Biological control of soil-borne pathogens comprises the decrease of inoculum or of
the disease producing activity of a pathogen through one or more mechanisms. Interest
in biological control of soil-borne plant pathogens has increased considerably in
the last few decades, because it may provide control of diseases that cannot or only
partly be managed by other control strategies. Recent advances in microbial and molecular
techniques have significantly contributed to new insights in underlying mechanisms
by which introduced bacteria function. Colonization of plant roots is an essential
step for both soil-borne pathogenic and beneficial rhizobacteria. Colonization patterns
showed that rhizobacteria act as biocontrol agents or as growth-promoting bacteria
form microcolonies or biofilms at preferred sites of root exudation. Such microcolonies
are sites for bacteria to communicate with each other (quorum sensing) and to act
in a coordinated manner. Elicitation of induced systemic resistance (ISR) by plant-associated
bacteria was initially demonstrated using Pseudomonas spp. and other Gram-negative
bacteria. Several strains of the species Bacillus amyloliquefaciens, B. subtilis,
B. pasteurii, B. cereus, B. pumilus, B. mycoides, and B. sphaericus elicit significant
reductions in the incidence or severity of various diseases on a diversity of hosts.
Elicitation of ISR by these strains has been demonstrated in greenhouse or field trials
on tomato, bell pepper, muskmelon, watermelon, sugar beet, tobacco, Arabidopsis sp.,
cucumber, loblolly pine, and two tropical crops (long cayenne pepper and green kuang
futsoi). Protection resulting from ISR elicited by Bacillus spp. has been reported
against leaf-spotting fungal and bacterial pathogens, systemic viruses, a crown-rotting
fungal pathogen, root-knot nematodes, and a stem-blight fungal pathogen as well as
damping-off, blue mold, and late blight diseases. This progress will lead to a more
efficient use of these strains which is worthwhile approach to explore in context
of biocontrol strategies.