 33% in compost-amended organic plots. Plots amended with a high compost rate had more ripe fruit than the nonamended control. The incidence of anthracnose and of total disease on fruit was less on the cultivar OH 8245 than on Peto 696. Total fruit yield of OH 8245 but not Peto 696 in organic plots was increased by amendment with composted cannery wastes. In conventional tomato production, composted yard wastes increased disease severity on foliage both years but reduced bacterial spot incidence on fruit in 1997, when disease pressure was high. The incidence of anthracnose was not affected by composted yard wastes. Marketable and total fruit yields of Peto 696 were not increased in compost-amended conventional plots. The plant activator Actigard reduced foliar disease severity and the incidence of bacterial spot and anthracnose on fruit, while increasing yield of marketable fruit.

Randomly amplified polymorphic DNA (RAPD) analysis and the PCR assay were used in combination with dilution plating on a semiselective medium to detect and enumerate propagules of Trichoderma hamatum 382, a biocontrol agent utilized in compost-amended mixes. Distinct and reproducible fingerprints were obtained upon amplification of purified genomic DNA of T. hamatum 382 with the random primers OPE-16, OPH-19, and OPH-20. Three amplified DNA fragments of 0.35 (OPE-16 _0.35 ), 0.6 (OPH-19 _0.6 ), and 0.65 (OPH-20 _0.65 ) kb were diagnostic for T. hamatum 382, clearly distinguishing it from 53 isolates of four other Trichoderma spp. tested. Some isolates of T. hamatum shared these low-molecular-weight fragments with T. hamatum 382. However, RAPD analysis of isolates of T. hamatum with all three random primers used in consecutive PCR tests distinguished T. hamatum 382 from other isolates of T. hamatum . These three RAPD amplicons were cloned and sequenced, and pairs of oligonucleotide primers for each cloned fragment were designed. Use of the primers in the PCR assay resulted in the amplification of DNA fragments of the same size as the cloned RAPD fragments from genomic DNA of T. hamatum 382. A combination of dilution plating on a semiselective medium for Trichoderma spp. and PCR, with the RAPD primers OPH-19, OPE-16, and OPH-20 or the three sequence-characterized primers, was used successfully to verify the presence of T. hamatum 382 propagules in nine different soil, compost, and potting mix samples. All 23 Trichoderma isolates recovered on semiselective medium from commercial potting mixes fortified with T. hamatum 382 were identified as T. hamatum 382, whereas 274 Trichoderma isolates recovered from the other nine samples were negative in the PCR assay. Thus, this highly specific combination of techniques allowed detection and enumeration of propagules of T. hamatum 382 in fortified compost-amended potting mixes. Sequence-characterized amplified region markers also facilitated the development of a very simple procedure to amplify DNA of T. hamatum 382 directly from fortified compost-amended potting mixes.

Epidemics of aster yellows in lettuce in Ohio are caused by at least seven distinct phytoplasma strains in the aster yellows (AY) group. Five of the strains are newly reported: AY-BW, AY-WB, AY-BD3, AY-SS, and AY-SG. All seven strains were characterized based on symptoms in aster and lettuce, and by polymerase chain reaction (PCR). Strain AY-BD2 (formerly ‘Bolt’) causes yellowing and leaf distortion in lettuce and bolting in aster, whereas strain AY-S (formerly ‘Severe’) causes stunting, leaf clustering, and phyllody. Strain AY-WB causes yellowing and wilting in lettuce and witches'-broom in aster. Strain AY-SG induces horizontal growth in lettuce and aster plants. Strain AY-BW causes chlorosis of emerging leaves and abnormally upright growth of leaf petioles. AY-SS causes symptoms similar to those caused by AY-S but has a different PCR-restriction fragment length polymorphism (RFLP) banding pattern. Strains AY-BD2 and AY-BD-3 cause mild leaf and stem distortion in lettuce but are differentiated by PCR-RFLP. All phytoplasma strains collected from lettuce in Ohio belong to the 16SrI group. AY-WB belongs to the 16SrI-A subgroup and the other six belong to the 16SrI-B subgroup. Five of the seven strains were distinguished from each other by primer typing. The results of phylogenetic analyses of sequences of the 16S rRNA genes were basically consistent with the classification based on PCR-RFLP, in which AY-WB clustered with phytoplasmas of the 16rIA subgroup and the other Ohio lettuce strains clustered with phytoplasmas in the 16SrI-B subgroup.