Strawberry is a high value crop grown in Egypt. It is widely infected by many pathogens with a consequent broad distribution of the black root rot disease. Control of such fungal diseases, using different chemical fungicides such as actamyl (used herein) has hazardous side effects on human beings and animals. Hence, other management tactics such as biocontrol agents can constitute a safe alternative of controlling fungal diseases.
It is logic to assume that the yield increase resulted, at least partly, from controlling the fungal pathogens. In this respect, Abd-Elbaky et al. (2012) found that B. pumilus applications significantly increased onion bulb yield, whereas Shalaby et al. (2013) reported that B. subtilis treatment enhanced the chlorophyll content, the development of the root and foliage systems, the dry matter of the foliage, and the bulb mass of onion plants grown under field conditions. Kim et al. (2003) reported that 2 Bacillus strains could inhibit the growth of several plant pathogens such as R. solani and Pythium sp. Also, Vasebi et al. (2013) showed that Bacillus sp. inhibited the mycelial growth of M. phaseolina by 63.3% with consequent increase of the peanut growth under 2 soil regimes.
Various mechanisms/modes of action have been proposed to explicate the role of such antagonistic organisms in suppressing the growth with consequent incidence and severity of such pathogens. These could include a number of approaches such as competition, antibiosis, cell wall degradation, mycoparasitism, induced resistance, and rhizosphere colonization capability. Herein, such mechanisms of the bacterial isolates related to Bacillus spp. are usually displayed in terms of production of antibiotic-type secondary metabolites and competition with the pathogens for nutrients and space (Sivanantham et al. 2013). Moreover, Bacillus sp. strains are well known from previous studies as plant growth-promoting agents (Wahyudi et al. 2011). Factually, many Bacillus spp. in addition to other fungal pathogens of plants, accounting for the induced system resistance (ISR), can significantly elicit decrease in the incidence of numerous diseases on major host crops (Kloepper et al. 2004). These authors reported that ISR by the Bacillus spp. has been demonstrated against various diseases in both greenhouse and field trials on crops like watermelon, muskmelon, tomato, tobacco, cucumber, bell pepper, sugar beet, Arabidopsis sp., and loblolly pine. Moreover, B. pumilus spores generally showed remarkable resistance not only to environmental stresses such as UV light exposure, desiccation, and the presence of oxidizers, but strains of B. pumilus were also found to be resistant to hydrogen peroxide (Kempf et al. 2005).
Surely, it is preferable that an antagonistic organism can antagonize these pathogenic fungi and others using multiple approaches so that it can more effectively control them. For example, 5 PGPR of different genera and characterized with phosphate solubilizing and root colonizing ability could significantly increase tomato seed germination, seedling vigor, and growth and fruit weight. Babu et al. (2015) speculated that such improvements in tomato plants might be partly attributed to the ability of the PGPR to produce indole acetic acid and enhance nutrient uptake and chlorophyll content in the treated plants. Moreover, IPM as a preferable strategy may be followed. So, the antagonistic organism could be applied in conjugation with other control measures. In such a case, a compatibility test should ensure that none of the involved measures are mutually suppressed.
Furthermore, the recent determination of the nucleic acid sequence for the whole B. pumilus GLB197 genome by Zeng et al. (2020) may aid in grasping biological traits relevant to biocontrol against plant pathogens. Eventually, production practices such as crop rotation, tillage, fallow periods, and pesticide uses can directly disrupt populations of antagonistic organisms. These practices can also indirectly and adversely affect antagonists by decreasing their pathogen host(s). Hence, a major confront of conservation biocontrol is that practices intended to protect or enhance suppression of pathogens may not be effective in all field sites because they are dependent on indigenous antagonists (Sharma 2011, Timper 2014). Therefore, indicators will need to be characterized. These may include the existing particular antagonists, which can direct judgments on where it is effective to use conservation biocontrol. In future research, B. pumilus should be examined for managing the root rot disease of strawberry on large scale in Egypt. It should focus on factors that limit suppression of pathogens causing black root rot disease of strawberry plants because changes in abundance of particular antagonists may not affect biocontrol of plant pathogens.