The in vitro results exhibited various relationships including competition, antibiosis and mycoparasitism of the respective test isolates against FORL. This fungus-on-fungus interaction is most likely attributed to the release of labile chemicals by respective fungi that affect the development and survival of the microorganisms with which they interact [18; 19]. The inhibitory action of A. fumigatus and P. concavorugulosum-2 on FORL was established to be due to mycoparasitism, an interaction that occurs when one fungal species parasitizes another [20]. Gliotoxin, a pleiotropic mycotoxin that is produced by A. fumigatus has been reported to cause cytotoxicity in other fungal isolates, including A. flavus, A. oryzae and F. graminearum [21]. This could have caused the rapid degradation and reduced growth of FORL. Myco-parasitism established on P. concavorugulosum-2 against FORL could be because FORL released extrolites that acted as a nutritional source for P. concavorugulosum-2 [22].
Aspergillus fumigatus and P. concavorugulosum-2 demonstrated rapid growth and a mycoparasitism mechanism against FORL as their mycelia expressed a degradation effect. This was observed with mycelia of these respective isolates overlaying FORL mycelia. The two test isolates could probably have catabolized metabolites produced by FORL to decrease its growth and survival rate. Chatterjee et al. [23] reported that species-specific metabolite catabolism by the producer or an organism in the same growth environment is possible. The good performance of P. concavorugulosum-2 against FORL is indicative of its potential use as a biocontrol agent against the pathogen. The Penicillium species could be used as a single agent or be used in combination with other Penicillium isolates such as P. oxalicum that also showed a suppressive effect against Fusarium [24]. It is expected that a synergistic effect of collective activity against FORL of the combined fungal species could be generated. Further studies should therefore be undertaken to determine whether such synergistic effect could be achieved.
The antibiosis interaction i.e., growth inhibition zone of P. commune-2 against FORL that was established (Fig. 1 (e)) was possible because of the production of metabolites that can weaken and reduce competitiveness and growth of other microorganisms. For example, Perincherry et al. [23] reported on the production of toxic secondary metabolites by Fusarium species during various interactions with plants or other microbes. These metabolites can therefore influence/weaken the growth of other microbial species. Fusarium species have been reported to produce mycotoxins such as zearalenone, deoxynivalenol, fumonisins and trichothecenes that can invade cell membranes, alter microbial growth patterns and increase pathogen multiplication [23]. Therefore, this might have given FORL growth and multiplication advantage over P. commune-2. Furthermore, the growth inhibition zone observed during the incubation stage may be explained by the production of inhibitory metabolites by FORL that led to the growth-free zone, allowing both species to grow but giving FOR competitive growth [24].
There was an observed strong variation of the relationship between test isolates and FORL, ranging from weakly negative to moderate positive associations. Correlation analysis between A. fumigatus and FORL showed a very weak positive relationship where r = 0.062 [Fig. 2(c)] compared to moderate correlation value between P. concavorugulosum-2 and FORL that showed a moderate negative relationship of r = \(-\)0.43 [Fig. 2 (e)]. This means P. concavorugulosum-2 expressed a range of fungal suppressive properties i.e., competitive, antibiotic and myco-parasitic properties. A similar expression was established between A. fumigatus and FORL albeit at a weaker level [Fig. 2(c)]. The moderate positive association between P. commune-1 and FORL showed no indication of pathogen suppressiveness [Fig. 2(a)]. There was no evident relationship between P. commune-1 and FORL on all the five assessments. There was, however, a relatively weak negative association of P. commune-2 and FORL indicating a decline of P. commune-2, and an increase of FORL.
The negative correlation established between P. concavorugulosum-2 and FORL is indicative of the inhibition ability of the test fungus against the pathogen. The inhibition characteristics of a test isolate exerted on FORL that was calculated and expressed in percentages showed P. concavorugulosum-2 had the highest inhibitory effect against FORL (67.9%) followed by A. fumigatus (65.36%) (Fig. 3). The other test isolates’ inhibition was not very significant as they ranged from 9–29%. The results demonstrated that the indigenous isolate, P. concavorugulosum-2, significantly inhibited FORL development on culture media; demonstrating that the test isolate may have had a direct effect, such as competition and mycoparasitism.