1. Introduction
Plant pathogen detection recognizes the presence of plant pathogens in a specific location or area. The process involves observing visible disease symptoms in plants, collecting plant samples for further laboratory analysis, or using remote sensing techniques to detect the presence of pathogens
[1][2]. Plant pathogen detection aims to identify the presence of plant pathogens as early as possible so control measures can be implemented to alleviate their impacts on crop production
[3]. On the other hand, plant pathogen diagnosis refers to identifying the specific cause of a plant disease. It involves the identification of specific disease-causing pathogens by biochemical, molecular, and other techniques
[4]. Plant pathogen diagnosis aims to find the specific pathogen responsible for the specific disease so proper control measures can be implemented to limit the further spread of the pathogen and disease. Plant pathogen detection and diagnosis are critical to understanding and managing plant diseases. They are associated with applying multiple techniques and approaches to identify and understand the presence and cause of plant diseases
[5][6].
Plant diseases are a significant constraint to crop production worldwide and exert particularly severe impacts in developing countries, where agricultural systems may be less resilient than in developed ones
[7][8][9][10]. Although bacteria evolved billions of years ago
[11], they had not been demonstrated to cause plant diseases until the late 19th century
[12]. Bacterial plant diseases can reduce crop yields and debase the quality of harvested crops, thus leading to significant quality and economic losses for farmers and agricultural industries
[13][14][15]. Studying bacterial plant pathogens helps identify the ways to detect, diagnose, prevent, and control those destructive plant diseases, such as using resistant crop varieties, applying chemicals or biological control agents, and implementing good agricultural practices
[9][16]. By further understanding the biology and epidemiology of bacterial plant diseases, researchers can upgrade their strategies to reduce the impacts of these diseases on crop production and improve global food security. Alongside developing control measures, it is also essential to study bacterial plant diseases to understand the factors contributing to their emergence and spread
[17]. These efforts can involve the identification of the genetic and environmental factors that influence plant disease development and the roles that different plant hosts, vectors, and reservoirs play in the transmission of bacterial plant pathogens.
Rapid detection and correct diagnosis of bacterial plant pathogens and diseases are increasingly essential for protecting global food security. By detecting and diagnosing these pathogens and diseases early, it is possible to implement control measures such as the application of chemicals or biological control agents or the implementation of other agricultural practices to reduce the impacts of these diseases on crop production
[18]. Furthermore, bacterial plant pathogens can sometimes result in the contamination of human food with harmful pathogens
[19]. By detecting and diagnosing these pathogens early, it is possible to implement the control measures to prevent food contamination and improve food safety
[20]. Bacterial plant diseases can also sometimes lead to the extinction of plant species, particularly rare or endangered species
[21]. By noticing these pathogens earlier, it is helpful to develop strategies to protect cultures or production directly.
There are challenges in detecting and diagnosing bacterial plant pathogens, such as
P. syringae [22]. One challenge is the need for rapid and correct diagnosis of bacterial plant pathogens. However, traditional methods, such as biochemical or molecular techniques, can be time-consuming and may not provide rapid results
[23]. Another challenge is adapting to the fluctuating environmental conditions
[24]. Bacterial plant pathogens can be influenced by various factors, including temperature, humidity, soil conditions, etc., which can vary over time and space
[20]. Therefore, it is difficult to accurately diagnose and control bacterial plant diseases as the effective control measures may vary, depending on the specific environmental conditions. Diagnosis and control require the development of flexible and adaptable diagnostic and control strategies tailored to the specific environmental conditions in which the diseases are occurring. There is also a need to account for the diversity of bacterial plant pathogens. A wide range of pathogens can cause bacterial plant diseases
[25]. This diversity can make it difficult to accurately diagnose and control bacterial plant pathogens and diseases. The effective diagnostic and control strategies may vary depending on the pathogen involved.
2. P. syringae as a Bacterial Plant Pathogen
P. syringae, a Gram-negative, rod-shaped bacterium that can cause severe damage to many plant species, is a significant concern for plant health and crop production
[26]. It is classified as a hemibiotrophic pathogen that initially feeds on living plant tissues and later causes the death of plant cells
[27]. The
P. syringae phylogenetic group includes more than 60 pathovars and 15 recognized bacterial species
[28]. Each pathovar of
P. syringae infects a distinct group of host plants and is known for its diverse host-specific interactions with the plants
[29][30]. As early as 1939, the
P. syringae pv.
primulae was reported to cause necrotic leaf spots on primrose plants in the USA (
Figure 1A)
[31]. In 1961, the
P. syringae pv.
tomato was reported to cause necrotic leaf spots on tomato plants in the UK (
Figure 1A)
[32]. The
P. syringae pv.
tomato DC3000 is also pathogenic to Arabidopsis plants and has become a model pathogen for probing disease susceptibility and hormone signaling in plants
[27]. Up to 2009, Japan witnessed the highest level of occurrence of plant diseases caused by
P. syringae, followed by the USA (
Figure 1B). Japan reported/deposited 18 different pathovars of
P. syringae to the National Collection of Plant Pathogenic Bacteria (NCPPB), and the USA reported/deposited 9 different pathovars of
P. syringae to NCPPB (
Figure 1B), which increased our understanding of the occurrence/distribution of
P. syringae on a world-scale view.
Figure 1. Occurrence of plant diseases caused by
P. syringae. Data were retrieved from NCPPB (National Collection of Plant Pathogenic Bacteria,
https://www.fera.co.uk/ncppb, accessed on 15 February 2023). (
A) Landmark discoveries of pathovars of
P. syringae.
The life cycle of
P. syringae involves a range of different stages and modes of transmission
[33].
P. syringae can be transmitted through seeds, water, vector insects, and infected plant debris. Once inside the plants,
P. syringae can multiply and produce toxins that harm plant tissues. The infected plants can develop characteristic symptoms, such as lesions or discoloration on diseased leaves and necrosis spots on diseased fruits.
P. syringae can survive in plant debris in the environment for extended periods and easily infect susceptible host plants through wounds or natural openings (
Figure 2). It is worth noting that the life cycle of
P. syringae can vary depending on the pathovar (strain) of the bacterium and the plant species it infects (
Table 1).
P. syringae is typically characterized by its ability to infect only specific areas of plants, such as foliar tissues and fruits. Some pathovars of
P. syringae are more virulent or have a broader host range than others, affecting how the bacterium spreads and causes diseases
[34].
Figure 2. The life cycle of
P. syringae. The diagram was adapted from
[27] with some modifications and updates. The figure was created with BioRender.com, accessed on 15 February 2023.
Table 1. Documentary records of plant diseases caused by
P. syringae. Data were retrieved from NCPPB (National Collection of Plant Pathogenic Bacteria,
https://www.fera.co.uk/ncppb, accessed on 15 February 2023).
P. syringae has been extensively studied since the early 1980s, and it is often used as a model for understanding various aspects of bacterial pathogenicity, including molecular mechanisms of plant-microbe interactions, microbial ecology, and epidemiology
[27][30]. Genomic studies have revealed specific genomic characteristics that contribute to the virulence of
P. syringae. Currently, it has been found that
P. syringae deploys three vital strategies to harm plants: it can survive and adapt to the surface of plants, it can suppress the plant’s immune system at different stages of infection, and it can establish a water-filled space in the plant tissues, which provides it with the access to water and nutrients
[30][92][93][94].
There are various techniques available for the detection and diagnosis of P. syringae. These techniques can be broadly classified into several categories: conventional (visual examination, microscopy, culture plate or phage typing), molecular (RPA, LAMP, NGS, FISH or PCR), serological (FCM, ELISA, IF or immunoStrip), biomarker-based (plant metabolite profiling, pathogen metabolite profiling, or microbiome analysis), vision-based (hyperspectral imaging or spectroscopic imaging) and AI (artificial intelligence). Different techniques have different advantages and limitations depending on the sample type, pathovar diversity, cost-effectiveness, etc. Conventional, molecular, and serological techniques are widely used nowadays for the detection and diagnosis of P. syringae.