Nine isolates of Aspergillus spp., isolated from soil and compost were tested in vitro and in vivo for their antifungal activity against Fusarium sambucinum and Phytophthora erythroseptica, the causal agents of the Fusarium dry rot and pink rot of potato tubers. Tested using the dual culture method, the pathogen growth of F. sambucinum and P. erythroseptica was inhibited by 27 to 68% and 16 to 25% by all Aspergillus species, respectively. The highest inhibitory activity against both pathogens was induced by the isolate CH12 of A. niger. A significant reduction of the mycelial growth of both pathogens tested using the inverse double culture method involves the presence of volatile antifungal metabolites. Their effectiveness was also evaluated as tuber treatment prior to inoculation with the pathogens. The highest effectiveness in reducing Fusarium dry rot severity was recorded on tubers treated with the isolate CH12 of A. niger. This study also revealed that the efficacy of Aspergillus spp. as biocontrol agents may be enhanced by varying the timing of their application. In fact, the lesion diameter of dry and pink rots was reduced by 54-70 and 52% with preventive application, respectively. However, this parameter decreased by 21-48 and 47% when the Aspergillus spp. were applied simultaneously with pathogens, respectively. Similarly, diseases' severity, estimated based on average penetration of F. sambucinum and P. erythroseptica, was reduced by 57-77 and 55% with preventive treatments and by 29-68 and 44% with simultaneous application, respectively. This study reveals that Aspergillus spp., isolated from compost and soil, exhibits an interesting antifungal activity toward F. sambucinum and P. erythroseptica and may represent a potential source of biopesticide. Testing of their culture filtrates, their organic extracts and their toxicity may give additional information on their safe use as biocontrol agents.
Two resistance inducers (RIs), chitosan and salicylic acid (SA), were assessed in vitro for their antifungal activity against ten tomato phytopathogenic fungi i.e. Fusarium oxysporum f. sp. lycopersici, F. oxysporum f. sp. radicislycopersici, F. solani, Verticillium dahliae, Rhizoctonia solani, Colletotrichum coccodes, Pythium aphanidermatum, Sclerotinia sclerotiorum, Botrytis cinerea, and Alternaria solani. The impact of these RIs, applied as soil drench, on Verticillium wilt, Fusarium wilt, and Fusarium Crown and Root Rot severity and on growth parameters of tomato cv. Rio Grande plants were also investigated. Chitosan (0.5-4 mg/ml) and SA (1-25 mM) inhibited mycelial growth of all pathogens in Potato Dextrose Agar (PDA) medium in a concentration-dependent manner, with the greatest inhibition achieved using the highest chitosan and SA concentrations. Inter specific variations in sensitivity to chitosan and SA were detected. P. aphanidermatum and S. Sclerotiorum were the most sensitive to both RIs. Single treatments with chitosan (4 mg/ml) and SA (10 mM) resulted in varied degree of protection against wilt diseases. Chitosan-and SA-based treatments resulted in 42.1-73.68, 60.86-78.26 and 45-50% reductions in wilt severity, as compared to VD-, FOL-and FORL-inoculated and untreated controls, respectively. All growth parameters noted were enhanced using RIs compared to pathogen-inoculated controls. In fact, SA-based treatment had significantly increased plant height, root and aerial part fresh weights by 17.94, 52.17 and 33.33%, by 23.01, 55.40 and 29.72%, and by 17.72, 50 and 46.84%, respectively, while compared to VD-, FOL-and FORL-inoculated and untreated plants. Chitosantreated plants showed increment in their height, root and aerial part fresh weights by 13.81, 62.16 and 38.97%, respectively, compared to FORL-inoculated and untreated control. Results from this investigation showed that SA and chitosan may be used as potential inducers of systemic acquired resistance for successfully controlling fungal tomato diseases in Tunisia.
Six isolates of culturable bacteria, isolated from stems of wild Solanaceae species (Datura metel, Solanum nigrum, S. elaeagnifolium, and Nicotiana glauca), were assessed for their antifungal activity against F. oxysporum f. sp. lycopersici (FOL), the causal agent of the tomato Fusarium wilt. Blast analysis of 16S rDNA sequencing genes homology showed that the isolates belonged to the genus Bacillus (Bacillus cereus str. S42, B. tequilensis str. SV39, B. subtilis str. SV41, B. methylotrophicus str. SV44, B. amyloliquefaciens subsp. plantarum str. SV65, and B. mojavensis str. SV104). The mycelium growth of FOL was significantly reduced by 36 to 46% by diffusible metabolites and by 18 to 21% by volatile compounds. Cell-free cultures were found to be mostly active when issued from 4 daysold cultures where FOL growth inhibition significantly varied from 31.1 to 59.5%. Active metabolites present in the cell-free cultures were extracted with n-butanol and chloroform. Both organic extracts exhibited antifungal potential towards FOL higher than that induced by the two commercial products i.e. Bavistin ® (50%, chemical fungicide) and Bactospeine ® (16000UI/mg, biopesticide). This study clearly indicates that endophytic Bacillus spp. from wild Solanaceae species can be used as natural sources of bioactive metabolites towards FOL. Wild Solanaceae are frequent in Tunisia and were not explored as potent sources of candidate antagonistic bacteria. In view of the endogenous progress of the pathogen via the vascular tissues, the use of endophytic bacteria can suppress tomato Fusarium wilt disease.
Potato is threatened by several soil-borne fungi causing wilt and root rots. In this study, two fodder radish (Raphanus sativus L.) (FR) cultivars (cvs. Boss and Defender), used as green manure preceding a potato crop, were evaluated for their suppressive effects against wilt incidence and severity, potato growth and yield as compared to animal manure. The essay was carried out in a completely randomised design with three types of organic amendment and two potato cultivars (cvs. Spunta and Royal). Incidence of potato wilting noted 100 days post planting (DPP) was high, exceeding 70%, for all soil amendments tested. The extent of vascular discoloration varied depending on amendments used where cv. Defender behaved as the control while the highest extent was noted on potato plants grown in cv. Boss amended plots. As compared to animal manure, the application of cvs. Boss and Defender had increased by 48.43 and 41.28% the incidence of vascular discoloration on cv. Spunta, respectively, while on cv. Royal, only cv. Defender had reduced this parameter by 16.32%. Fungal isolations performed from roots and stems revealed the involvement of several soil-borne pathogens in the recorded plant wilting. Soil manuring using cvs. Boss and Defender FR resulted in significant increment in average stem number per plant and aerial part fresh weight by 22.79 and 21.32% and by 34.62 and 27.03%, respectively, as compared to animal manure. At 100 DPP, potato root fresh weight increase by 8.7 and 33.49% was noted on cv. Spunta compared to 30.34 and 23.48% recorded on cv. Royal. Potato tuber yield was improved by 38.28 and 10.7% and by 28.44 and 27.62% in cvs. Spunta and Royal, respectively, relative to animal manure. The use of FR as green manure may be implemented in the integrated management of soil-borne diseases for the enhancement of potato yield.
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