Pseudomonas cichorii was accepted in the Approved Lists of Bacterial Names (Skerman et al., 1980). There have been no revisions of the name since then. Phylogenetic investigations showed that the genus Pseudomonas belongs to the gamma subclass of Proteobacteria (Moore et al., 1996). P. cichorii belongs to the ‘Pseudomonas syringae group’ of the genus using the 16S rRNA sequence analyses (Anzai et al., 2000), while it belongs to the ‘P. syringae subgroup’ of Palleroni rRNA group I based on the rRNA-DNA hybridization analyses (Palleroni, 1984). Recently, multilocus sequence analysis (MLSA) revealed high genetic diversity among the P. cichorii strains isolated from various host plants in different geographical areas. It has been shown that the species consists of a cluster of closely related phylogroups that significantly differ from the type strain in their phenotypic and genotypic characteristics (Trantas et al., 2013; Timilsina et al., 2017).
P. cichorii is a Gram-negative rod, about 0.8-1.3 µm, with one to several polar flagella. Colonies on nutrient agar are round, white, slightly raised, translucent, with slightly irregular margins; on King's medium B, a fluorescent greenish pigment diffuses into the medium (Bradbury, 1981). Typical differentiative phenotypic characteristics for the identification of P. cichorii based on the LOPAT scheme consisted of negative levan, positive oxidase, negative potato soft rot, negative arginine dihydrolase and positive hypersensitive reaction features (Schaad et al., 2001).
P. cichorii is probably ubiquitous. It causes serious diseases in all regions characterized by mild temperatures and high humidity or in protected environments. It is widespread in France, Germany, Italy, Russia, UK, Japan, Taiwan, South Africa, Tanzania, Barbados, Brazil, Cuba and New Zealand. In the USA, it occurs most frequently in Florida.
The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.
Symptoms caused by P. cichorii may vary depending on the host and the infected part of the plant. Symptoms usually start with the appearance of water-soaked lesions that develop either at the leaf margin, near the midvein and/or are randomly distributed as leaf spots. Developing lesions are roughly circular and are rarely confined to the inter-veinal areas. The lesions enlarge and turn dark brown or black. Lesions are sometimes surrounded by bright yellow halos. Lesions may coalesce to form very large necrotic areas and may affect the whole leaf. In some hosts symptoms may also be present on petioles, pedicels and/or stems (celery, chrysanthemum, coffee, aubergine, geranium and tomato) and flower buds (chrysanthemum, aubergine and geranium). The size of the lesions varies depending on the environmental conditions; in high humidity conditions larger lesions and rotting of infected tissues are observed, whereas in low humidity conditions lesions may only be a few millimetres diameter and the disease may eventually cease development. Abscission of severely infected leaves (dwarf schefflera (Schefflera arboricola), mint, Ficus lyrata and geranium) and, rarely, the death of the plant (ginseng, chrysantemum, celery seedlings and F. lyrata) have also been reported.
Host-specific symptoms have also been described on a number of plant species. For instance, 'varnish spot of lettuce' affects the blades and petioles of the inner leaves of head lettuce varieties, and is characterized by shiny, dark-brown, necrotic lesions. Lesions range in size from a few millimetres to very large and are not delimited by veins (Grogan et al., 1977). Vein blackening of the outer leaves is possible on lettuce grown in severely infested soils.
Disease on field-grown lettuce is frequently reported as lettuce rot or varnish spot, while it is called midrib rot when infects greenhouse-grown lettuce plants (Cottyn et al., 2009). In the latter case, symptoms consist of small, brown spots that coalesce and expand into moist, dark brown to greenish black rotted lesions along the midrib of inner head leaves (Cottyn et al., 2011).
On celery, P. cichorii causes 'bacterial blight' (leaf and petiole lesions) and 'brown stem of celery'. Most of the symptoms occur on leaves. Leaf lesions are at first circular to angular, dark green, water-soaked spots 1-2 mm diameter, while in the large number of spots, the whole leaf becomes chlorotic. Symptoms on stem and petiole appear as elongated, rusty brown lesions 1-2 mm wide by 2-5 mm long (Wilkie and Dye, 1974; Pernezny et al., 1994).
'Leaf rot of pepper' causes watery spots on the leaves, stem and apical regions resulting in complete decay (Rivera et al., 1981). 'Stem melanosis of spring wheat' is characterized by bleached, empty heads and blackening of the rachis, peduncle and stem immediately beneath the nodes (Piening and McPherson, 1985). Chrysanthemum leaf spot and bud blight (McFadden, 1961) produces symptoms on the leaves, buds or stem. Irregular, dark brown to black necrotic lesions develop on the leaves. The bacterium moves from the leaf through the petiole and causes a dark-brown stem necrosis. Flower buds turn dark brown and die prematurely. On chrysanthemum, P. cichorii causes 'stem necrosis', characterized by dark blue to black, water-soaked lesions along the stem, without lesions on the leaves; severely affected plants may die (Jones et al., 1983). 'Leaf spot of geranium' (Engelhard et al., 1983) causes irregularly-shaped, water-soaked spots that become dark brown to black. Yellowing of adjacent tissues invariably occurs a few days after the lesion appears and infected leaves die. Flower buds turn black and fail to open and the necrosis extends down the peduncle. Basal rot of geranium cuttings has been reported as softening and blackening of stems. The leaf margins of affected cuttings develop chlorosis and wilting symptoms (Semer and Raju, 1984).
On gerbera (Miller and Knauss, 1973), circular to irregular, brownish-black spots with or without a concentric ring are usually observed. The lesions extending from the margins become narrow as they reach the midvein. On Hibiscus rosa-sinensis, symptoms consist of tan spots with purple or black margins (Chase, 1986). The disease is called pith necrosis on tomato when infects the inner parts of the stems. Initial symptoms of pith necrosis consist of yellowing and wilting of young leaves, while serious infection can lead to dark green to dark brown, irregular blotches on the leaf blades and elongated streaks along the stem (Wilkie and Dye, 1974). Longitudinal cut of the symptomatic stems will reveal extensive discoloration of the centre of the stem (pith). Infected stems may crack or collapse due to the destruction of the tissues (Trantas et al., 2013).
On Mentha arvensis, small, water-soaked, yellow spots on the leaf edges are observed, which cause leaf abscission at the advanced stages of disease. On ginseng, small, dark-brown spots on the central vein of the leaf that spread across the surface, while leaves lose turgor and wilt. On F. lyrata (Chase, 1987b), angular, black lesions with purplish margins on leaves, frequently initiated at the junction of the blade and petiole, expanding along the veins into the leaf blade. The leaves may abscise and, in severe cases, the stem rots and the plant dies. On nectarines, gummy and corky spots on fruits are observed (Pinto de Torres and Carreno Ibanez, 1983). A disease of mushrooms called oozing gills and brown blotch, which is similar to drippy gill caused by Pseudomonas agarici, is caused by strains with biochemical characteristics similar to P. cichorii (Bateson et al., 1972). P. cichorii has been also isolated from rots of bananas, carrot, cauliflower, lettuce, in mixed infections with other pseudomonads and/or Pectobacterium species. P. cichorii is associated with the postharvest bacterial rot of lettuce together with other fluorescent pseudomonads.
Until recently, the genomic data of P. cichorii has rarely been available (Ramkumar et al. 2015), hence, most of the pathogen-plant interaction mechanisms remain undetermined. P. cichorii has a broad host range and low host specificity. Under favourable environmental conditions, this constitutes a high potential for the development of epidemics. Infection occurs through stomata or wounds. Wounding is not required for infection of most hosts when plants are subject to high levels of free moisture, such as under misting, while inoculum of P. cichorii is present. Under environmental conditions that favour the pathogen, lettuce may be infected through epidermal hairs (Shirata et al., 1982). Wounds caused by the feeding and oviposition of Lyriomiza trifolii provide entry sites for P. cichorii in chrysanthemum (Matteoni and Broadbent, 1988). The bacterium multiplies in the intercellular spaces of the epidermis and then colonizes the intercellular spaces of the mesophyll (Hikichi et al., 1996a). Browning of the tissue develops after bacterial multiplication. Extracts of a phytotoxin, isolated from several P. cichorii strains and partially characterized (Hu et al., 1998), has been shown to induce lesions when injected into lettuce tissues, similar to those induced by the bacterium, and may be responsible for disease symptoms (Shirakawa et al., 1998).
Huang et al. (2015) showed that P. cichorii strain SF1-54 causing lettuce midrib rot disease, produces seven bioactive compounds with biosurfactant properties among which two phytotoxic compounds 'cichopeptin A and B' contribute to virulence of the pathogen on lettuce. Although the cipA-mutant strain (lacking cichopeptin A and B) exhibited significantly less virulence and rotten midribs than the wild type strain, the two strains showed the same level of leaf multiplication on lettuce (Huang et al., 2015). Furthermore, it has been shown that the gene hrpW is involved in the virulence of P. cichorii and it contributes in the induction of hypersensitivity reaction in tobacco (Kajihara et al., 2012). On the other hand, it has been shown that induction of apoptotic cell death by P. cichorii within the infected lettuce tissues initiates the development of rotting symptoms on the plant (Kiba et al., 2006).
Periods of high humidity and leaf wetness are necessary for infection and disease development. The longer the period of moisture the greater is the incidence of the disease (Jones et al., 1984). Lesions cease to expand in low moisture conditions. Thus, plants grown in polyethylene-covered greenhouses or tunnels, in nurseries as well as plants subjected to rainfall or sprinkle-irrigation tend to develop the most severe symptoms. The greenhouse is an ideal environment for disease development because of the dense plant canopy and high humidity.
Disease develops over a wide temperature range, while the optimum temperature is 20-28°C. At higher temperatures, lesion expansion declines and ceases at temperatures above 36°C (Jones et al., 1984).
Experiments on the effect of host nutrition showed that high fertilizer application, stimulating rapid and succulent plant growth, results in an increased number of lesions after inoculation with P. cichorii (Jones et al., 1984). Contrasting results have been obtained on dwarf schefflera (Schefflera arboricola) (Chase, 1987a). Different host species (chrysanthemum, lettuce and coffee) are known to carry epiphytic populations of P. cichorii. On lettuce, the bacterial population increased on the outer and head leaves in the early and middle stages of head formation, respectively (Hikichi et al., 1996b). The bacterium may also be present in apparently healthy chrysanthemum buds (Jones et al., 1990). It has been shown that infections occur only when the pathogen reaches populations on leaves that are greater than 100,000 CFU/g of fresh weight (Hikichi, 1996b). Disease incidence is also correlated with the population level of the bacterium on lettuce leaves (Hikichi et al., 1996b).
P. cichorii can survive in soil for short periods, while it survives for up to 6 months in buried infected debris, but this is not the case under conditions where they are subject to rapid decay (Ohata et al., 1982; Bazzi et al., 1984). The bacterium is frequently associated with other fluorescent pseudomonads in soils and in the rhizosphere of vegetables and weeds (Grogan et al., 1977). It has also been isolated from dried, diseased lettuce leaves that had overwintered for 4 months in a vinyl house, from rotted leaves of Senecio vulgaris and Capsella bursa-pastoris growing in lettuce fields affected by the disease and from their root surfaces, and from those of lettuce and Artemisia princeps (Ohata et al., 1982). P. cichorii which survives within an infected field is able to colonize new plants in the following year. Populations of the pathogen may be present but the disease does not always occur unless environmental conditions favour bacterial multiplication in the host. The bacterium has been isolated from artificially inoculated seeds stored at 23 and 5°C after 50 and 93 days, respectively.
Splashing water (rain, sprinkler irrigation, water dripping from the roof of greenhouses, etc.) results in dispersal of P. cichorii within the crop, from soil to plant or from plant to plant. It is likely that this pathogen can be dispersed between adjacent crops by aerosols. The very wide host range of P. cichorii suggests that there may exist many weed plants that act as alternative hosts. Prevention of infection of crops when adjacent plants are hosts to the pathogen may be impossible.
Vector Transmission
Adults of Liriomyza trifolii are able to acquire and transmit P. cichorii from infected to non-infected chrysanthemums (Broadbent and Matteoni, 1991).
Seedborne Spread
P. cichorii has been shown to survive on artificially inoculated lettuce seeds (Ohata et al., 1982). Many authors have reported P. cichorii as seed transmitted but, despite field observation of early infections in the crop and in the nursery that support this suggestion, there is no experimental data on transmission by seed.
Agricultural Practices
Sprinkler irrigation provides dispersal of inoculum within the crop.
Movement in Trade
The exchange of propagative material can distribute P. cichorii over long distances as the bacterium may be carried in commodities either in tissue lesions (symptomatic plants) or epiphytically on asymptomatic plants.
Under environmental conditions favourable to the pathogen, diseases incited by P. cichorii cause severe damage to the host and can result in outbreaks. Outbreaks in the nursery or in the field during warm winters in Florida, USA, can lead to widespread disease affecting thousands of plants (Jones et al., 1983; Chase and Brunk, 1984; Uddin and McCarter, 1996). Diseases caused by P. cichorii can appear sporadically over a number of years then cause severe outbreaks such as the outbreak of 'varnish spot of lettuce' in California (Grogan et al. 1977) and 'brown stem of celery' in Florida, USA (Pernezny et al., 1994). Severe disease outbreaks on lettuce leading to losses of up to 100% have been reported in California (Grogan et al., 1977), Italy (Bazzi and Mazzucchi, 1979) and Portugal (Ferreira-Pinto and Oliveira, 1993). In Italy and France, P. cichorii is a recurring problem on endive and lettuce (Allex and Rat 1990; D'Ascenzo et al., 1997). In Florida, brown stem of chrysanthemum only occurs about every 5 to 6 years whereas leaf spot occurs annually (Pernezny et al., 1994).
Isolation is made from early infection stages, small pieces of tissue being excised from the margins preferably of the youngest lesions. These are comminuted in small quantities of sterile water and streaked on plates of surface-dried King's medium B and incubated at 25-27°C for 2 days. The development of largely unmixed colonies, green fluorescent under UV light, is a good indication that the pathogen has been isolated. Single colonies are sub-cultured onto nutrient agar for storage and confirmation of the identity of the pathogen using LOPAT tests (Lelliott and Stead, 1987; Schaad et al., 2001). Presumptive diagnosis of the pathogen can be made if green fluorescent bacteria with the LOPAT Group III characteristics are obtained (being levan negative, oxidase positive, potato rot negative, arginine dihydrolase negative, hypersensitivity on tobacco positive). However, variable results for the LOPAT scheme were reported for the Pseudomonas strains designated as P. cichorii (Aysan et al., 2003; Cottyn et al., 2009). Culturally, P. cichorii most resembles the common plant saprophytes, P. fluorescens and P. putida. These are distinguished from P. cichorii because they are positive in the test for arginine dihydrolase and negative in the test for tobacco hypersensitivity. Further biochemical analysis can be performed according to Schaad et al. (2001) or by mean of galleries for biochemical characterization and identification. Pathogenicity tests could be performed on the host of isolation and preferably on one known host (e.g., lettuce or chrysanthemum). A bacterial suspension of no more than 10,000,000 CFU/ml is sprayed on plants until run off. Wounding is not necessary for leaf infection but high humidity levels (for example, enclosing plants in polyethylene bags) are essential until 24 h post inoculation. The plants are incubated at 26°C, being maintained for 3 days after inoculation in high humidity conditions, either by being enclosed with a polythene bag or, preferably, held in a mist cabinet. Plants may also be inoculated on stems using a needle laden with inoculum.
Two semi-selective media containing fungal and bacterial inhibitors have been developed for ecological studies on P. cichorii (Jones et al., 1990).
An ELISA procedure has been evaluated to detect P. cichorii in lettuce leaves (Ogiso et al., 1997). Polyclonal antibodies are commercially available for the identification of P. cichorii by immunofluorescence and ELISA. Any unusual report or diagnosis on plants or plant parts moving in trade should be confirmed by a pathogenicity test. Highly specific and sensitive molecular probes based on unique DNA sequences obtained by DNA subtraction methods can readily be developed to confirm the identity of the pathogen including strains obtained from symptomless plants. These methods are only used when a high level of confidence in identification is necessary to support eradication programmes or quarantine restrictions that may involve compensation.
Recently, a sensitive real‐time PCR assay was developed for the specific detection of P. cichorii using a Taqman Minor Groove Binding probe based on a 90‐bp amplicon from the pathogenicity gene cluster hrcRST. The method was shown to be capable of detecting 1 CFU/mL of irrigation water, which is well below the concentration needed for lettuce midrib rot infection in commercial lettuce greenhouses (Cottyn et al., 2011).
Detection and Inspection
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Inspect the plants or plant parts (e.g. cuttings) for the presence of leaf and/or stem lesions. Water-soaked lesions and bright-yellow halos at the margin of the lesions are indicative of P. cichorii. Symptoms on lettuce in the field are not always visible because the bacterium infects the inner leaves of mature heads but are evident when the heads are cut open (Grogan et al., 1977). Diseased specimens must be collected and analysed for the presence of the bacterium by isolation in culture and confirmation of identity.
Symptoms induced by P. cichorii can be confused with those caused by other bacterial pathogens that cause leaf spot or blight on vegetables and ornamentals. In tomato for instance, initial field symptoms caused by P. cichorii could be confused with the symptoms caused by P. syringae pv. tomato, P. corrugata as well as the leaf spot pathogen Xanthomonas euvesicatoria (Jones et al., 2016). In the laboratory, fluorescence of colonies on King's B medium restricts the investigation to fluorescent pseudomonads that can be identified by LOPAT tests (see Diagnosis).
Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.
Control of disease caused by P. cichorii is difficult to achieve when conditions favour the pathogen. For greenhouse operations, the ability to regulate free water on plant surfaces offers some chance of success. Soil irrigation, rather than overhead watering, will reduce the likelihood of infection and disease development (Pauwelyn et al., 2011). Greenhouse ventilation is beneficial. Planting material should be from sources free of the pathogen. Plants with symptoms should be isolated if of high value, or discarded. The spread of the bacterium may be reduced by decontaminating tools and hands after handling diseased plants. High plant densities and over-fertilization, especially with nitrogen fertilizers, should be avoided. Inoculum in the field may be reduced by ploughing, removing and burning infected plant debris, removing weeds near growing areas. Different cultivar susceptibility has been observed in chrysanthemum (Strider and Jones, 1986), basil (Holcomb and Cox, 1998), endive, lettuce and chicory (Matta and Garibaldi, 1970). Chemical control using copper compounds or antibiotics has been demonstrated in trials, but success in commercial practice, especially in the field, is problematic. Spray costs and the variability of efficacy may mean that applications are often not cost effective.
Furthermore, resistance to streptomycin and copper-based chemicals was reported within the P. cichorii strains isolated from celery seedbeds in Florida, USA.
It has been shown that tomato seedlings inoculated with P. cichorii strain JBC1 and exposed to green or red light showed a significant reduction in disease incidence compared to the seedlings grown under white light or dark conditions. Inoculated seedlings grown under green or red light showed significant up-regulation in the defense-related genes, phenylalanine ammonia-lyase (PAL) and pathogenesis-related protein 1a (PR-1a) (Nagendran and Lee, 2015). Furthermore, Rajalingam and Lee (2018) showed that the expression of genes involved in the production of phytotoxic lipopeptides and siderophores were significantly decreased under green light in comparison to the dark conditions.
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Patel, N., Kobayashi, D. Y., Noto, A. J., Baldwin, A. C., Simon, J. E., Wyenandt, C. A., 2019. First report of Pseudomonas cichorii causing bacterial leaf spot on sweet basil (Ocimum basilicum) in New Jersey. Plant Disease, 103, 2666.
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Patel N, Kobayashi DY, Noto AJ, Baldwin AC, Simon JE, Wyenandt CA, 2019. First report of Pseudomonas cichorii causing bacterial leaf spot on sweet basil (Ocimum basilicum) in New Jersey. Plant Disease. 2666.
She XM, He ZF, Tang YF, Lan GB, 2016. First report of tomato leaf spot and stem necrosis caused by Pseudomonas cichorii in Guangdong Province, China. Plant Disease. 2319.
