Pseudomonas syringae

What is Pseudomonas syringae?

The bacteria Pseudomonas syringae is a common plant pathogen mostly found in stems and leaves [1]. It infects a wide range of plants—including fruit trees, greenhouse vegetables, ornamentals, and field crops. Annually, the bacteria cause billions of dollars of damage worldwide from lost fruits, damaged trees, unmarketable ornamentals, and more. In 2010, farmers in New Zealand experienced a loss of $450 million worth of kiwi [2–7]. As an opportunistic pathogen, the bacteria can only infect a plant via openings like the stomata and wounds. Once inside the host, it employs various ways to attack the plant with virulence factors [1]. The most reported methods are releasing plant toxins and producing a protein that encourages ice formation in the plant [1,2,8]. The P. syringae species complex comprises over 50 pathovars, each infecting a specific group of plants [7,9,10].

Virulence factors – molecules that help a pathogen invade and cause disease in a host

Pathovars – strains of an organism that can cause infections and disease in a host

Table 1. A short list of Pseudomonas syringae pathovars and their target plant examples.*

*Information summarized from [11]. Additional information added to this table are from [11–23].

Historically, P. syringae is known as the model plant pathogen; studying the bacteria created many advancements in agriculture and plant protection [1]. However, the study of bacteria is shifting from disease to ecology. Recent studies show that P. syringae is not just a plant pathogen but a bacterium that lives and influences the Earth’s water cycle [24]. Therefore, to understand how to control the bacteria in an agricultural setting, we must understand that it is a part of all naturally occurring bodies of water and the Earth’s water cycle.

Symptoms of Pseudomonas syringae

Symptoms of P. syringae depend on the host, with most symptoms appearing above the ground: stems, leaves, flowers, and fruit. Characteristic symptoms can include the following [2,11,12,16,17]:

• Stem and trunk cankers with a gummy substance are often found coming from the open wound
• Blackened twig (shoot-tip dieback)
• Discoloured or blackened leaves
• Bacterial leaf blight
• Peppery leaf spots
• Necrotic leaf spots
• Brown/blackened flowers (blossom blast)
• Dead, dormant buds
• Bacterial canker on fruits

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Life Cycle and Infection Stages

Before understanding the life cycle and the infection stages of P. syringae, it is essential to remember that the disease only occurs after P. syringae enters the plant. There are two phases of the life cycle:

1. Epiphytic Phase

The bacteria live outside the plant, typically on the surface of stems, leaves, flowers, fruits, and other above-ground plant parts. It lives in balance with other microorganisms on the plant’s surface [1,25]. As a result, the bacteria can complete its lifecycle and multiply. The bacteria can then transfer to other plants via water, insects, or other mechanical modes of transportation, such as agricultural tools [26,27]. 

Interestingly, P. syringae can produce ice nucleation proteins that help water crystalize and penetrate the plant’s surface forming wounds [25,28]. Note that not all pathovars of P. syringae can induce ice crystals to form. While P. syringae pv. syringae have been found to have the ice-forming ability, P. syringae pv. tomato does not [25].

2. Endophytic Phase

The endophytic phase is the disease state. First, P. syringae enters the plant through the stomata or open wounds. It can produce a compound called coronatine that tricks the stomata into opening [29]. Next, it grows and multiplies in the space between plant cells called the apoplast. Finally, to protect itself from the plant’s defences, it uses T3SS effectors (T3Es) and other virulence factors to interfere with the plant’s immune system [1,25,30]. As the bacterial population multiplies, it grows beyond the capabilities of the plant’s local immune system. the infected area will show visual symptoms, and the plant will start to die. The bacteria can also spread inside the plant through the plant’s vascular system [31].

apoplast – the space between cells where water and materials can freely flow, including the cell wall

T3SS effectors (T3Es) – needle-like compounds produced by bacteria that secrete molecules that interfere with the plant’s immune system

Growth conditions for Pseudomonas syringae

P. syringae is a ubiquitous bacterium that grows in many different conditions. The disease phase of the bacteria favours wet and humid conditions because it helps with the following[1,32,33]:

• penetration into a plant by keeping the stomata open
• quicker rate of infection due to the increased reproduction rate in the apoplast
• movement of the bacteria to different plants

It favours cooler temperatures, with bacteria preferring 18 to 24°C [34]. Furthermore, it also can grow between 0°C to 4°C [35,36]. For outdoor plants susceptible to frost, P. syringae can invade and provoke a disease phase during wet and cooler times of the year, especially in spring [37].

Methods of Prevention and Control

Best Practices against Pseudomonas syringae

P. syringae spreads through water, insects, and contaminated surfaces. The bacteria is found everywhere with naturally occurring water, and it is a common bacteria found in the natural environment. Therefore, prevention is the best way to control it. Here are a couple of ways that growers can decrease the risk of infection [2,3,23,37–40]

• Use new containers or properly sanitize previously used containers
• Minimize contact with soil outside the growing areas or untreated water from the outside water sources (rain, wind, pond water, etc.)
• Prune infected plants to prevent the spread of the bacteria to the rest of the crop
• Avoid hose ends and other equipment from touching the ground or soil
• Avoid using contaminated equipment with cuttings
• Water your crops in the morning so they can dry during warmer times of the day
• Increase the airflow between plant leaves 
• Increase water drainage
• Be diligent about IPM management, as insects can also be a source of contamination
• Be vigilant and diagnose quickly to allow time to apply control strategies

Resistant Varieties

Plant varieties resistant or less susceptible to P. syringae infections are commercially available. For example, lilac varieties Syringae josikaea, S. komarowii, S. microphylla, S. pekinensis, and S. reflexa are less susceptible than the common lilac cultivar [2]. For tomatoes, Ontario 7710 breeding line has resistance to P. syringae pv tomato, which is reported to be a source of resistance for other cultivars [41, 42]. Some commercially available cucumber varieties have intermediate resistance to Pseudomonas syringae pv. lachrymans [43].

Treatments for Pseudomonas syringae

A preventative method that growers should consider are biocontrols that actively work against pathogens. Currently, only Bacillus subtilis and rhizobacteria are commercially available [44]. Traditional chemical treatments are available for treating infections. Typical commercially available controls are either copper-based or bactericides [2, 40, 44]. Please always read the labels and note that different treatments exist for plant types and geographical regions. In some plants, spraying during the fall helps decrease bacterial populations and prepares the plant for when it is more vulnerable in spring. However, it is essential to note that copper treatments can damage young plants and that some P. syringae pathovars are resistant to copper [2].

Conclusion

 Pseudomonas syringae is an opportunistic pathogen that can be irritating to control once the disease phase starts. The symptoms range from treatable to systemic disease. Therefore, preventative practices are the most effective way of keeping it at bay.  learn more about how early detection can help, feel free to email us at [email protected].

Disclaimer:

The information we present in Pathogen Profile is based on collating published peer-reviewed scientific literature and sources we think are reliable. This is by no means an exhaustive review of pathogens. Pathogen Profile gives a small glimpse of what is known about pathogens. We encourage growers to do more research on the pathogens concerning their crops and hydroponic systems. We are not plant pathologists; thus, the information presented in the Pathogen Profile should not be used as professional advice to treat pathogens or operate your hydroponic system.

References

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David Santos is a Biotech and Agriscience Advisor at Healthy Hydroponics