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 lipopeptides have a major role in the antagonism of B. subtilis toward P. fusca.

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Diseases of fruit trees caused by pathovars of Pseudomonas syringae van Hall are of major concern in fruitproducing areas worldwide, are exceedingly difficult to control, and result in significant economic losses. The pathogen has the ability to kill both young and older trees. Systemic infection and death of young trees is a perennial problem in nurseries, and canker development leading to the girdling and death of scaffold limbs and entire trees is a common event that can lead to the rapid demise of older orchards. For example, bacterial canker of plum caused by P. syringae pv. syringae causes annual tree mortality rates as high as 30% in Germany (43), and bacterial canker of hazelnut has resulted in the mortality of large numbers of trees in Italy and other European countries (77). Yield losses due to lesions on fruit are typically more sporadic in occurrence and variety dependent, but significant yearly losses have been reported in some instances. Finally, the pathogen's trait of ice nucleation activity also could exacerbate the importance of frost injury as a predisposing factor for infection. Frost damage in commercial orchards is a sporadic problem that facilitates colonization and the initiation of pathogenesis for these opportunistic pathogens.P. syringae and its close relatives cause diseases of monocots, herbaceous dicots, and woody dicots, and these pathogens utilize an impressive array of virulence factors such as effectors, toxins, and phytohormones to incite disease symptoms. Aspects of the systematics, ecology, and genetics of P. syringae have been reviewed and discussed by many authors (cited literature examples: 6,27,44,47,52). Extensive recent research efforts have focused on Pseudomonas diseases of herbaceous plants, and our understanding of P. syringae-host interactions has been facilitated by genome sequencing, comparative genomics, and functional studies (cited literature examples: 21,31,96).Our understanding of and ability to manage P. syringae diseases of fruit trees is relevant to our understanding of P. syringae diseases of herbaceous hosts, and vice versa. The exciting new insights into host-pathogen interactions from the model systems can give us clues on how to pro-ceed with the less genetically pliable fruit tree systems. Studies of the fruit tree systems, and comparisons with the model systems, can provide insights into the interactions of P. syringae strains with woody hosts and address relevant questions, such as: What is the role of type III secretion in infection of woody tissues? What virulence traits enable the colonization of woody tissue by P. syringae? Are there differences in the roles of toxins in diseases of woody hosts and herbaceous hosts? How did some pathogens evolve the ability to induce galls in woody hosts? Can resistance mechanisms characterized in the model-system herbaceous hosts be exploited in fruit trees?The objectives of this article are (i) to provide an overview of several Pseudomonas diseases of fruit trees, (ii) to discuss the current and emerging unde...

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.

The biocontrol activity of many Bacillus species has been traditionally related to the direct antagonism of pathogens. In previous works, we reported that B. subtilis strain UMAF6614 was an efficient biocontrol agent that produced bacillomycin, fengycin and surfactin lipopeptides. Bacillomycins and fengycins were shown to have antagonistic activity towards fungal and bacterial pathogens of cucurbits; however, the functionality of surfactin remained unclear. In this study, the role of surfactin in the biocontrol activity of this strain was investigated. We observed that a deficiency in surfactin production led to a partial reduction of disease suppression by this biocontrol agent, which coincided with a defect in biofilm formation and the colonization of the melon phylloplane. These effects were due to a dramatic reduction in the production of exopolysaccharide and the TasA protein, which are the two major components of the extracellular matrix. We propose that the biocontrol activity of this strain is the result of the coordinated action of the three families of lipopeptides. B. subtilis UMAF6614 produces surfactin to trigger biofilm formation on melon phylloplane, which ensures the long-term persistence and the adequate secretion of suppressive lipopeptides, bacillomycins and fengycins, which efficiently target pathogens.