Bioguide #8

How microbe-stimulating foliar sprays support plant resilience

Plant leaves are critical for photosynthesis – the process that provides the plant with energy to live.  Plant surfaces such as leaves and fruit are covered with microbes, including both bacteria and fungi.  This thin layer of life is referred to as the biofilm and is critical to the plant’s health.

Some of these biofilm microbes, however, can have adverse effects on plant health. Examples include fungi (such as Botrytis cinerea, causing botrytis on leaves and fruit) or bacteria (like Pseudomonas syringae, which causes leaf blotching in many species). Other species of microbes are beneficial or benign to the plant and do not cause disease.

Growers often use foliar sprays on their crops with fungicides or bactericides to either prevent or control the disease-causing organisms living on the plant surfaces.  These fungicide and bactericide sprays change the composition of the leaf biofilm – controlling the disease-causing microbe but also killing off non-target species living on the plant surface, including beneficial microbes.  This creates gaps in the leaf biofilm where disease-causing microbes can land and re-establish to start a new infection.

Taking a biological approach to the problem of foliar health means repairing the leaf biofilm when applying foliar sprays – ensuring ongoing resilience against disease. 

Plant defence systems

Plants are continually subjected to attacks by hostile microbes, both above and below ground, but have developed a defence system to protect themselves from these attacks.

These plant defence systems are like the immune system in animals and, once activated, provide systemic control throughout the whole plant. The plant has two different defence systems – Systemic Acquired Resistance (SAR) and Induced Systemic Resistance (ISR). SAR is activated through foliar biotic (organisms) and abiotic (physical or chemical) stress events, whereas ISR is induced by soil microbes.

Systemic Acquired Resistance (SAR)

SAR is a plant’s response after localized exposure to a pathogen or insect.  During this attack, the bacteria, fungi, or insect will secrete some of its material in order to get nutrition from the plant.

The plant can “recognize” some of these “molecular signatures” (specific proteins, enzymes, etc) – triggering the “immune response” or SAR in the plant. This immune response is controlled by salicylic acid, which activates disease resistance pathways within the plant.

This resistance response includes the production of pathogenesis-related (PR) proteins, antimicrobial phytoalexins or cell wall fortification with callose or lignin.  Once local defences are established, the plant produces signals (transported via xylem around the plant) that lead to the production of defence compounds throughout the plant.

Once the SAR process is activated, it produces resistance against a wide range of pathogens (broad-spectrum) not just the organism that attacked it in the first place. 

Induced Systemic Resistance (ISR)

ISR is a plant defence system complementary to SAR triggered by soil microbes interacting or colonising the plant roots. These rhizosphere microbes are referred to as plant-growth-promoting rhizobacteria (PGPR) or fungi (PFPF). Mycorrhizal fungi are a well-known inducer of ISR in plants (another benefit of encouraging these fungi to associate with plant roots).

ISR in the roots leads to the release of jasmonic acid (JA) and ethylene (ET) – key signalling molecules that activate the immune response throughout the plant.

Plant elicitors

Elicitors are compounds that can trigger SAR or ISR in a plant in the absence of a pathogen. Once the plant’s SAR/ISR system is activated, it is primed to fight any future pathogen attacks.

The key to good plant health requires the maintenance of a healthy leaf biofilm as well as eliciting SAR/ISR in the plant – priming it to protect itself against attack.  One way to achieve this is to use elicitor compounds via regular foliar spray and soil biostimulant applications.

Two BioStart products that help activate the plant defence systems and maintain biofilm are: 

• Foliacin – containing a range of elicitor compounds that induce SAR in the plant.
• Mycorrcin – containing a range of elicitor compounds that induce ISR in the plant.