Podosphaera fusca is the main causal agent of cucurbit powdery mildew in Spain. Four Bacillus subtilis strains, UMAF6614, UMAF6619, UMAF6639, and UMAF8561, with proven ability to suppress the disease on melon in detached leaf and seedling assays, were subjected to further analyses to elucidate the mode of action involved in their biocontrol performance. Cell-free supernatants showed antifungal activities very close to those previously reported for vegetative cells. Identification of three lipopeptide antibiotics, surfactin, fengycin, and iturin A or bacillomycin, in butanolic extracts from cell-free culture filtrates of these B. subtilis strains pointed out that antibiosis could be a major factor involved in their biocontrol ability. The strong inhibitory effect of purified lipopeptide fractions corresponding to bacillomycin, fengycin, and iturin A on P. fusca conidia germination, as well as the in situ detection of these lipopeptides in bacterial-treated melon leaves, provided interesting evidence of their putative involvement in the antagonistic activity. Those results were definitively supported by site-directed mutagenesis analysis, targeted to suppress the biosynthesis of the different lipopeptides. Taken together, our data have allowed us to conclude that the iturin and fengycin families of lipopep-tides have a major role in the antagonism of B. subtilis toward P. fusca.

Aim: This study was undertaken to isolate Bacillus subtilis strains with biological activity against soil-borne phytopathogenic fungi from the avocado rhizoplane. Methods and Results: A collection of 905 bacterial isolates obtained from the rhizoplane of healthy avocado trees, contains 277 gram-positive isolates. From these gram-positive isolates, four strains, PCL1605, PCL1608, PCL1610 and PCL1612, identified as B. subtilis, were selected on the basis of their antifungal activity against diverse soil-borne phytopathogenic fungi. Analysis of the antifungal compounds involved in their antagonistic activity showed that these strains produced hydrolytic enzymes such as glucanases or proteases and the antibiotic lipopeptides surfactin, fengycin, and/or iturin A. In biocontrol trials using the pathosystems tomato/Fusarium oxysporum f.sp. radicis-lycopersici and avocado/Rosellinia necatrix, two B. subtilis strains, PCL1608 and PCL1612, both producing iturin A, exhibited the highest biocontrol and colonization capabilities. Conclusions: Diverse antagonistic B. subtilis strains isolated from healthy avocado rhizoplanes have shown promising biocontrol abilities, which are closely linked with the production of antifungal lipopeptides and good colonization aptitudes. Significance and Impact of the Study: This is one of the few reports dealing with isolation and characterization of B. subtilis strains with biocontrol activity against the common soil-borne phytopathogenic fungi F. oxysporum f.sp. radicis-lycopersici and R. necatrix.

SUMMARY: Numerous vegetable crops are susceptible to powdery mildew, but cucurbits are arguably the group most severely affected. Podosphaera fusca (synonym Podosphaera xanthii ) is the main causal agent of cucurbit powdery mildew and one of the most important limiting factors for cucurbit production worldwide. Although great efforts have been invested in disease control, by contrast, many basic aspects of the biology of P. fusca remain unknown. Taxonomy: Podosphaera fusca (Fr.) Braun & Shishkoff. Kingdom Fungi; Phylum Ascomycota; Subdivision Pezizomycotina; Class Leotiomycetes; Order Erysiphales; Family Erysiphaceae; genus Podosphaera ; species fusca. Identification: Superficial persistent mycelium. Conidia in chains, hyaline, ellipsoid to ovoid or doliform, about 24–40 × 15–22 µm, with cylindrical or cone‐shaped fibrosin bodies, which often germinate from a lateral face and produce a broad, clavate germ tube and cylindrical foot‐cells. Unbranched erect conidiophores. Cleistothecia globose, mostly 70–100 µm in diameter, dark brown/black. One ascus per cleistothecium with eight ascospores. Host range: Angiosperm species that include several families, such as Asteracea, Cucurbitaceae, Lamiaceae, Scrophulariaceae, Solanaceae and Verbenaceae. Disease symptoms: White colonies develop on leaf surfaces, petioles and stems. Under favourable environmental conditions, the colonies coalesce and the host tissue becomes chlorotic and usually senesces early. Control: Chemical control and the use of resistant cultivars. Resistance has been documented in populations of P. fusca to some of the chemicals registered for control.

Summary: Biological control of plant diseases has gained acceptance in recent years. B acillus subtilis UMAF 6639 is an antagonistic strain specifically selected for the efficient control of the cucurbit powdery mildew fungus P odosphaera fusca , which is a major threat to cucurbits worldwide. The antagonistic activity relies on the production of the antifungal compounds iturin and fengycin. In a previous study, we found that UMAF 6639 was able to induce systemic resistance ( ISR ) in melon and provide additional protection against powdery mildew. In the present work, we further investigated in detail this second mechanism of biocontrol by UMAF 6639. First, we examined the signalling pathways elicited by UMAF 6639 in melon plants, as well as the defence mechanisms activated in response to P . fusca . Second, we analysed the role of the lipopeptides produced by UMAF 6639 as potential determinants for ISR activation. Our results demonstrated that UMAF 6639 confers protection against cucurbit powdery mildew by activation of jasmonate‐ and salicylic acid‐dependent defence responses, which include the production of reactive oxygen species and cell wall reinforcement. We also showed that surfactin lipopeptide is a major determinant for stimulation of the immune response. These results reinforce the biotechnological potential of UMAF 6639 as a biological control agent.