New pest management strategies are based on decrease the use of synthetic pesticides and increase the use of natural alternatives. Therefore, introducing new natural products with low risk on human and environment, and new modes of action is highly needed in the field pest management nowadays32. In this regard, microorganisms are a rich source for natural compounds with successful use in pest management33,34. In this study, the herbicidal, fungicidal, insecticidal and antifeedant activities of twelve secondary metabolites isolated from fungi were investigated to explore their possible use in pest control programs.
The results showed that most of the tested compounds reduced seed germination and seedling growth of S. marianum at concentration of 500 mg/l. Among the eleven tested metabolites, compounds 6 and 11 revealed excellent herbicidal activity with complete inhibition of germination and seedling growth at this concentration. Moreover, compound 6 showed strong herbicidal activity at 25 mg/l. Based on the obtained results, this compound is among the most effective natural herbicides reported so far. In our previous study, compounds 6 and 11 were also the most potent inhibitors of seed germination, and growth of root and shoot of Echinochloa crus-galli at concentration of 2mM35. Also, compound 6 was found to be more effective than compound 11 against E. crus-galli. The herbicidal activity of compound 11 was further supported by the studies of Shiono and Murayama36 who stated that this compound inhibited the root growth of lettuce by 46% at concentration of 50 mg/l. In addition, Compound 6 has been described to inhibit wheat germination and the root growth of Allium cepa37.
Comparing the inhibitory effects of tested compounds on the seed germination with seedling growth of S. marianum indicated that the tested compounds were more effective against seedling growth than germination. These findings are in agreement with previous studies on many natural compounds in which the seedling growth were more sensitive than germination38–40. Also, the results of current study revealed that the tested compounds were more potent inhibitors to root than to shoot with few exceptions. The higher sensitivity of roots could be due to the roots are the first to expose and absorb tested compounds from growing media41. Similar observations were reported on the inhibitory effects of other natural compounds on seedling growth42–44.
Antifungal activity experiments showed that compounds (2, 4 and 7) and compounds (2 and 3) exhibited pronounced inhibitory effect on the mycelial growth of F. oxysporum and A. solani, respectively. To the best our knowledge, this is the first report on the antifungal properties of isolated secondary metabolites on plant pathogenic fungi. However, compound 6 has been reported to possess antifungal activity against Aspergillus niger, Candida albicans, and Trichophyton rubrum45. Also, da Silva et al.46 found that compound 12 had antifungal potential against C. albicans and C. tropicalis (MIC = 125 mg/l). On the other hand, some of the tested compounds have been described to have antibacterial activity against plant pathogenic bacteria. For example, compounds 3 and 11 inhibited the growth of Rhizobium radiobacter, Ralstonia solanacearum, Pectobacterium carotovorum subsp. carotovorum and Pseudmonas syringae pv. syringae with MIC values ranged between 125 and 500 mg/l. Furthermore, compounds 2 and 4 inhibited the growth of R. solanacearum and P. carotovorum subsp. carotovorum, respectively, with MIC value of 125 mg/l35.
The results of larvicidal bioassays against C. pipiens demonstrated that among the tested metabolites, compounds 9, 4 and 5 had a promising larvicidal activity. In particular, compound 9 which displayed the highest toxicity with LC50 = 3.27 mg/l. Based on our results, this compound is among the most toxic natural compounds reported so far against C. pipiens or other mosquito species. This finding is supported by previous study of Geris et al.47 who found that compound 9 was highly toxic against third instar larvae of Aedes aegypti with LC50 value of 2.9 mg/l. Compound 9 was more toxic to C. pipiens larvae than thymol (LC50 = 37.95 mg/l), carvacrol (LC50 = 44.38 mg/l), cinnamaldehyde (LC50 = 58.97 mg/l), eugenol (LC50 = 86.22 mg/l) and cuminaldehyde (LC50 = 38.94 mg/l)48,49. Similarly, compound 9 showed higher toxicity than curcumin (LC50 = 19.07 mg/l) isolated from Curcuma longa50 (Sagnou et al. 2012). Moreover, this compound showed similar toxicity to caulerpin (LC50 = 1.99 mg/l) and caulerpinic acid (LC50 = 4.89 mg/l) isolated from Caulerpa racemosa51.
The results indicated that the tested compounds had weak antifeedant activity with only compounds 4, 5, 7 and 11 being active antifeedants against S. littoralis at 1000 mg/l. Similar findings were concluded by Li et al.52 who examined the antifeedant activity of thirty-nine fungal metabolites isolated from Aspergillus fumigatus LN-4 against larvae of Mythimna separata Walker and found that only two compounds (fumitremorgin B and verruculogen) were active antifeedants.
The results of the current study indicate that the tested compounds showed high selectivity against the examined pests. For example, compounds 6 and 11 showed promising herbicidal activity against S. marianum while had no antifungal or insecticidal activity. Likewise, compounds 5 and 9 possessed potent insecticidal activity against C. pipiens but were not active against tested weed or fungi. Finally, compounds 2 and 7 had remarkable antifungal activity, while they showed no insecticidal or herbicidal activity. This selectively is crucial for the development of these compounds as new bio-pesticides.
In conclusion, this study described, for the first time, the herbicidal, antifungal, insecticidal and antifeedant activities of twelve fungal metabolites against the economic agricultural pests, S. marianum, F. oxysporum, A. solani, C. pipiens and S. littoralis. Among the tested metabolites, brefeldin A (6) and 6-eopxy-4-hydroxy-3-methoxy-5-methyl-cyclohex-2-en-1-one (11) revealed promising herbicidal activity against S. marianum, in particular, brefeldin A (6) which could be developed as a new natural herbicide. In addition, dehydroaustin (9) exhibited remarkable larvicidal activity against C. pipiens indicating its possible use in integrated management programs of mosquitoes.