Exploration of non-phytopathogenic Aspergillus spp. isolates recovered from soil and compost as potential source of bioactive metabolites for potato Fusarium dry rot control.

During storage, infected potato tubers by Fusarium species leads to significant losses. Searching natural-based alternatives to chemical fungicides for the control of tuber dry rot pathogens is becoming essential. Nine Aspergillus spp. (A. niger, A. terreus, A. flavus, and Aspergillus sp.) isolates, recovered from soil and compost samples, were explored and evaluated for their ability to suppress Fusarium sambucinum the main causal agent of potato tuber dry rot disease in Tunisia. All conidial suspensions of Aspergillus spp. tested and their cell-free culture filtrates had significantly inhibited the in vitro pathogen growth by 18.5 to 35.9% and by 9 to 69% compared to control, respectively. A. niger CH12 cell-free filtrate was the most active against F. sambucinum at the three concentration tested (10, 15, and 20% v v-1). Chloroform and ethyl acetate extracts from four Aspergillus spp., tested at 5% v v-1, had limited F. sambucinum mycelial growth by 34-60% and 38-66%, respectively, compared to control, with A. niger CH12 ethyl extract being the most active. Tested on potato tubers inoculated with F. sambucinum, all tested Aspergillus spp. isolates, their cell-free filtrates and organic extracts had significantly decreased the external diameter of dry rot lesion compared to pathogen-inoculated and untreated control tubers. For the rot penetration, all Aspergillus spp. isolates, their organic extracts and only filtrates from A. niger CH12 and MC2 isolates had significantly limited dry rot severity compared to pathogen-inoculated and untreated control. The highest reductions in the external diameter of dry rot lesion (76.6 and 64.1%) and the average rot penetration (77.1 and 65.1%) were achieved using chloroform and ethyl acetate extracts from A. niger CH12, respectively. These results clearly demonstrated the presence of bioactive compounds in Aspergillus spp. that can be extracted and explored as an eco-friendly alternative for the control of the target pathogen.

Ability of endophytic fungi associated with Withania somnifera L. to control Fusarium Crown and Root Rot and to promote growth in tomato.

Fusarium crown and root rot (FCRR), caused by Fusarium oxysporum f. sp. radicis-lycopersici (FORL), is a soilborne tomato disease of increased importance worldwide. In this study, Withania somnifera was used as a potential source of biological control and growth-promoting agents. Seven fungal isolates naturally associated with W. somnifera were able to colonize tomato seedlings. They were applied as conidial suspensions or a cell-free culture filtrate. All isolates enhanced treated tomato growth parameters by 21.5-90.3% over FORL-free control and by 27.6-93.5% over pathogen-inoculated control. All tested isolates significantly decreased by 28.5-86.4% disease severity over FORL-inoculated control. The highest disease suppression, by 86.4-92.8% over control and by 81.3-88.8% over hymexazol-treated control, was achieved by the I6 isolate. FORL radial growth was suppressed by 58.5-82.3% versus control when dual cultured with tested isolates and by 61.8-83.2% using their cell-free culture filtrates. The most active agent was identified as Fusarium sp. I6 (MG835371), which displayed chitinolytic, proteolytic, and amylase activities. This has been the first report on the potential use of fungi naturally associated with W. somnifera for FCRR suppression and for tomato growth promotion. Further investigations are required in regard to mechanisms of action involved in disease suppression and plant growth promotion.

Efficacy of Bacillus subtilis V26 as a biological control agent against Rhizoctonia solani on potato.

The aim of this study is to evaluate the efficacy of the strain Bacillus subtilis V26, a local isolate from the Tunisian soil, to control potato black scurf caused by Rhizoctonia solani. The in vitro antifungal activity of V26 significantly inhibited R. solani growth compared to the untreated control. Microscopic observations revealed that V26 caused considerable morphological deformations of the fungal hyphae such as vacuolation, protoplast leakage and mycelia crack. The most effective control was achieved when strain V26 was applied 24h prior to inoculation (protective activity) in potato slices. The antagonistic bacterium V26 induced significant suppression of root canker and black scurf tuber colonization compared to untreated controls with a decrease in incidence disease of 63% and 81%, respectively, and promoted plant growth under greenhouse conditions on potato plants. Therefore, B. subtilis V26 has a great potential to be commercialized as a biocontrol agent against R. solani on potato crops.