Natural defenses: tomatoes have nothing to blush about!

The flora that covers the leaves of tomato plants is thought to help them to better resist Pst*, a disease that affects crops. An American team discovered that this protection could be impaired by the addition of fertilizer.


In tomatoes as in humans, probiotics are in great demand to boost natural defenses. Two Californian researchers wanted to find out whether the bacteria living on the plants influenced their capacity to resist Pst, a bacterium of the Pseudomonas genus that causes bacterial speck, a disease that produces spots on the leaves, stems and fruits, and affects both photosynthesis and market value.

Protective “leaf baths”

The researchers focused their attention on the leafy part of the plant–the phyllosphere: in a farm near their laboratory, they watered the leaves of different plant varieties, then collected these “leaf baths”, rich in bacteria. In the laboratory, plants of the aptly named old variety “Moneymaker” were sprayed with a sterile solution containing this mixture at different dilutions, therefore varying concentrations of bacteria, then were infected one week later with Pst. Verdict after 24 hours: the surface area affected by spotting on the leaves was limited, an indication of the protective efficacy of the bacteria collected on the farm.

Disruptive fertilizers

More extensive experiments showed that this protective effect did not seem linked to the quantity or diversity of bacteria used, nor even to the presence of one or more specific strains. Another important result:  the addition of fertilizer weakened the resistance provided by the plant microbiota, allowing Pst to gain ground. The mechanisms at work seem complex: competition between resident and foreign bacteria for the nutrients present in the ground or provided by the fertilizer, stimulation of the plant immune system, without forgetting the rhizosphere–the underground part of the plant–which is also likely to play a role. The only certainty is that tomato plants will have to wait before probiotic sprays that boost plant immunity can be used, as you cannot say “tomayto, tomahto” when it comes to bacteria.


*Pseudomonas syringae pv. tomato



Berg M, Koskella B. Nutrient- and Dose-Dependent Microbiome-Mediated Protection against a Plant Pathogen. Curr Biol. 2018 Aug 6;28(15):2487-2492.e3