Bacterial leaf spot is caused by Pseudomonas syringae; it is shown here using tomato as an example.

Landing. Splashing water and rain carry the bacteria onto leaves and fruit. Tools, infected planting material and cultivation work can also promote their spread.

Infection. The bacteria enter through stomata, other natural openings and small injuries, for example from insect bites or hail.

Survival. The bacterium survives on infected plant residues, seeds and leaf surfaces, particularly in cool, damp weather. In soil it survives only briefly.

The consequence. Lower yield, delayed ripening and reduced market quality.

Economic significance. Economic damage arises from crop losses due to leaf fall — and hence reduced photosynthesis — and, where fruit is affected, from the produce becoming unsaleable.

Research into plant–microbe interaction helps to control Pseudomonas syringae specifically and can therefore have a positive effect on several UN Sustainable Development Goals.

Goal 2 (Zero Hunger): the pathogen causes disease in numerous crop plants and can thereby endanger yields and food production; research investigates how it infects plants and how beneficial microbes or resistant plants can contain the disease.

Goal 6 (Clean Water and Sanitation): the pathogen is often controlled with chemical crop protection measures whose use can burden the environment and water bodies; research into microbial antagonists and the plant microbiome looks for ways to manage with fewer chemicals.

Goal 12 (Responsible Consumption and Production): research is developing biological alternatives that conserve resources.

Goal 15 (Life on Land): whether an infection succeeds also depends on the plant's natural microbiome; research examines these communities in order to promote natural antagonists of the pathogen.

Pseudomonas syringae causes bacterial leaf spot on tomato (Solanum lycopersicum). Splashing water and rain carry the bacteria onto leaves and fruit. Tools, infected planting material and cultivation work can also promote their spread.

The bacteria enter through stomata, other natural openings and small injuries, for instance from insect bites or hail. Unlike fungi, they do not break through the leaf surface but use existing points of entry, and subsequently multiply in the intercellular space. Scanning electron micrographs of thale cress (Arabidopsis thaliana), a model plant, show rod-shaped Pseudomonas bacteria around a stoma. The bacteria release signalling substances; in response the plant opens the pore further, which makes it easier for the bacteria to reach the interior of the leaf.

Characteristic first signs are small, water-soaked spots that turn dark brown to black and are often surrounded by a yellowish margin. Under heavy infection the spots merge, the affected tissue dies and the leaf dries out; sunken lesions remain on the leaf.

The bacterium survives on infected plant residues, seeds and leaf surfaces, particularly in cool, damp weather. In soil it survives only briefly. Damp conditions and persistent leaf wetness favour its spread.

The consequences are lower yield, delayed ripening and reduced market quality. Damaged leaves can carry out less photosynthesis, so the plant's growth and yield decline. Economic damage arises on the one hand from crop losses due to leaf fall, and on the other, where fruit is affected, from the produce becoming unsaleable.