Giampaolo Buriani

New insights on the bacterial canker of kiwifruit (Pseudomonas syringae pv. actinidiae)

Since 2008, Pseudomonas syringae pv. actinidiae, the causal agent of bacterial canker of kiwifruit has become the main pathogen of yellow and green fleshed kiwifruit. All major kiwifruit producing countries in the world have been affected by this bacterial pathogen, leading to substantial economic losses. This review presents the current knowledge on various aspects about the origin, epidemiology, detection and control strategies of Pseudomonas syringae pv. actinidiae.

Characterization of volatile organic compounds emitted by kiwifruit plants infected with Pseudomonas syringae pv. actinidiae and their effects on host defences

Key messageSpecific VOC emissions from infected plants allow their recognition and elicit defence responses in neighboring plants, which are, however, insufficient to induce resistance.AbstractA wide range of volatile organic compounds (VOCs) is released during plant–pathogen interactions both by the pathogens and the hosts. Some of these VOCs are specific for the different diseases and are known to play a role in the pathogenicity or in plant defence responses. Besides, disease-specific VOCs may serve as markers for diagnostic protocols, which allow a non-destructive and rapid screening of bulk samples of plant material. This work aimed to verify the feasibility of a VOC-based diagnosis and to investigate the possible biological role of VOCs released during the plant–pathogen interactions. The volatile emissions from Pseudomonas syringae pv. actinidiae in axenic cultures and from inoculated in vitro kiwifruit plants were characterized by gas chromatography–mass spectrometry (GC–MS) and proton transfer reaction–time-of-flight-mass spectrometry (PTR–ToF-MS). By GC–MS, several putative biochemical markers, such as 1-undecene, were identified. PTR–ToF-MS resulted highly effective in screening the plant material for latent infections. To develop a more user-friendly, portable and less expensive diagnosis system, two different electronic nose models were tested for the early diagnosis of P. syringae pv. actinidiae in asymptomatic plant material. Our experiments demonstrated the feasibility of the electronic nose-based screening of infected plant material. Concerning the biological role of the VOCs released during the plant–pathogen interactions, the exposure of healthy plants to VOCs from infected ones influences the plant growth and induces the stimulation of protective responses. However, after the infection, P. syringae pv. actinidiae is able to selectively inactivate the induced plant defences.

First Report of Pseudomonas syringae pv. actinidiae on Kiwifruit Pollen from Argentina

During last years, in Mar del Plata (Argentina) area, many kiwifruit orchards composed by only male plants (Actinidia deliciosa cv. Chieftain) have been established for pollen production and commercialization, since the country was considered unaffected by Psa. When mature, the anthers are detached and dried, and grains of pollen collected with a portable cyclonic pollen collector. Then, routine controls aimed to assess the risk of infection by Pseudomonas syringae pv. actinidiae (Psa), the causal agent of kiwifruit bacterial canker (Takikawa et al., 1989), are carried out by plating 1 gram aliquots of pollen on semi-selective medium (modified KB) and incubating plates for 48-72 h at 27 °C. During one of these controls, in February 2015, some bacterial colonies with morphological features similar to Psa were detected. After purification, four of them (labeled as Arg1.1 to Arg2.3) were found to be gram negative, non-fluorescent, positive for levan production, and caused hypersensitive response on tobacco. ...

Biological relevance of volatile organic compounds emitted during the pathogenic interactions between apple plants and Erwinia amylovora

Volatile organic compounds emitted during the infection of apple (Malus pumila var. domestica) plants by Erwinia amylovora or Pseudomonas syringae pv. syringae were studied by gas chromatography-mass spectrometry and proton transfer reaction-mass spectrometry, and used to treat uninfected plants. Infected plants showed a disease-specific emission of volatile organic compounds, including several bio-active compounds, such as hexenal isomers and 2,3-butanediol. Leaf growth promotion and a higher resistance to the pathogen, expressed as a lower bacterial growth and migration in plant tissues, were detected in plants exposed to volatile compounds from E. amylovora-infected plants. Transcriptional analysis revealed the activation of salicylic acid synthesis and signal transduction in healthy plants exposed to volatiles produced by E. amylovora-infected neighbour plants. In contrast, in the same plants, salicylic acid-dependent responses were repressed after infection, whereas oxylipin metabolism was activated. These results clarify some metabolic and ecological aspects of the pathogenic adaptation of E. amylovora to its host.

Pathways of flower infection and pollen-mediated dispersion of Pseudomonas syringae pv. actinidiae, the causal agent of kiwifruit bacterial canker

Flowers can provide a protected and nutrient-rich environment to the epiphytic microflora, thus representing a sensible entry point for pathogens such as Pseudomonas syringae pv. actinidiae (Psa). This bacterium can colonize both male and female Actinidia flowers, causing flower browning and fall, and systemic invasion of the host plant, eventually leading to its death. However, the process of flower colonization and penetration into the host tissues has not yet been fully elucidated. In addition, the presence of Psa in the pollen from infected flowers, and the role of pollination in the spread of Psa requires confirmation.The present study employed a Psa strain constitutively expressing the fluorescent GFPuv protein, to visualize in vivo flower colonization. Microscopy observations were performed by means of confocal laser scanning and wide-field fluorescent microscopy, and were coupled with the study of Psa population dynamics by quantitative PCR (q-PCR). The pathogen was shown to colonize stigmata, move along the stylar furrow, and penetrate the receptacles via the style or nectarhodes. Once the receptacle was invaded, the pathogen migrated along the flower pedicel and became systemic. Psa was also able to colonize the anthers epiphytically and endophytically. Infected male flowers produced contaminated pollen, which could transmit Psa to healthy plants. Finally, pollinators (Apis mellifera and Bombus terrestris) were studied in natural conditions, showing that, although they can be contaminated with Psa, the pathogen’s transmission via pollinators is contrasted by its short survival in the hive.Kiwi fruit: Insights to combat bacterial cankerStudying the transmission pathways of a bacterium that infects and kills kiwi fruit plants suggests that modified plant protection strategies could minimize the risk of disease. Researchers in Italy and New Zealand, led by Francesco Spinelli at the University of Bologna, investigated Pseudomonas syringae pv. Actinidiae (Psa), which causes kiwi fruit bacterial canker. This has been a major problem worldwide since a pandemic outbreak in 2008. The results show that flower tissues, especially the stigmata which receive transmitted pollen, are crucial sites allowing Psa to grow and penetrate into the plants. Evidence is presented of bacterial transmission via the pollen dispersed from plants that do not themselves show signs of infection. Suggested new control strategies include inspecting and treating pollen-donating plants, and using biological agents to compete with Psa growing inside kiwi fruit flowers.

Optimization of cultural practices to reduce the development of Pseudomonas syringae pv. actinidiae, causal agent of the bacterial canker of kiwifruit

BACKGROUND: The bacterial canker of kiwifruit, caused by Pseudomonas syringae pv. actinidiae (Psa), affects several cultivated Actinidia species, including A. chinensis and A. deliciosa. Its development is related to permissive environmental conditions, such as temperature, humidity, presence of entry points, genetic and physiological features of the host plant. Moreover, also cultural practices influence, directly or indirectly, the disease development. OBJECTIVE: The role of agricultural practices on disease development and spread was studied. METHODS: Irrigation, pruning and training systems were tested in the field according to conventional orchard management. Experiments on mineral nutrition, use of bio-regulators and rootstock susceptibility were performed in controlled conditions. Bacterial growth, symptom development and disease incidence were assessed in relation to the different practices. RESULTS: High nitrogen fertilization, iron deficiency and water stress were related to more severe symptoms. Open canopies allow a better irradiation, aeration, and penetration of phytosanitary treatments. Synthetic gibberellins reduced disease incidence and severity in controlled conditions. Fruits from diseased plants showed a lower quality and storability. CONCLUSIONS: Dense canopies are harder to manage and more exposed to bacterial canker. Pruning tools and irrigation water are relevant for the bacterial spread.