2.2. Competition for niche and nutrition

Competition for niche or competitive exclusion is a restriction of access to the habitat of a pathogen on the plant or seeds by another microorganism while competition for nutrients happens when two or more microorganisms compete for the same source of macro- and micro-nutrients required for growth and secondary metabolites production [7].

One of the most famous and promising examples on competition for ecological niche and nutrition is found in A. flavus control [26]. However, competition of other mycotoxigenic pathogens such as F. pseudograminearum through nutrient competition [50] and F. culmorum and F. graminearum [51] were also reported. It has been demonstrated that atoxigenic A. flavus strains are powerful BCAs to control the toxigenic strains of A. flavus in cottonseed [52–54], maize [27, 55–57] and various types of nuts [58–61]. Currently, different strains of atoxigenic A. flavus are being used depending on the endemic area and sometimes a mixture of strains is used in the field. This competitive exclusion theory has been recently confirmed in situ by coinoculating corn kernels with GFP-labeled AF70 and wild-type AF36. The study showed that there is a population difference (up to 82% reduction) between the co-inoculated kernels with both fungi and the control one inoculated only with GFP-labeled AF70 after visualizing under UV. Furthermore, aflatoxins (AFs) analysis showed a 73% reduction compared to the control [62].

However, AFs are not the only toxic compounds produced by A. flavus. Cyclopiazonic acid (CPA) is another mycotoxin produced by certain strains of A. flavus, including the atoxigenic strains, affecting mainly the liver and muscles of livestock [63, 64]. As an example, the commercially registered BCAs AF36, while it is effective against toxigenic A. flavus, it has been confirmed for its CPA production in cottonseeds. Therefore, researchers screened and tested new strains lacking the production of both toxins for the same previously mentioned crops [65–67]. Testing atoxigenic strains of A. flavus against other AFs producing fungi such as A. parasiticus was less common because A. parasiticus is less virulent and not predominantly occurs in the soil as A. flavus [59].

Competition for nutrient and niche can also be seen in Trichoderma and Clonostachys spp. when they are applied before pathogen occurrence [11, 68]. Trichoderma spp., especially T. harzianum, produce siderophores, low-molecular-mass ferric-iron-specific chelators, when the available iron in the environment is low [23]. Siderophores chelate the oxidized ferric ions (Fe + 3) making it available as an iron source [24, 37, 69] and this enables Trichoderma spp. to compete for iron which is an essential element for the development of many plant pathogens [24, 68].
