Disease suppressive soils function to prevent the establishment of

pathogen In biology, a pathogen (, "suffering", "passion" and , "producer of"), in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a Germ theory of d ...

s in the

rhizosphere The rhizosphere is the narrow region of soil or Substrate (biology), substrate that is directly influenced by root secretions and associated soil microorganisms known as the root microbiome. Pore space in soil, Soil pores in the rhizosphere can ...

of plants. These

soil Soil, also commonly referred to as earth, is a mixture of organic matter, minerals, gases, water, and organisms that together support the life of plants and soil organisms. Some scientific definitions distinguish dirt from ''soil'' by re ...

s develop through the establishment of beneficial microbes, known as

plant growth-promoting rhizobacteria Rhizobacteria are root-associated bacteria that can have a detrimental (parasitic varieties), neutral or beneficial effect on plant growth. The name comes from the Greek language, Greek ''rhiza'', meaning root. The term usually refers to bacteria ...

(PGPR) in the rhizosphere of plant roots. These mutualistic microbes function to increase plant health by fighting against harmful soil microbes either directly or indirectly. As beneficial bacteria occupy space around plant roots they outcompete harmful pathogens by releasing pathogenic suppressive metabolites. Recent research has demonstrated that disease-suppressive soils not only inhibit soilborne pathogens but also protect plants from insect pests. In particular, these soils can enhance plant resistance to leaf-feeding insects by promoting beneficial rhizosphere bacteria and priming plant defense mechanisms.

PGPR and harmful soil microbes

Plant growth promoting rhizobacteria are bacteria that promote plant growth, through fixing nitrogen, producing growth hormones, or even suppressing pathogens. A variety of PGPR genera provide a wide array of functions that directly improve plant health. Bacteria genera ''

Rhizobium ''Rhizobium'' is a genus of Gram-negative soil bacteria that fix nitrogen. ''Rhizobium'' species form an endosymbiotic nitrogen-fixing association with roots of (primarily) legumes and other flowering plants. The bacteria colonize plant ce ...

'' and ''

Mesorhizobium ''Mesorhizobium'' is a genus of Gram-negative soil bacteria. At least one, the nitrogen fixing species, ''Mesorhizobium loti'', forms symbiotic root nodule Root nodules are found on the roots of plants, primarily legumes, that form a symbiosi ...

'' work to fix nitrogen into a usable form. ''

Actinomyces ''Actinomyces'' is a genus of the Actinomycetia class of bacteria. They all are Gram-positive and facultatively anaerobic, growing best under anaerobic conditions. ''Actinomyces'' species may form endospores, and while individual bacteria are r ...

'' and ''

Azospirillum ''Azospirillum'' is a Gram-negative, microaerophilic, non-fermentative and nitrogen-fixing bacterial genus from the family of Rhodospirillaceae. ''Azospirillum'' bacteria can promote plant growth. Characteristics The genus ''Azospirillum'' bel ...

'' produce growth hormones that increase root growth and uptake of nutrients. While many of these genera increase plant health directly some PGPR promote plant health indirectly through pathogen suppression. Pathogenic

fungi A fungus (: fungi , , , or ; or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified as one ...

create a complex of

hyphae A hypha (; ) is a long, branching, filamentous structure of a fungus, oomycete, or actinobacterium. In most fungi, hyphae are the main mode of vegetative growth, and are collectively called a mycelium. Structure A hypha consists of one o ...

that migrate through the soil. When these fungi reach the rhizosphere they release enzymes that degrade the cell wall of plant root cells. This allows them to enter and infect the host plant and prevent the uptake of nutrients. Beneficial ''

Pseudomonas ''Pseudomonas'' is a genus of Gram-negative bacteria belonging to the family Pseudomonadaceae in the class Gammaproteobacteria. The 348 members of the genus demonstrate a great deal of metabolic diversity and consequently are able to colonize a ...

'' and ''

Bacillus ''Bacillus'', from Latin "bacillus", meaning "little staff, wand", is a genus of Gram-positive, rod-shaped bacteria, a member of the phylum ''Bacillota'', with 266 named species. The term is also used to describe the shape (rod) of other so-sh ...

'' produce fungal suppressing metabolites that break up this migrating fungal

. Over time, plants are able to create disease-suppressive soils in response to these pathogens through the increasing the abundance of these PGPR in their rhizosphere area.

Establishment of suppressive soils

Plants respond to pathogens by recruiting PGPR to their root rhizosphere from the bulk soil to fill in and prevent pathogen establishment. This ultimately leads to the development of disease-suppressive soils. PGPR are recruited through the natural release plant exudates from root cells as they push through the soil. Different species of plants release different exudates and therefore recruit different microbial communities to their rhizosphere from the microbes already present in the surrounding bulk soil. If a beneficial microbe genera is not present in a soil plants are not able to recruit it as a defense. Therefore, suppressive soils are a function of the microbes already present in a soil and able to be recruited. The creation of these soils must be done by increasing PGPR in the overall bulk soil. Specific plants are able to recruit beneficial bacteria through the secretion of different root exudates. A greater diversity of plants in a soil leads to a greater diversity of microbes in the rhizosphere and furthermore can lead to greater suppression of soil diseases. Management, such as informed

crop rotation Crop rotation is the practice of growing a series of different types of crops in the same area across a sequence of growing seasons. This practice reduces the reliance of crops on one set of nutrients, pest and weed pressure, along with the pro ...

and soil solarization, can create suppressive soils that naturally suppress pathogens.Peters, R. D., Sturz, A. V., Carter, M. R., & Sanderson, J. B. (2003). Developing disease-suppressive soils through crop rotation and tillage management practices. ''Soil and Tillage Research'', ''72''(2), 181-192.

References

{{reflist Soil Phytopathology