Antibiosis

This study assessed the ability of two bio-based films, obtained from sodium alginate (NaAlg) and locust bean gum (LBG), to protect the viability of Wickerhamomyces anomalus cells and control the growth of Penicillium digitatum. The effect of microbial cell incorporation on physical properties of the developed films was evaluated in terms of barrier, mechanical and optical properties. Furthermore, the application of these two matrices as bioactive coatings was investigated in order to evaluate their efficacy in preserving the postharvest quality of 'Valencia' oranges and inhibiting the growth of P. digitatum on artificially inoculated fruits. Results showed that NaAlg and LBG films were able to maintain more than 85% of the initial W. anomalus yeast population and that the developed films incorporating the killer yeast completely inhibited the growth of P. digitatum in synthetic medium. Likewise, NaAlg and LBG coatings enriched with W. anomalus yeast were effective at reduci...

Dieback caused by the fungus Lasiodiplodia theobromae is an important disease on mango plantations in the United Arab Emirates (UAE). In this study, 53 actinobacterial isolates were obtained from mango rhizosphere soil in the UAE, of which 35 (66%) were classified as streptomycetes (SA) and 18 (34%) as non-streptomycetes (NSA). Among these isolates, 19 (12 SA and 7 NSA) showed antagonistic activities against L. theobromae associated with either the production of diffusible antifungal metabolites, extracellular cell-wall-degrading enzymes (CWDEs), or both. Using a " novel " mango fruit bioassay, all isolates were screened in vivo for their abilities to reduce lesion severity on fruits inoculated with L. theobromae. Three isolates, two belonging to Streptomyces and one to Micromonospora spp., showed the strongest inhibitory effect against this pathogen in vitro and were therefore selected for tests on mango seedlings. Our results revealed that the antifungal action of S. samsunensis UAE1 was related to antibiosis, and the production of CWDEs (i.e., chitinase) and siderophores; whilst S. cavourensis UAE1 and M. tulbaghiae UAE1 were considered to be associated with antibiotic-and CWDE-production, respectively. Pre-inoculation in greenhouse experiments with the most promising actinobacterial isolates resulted in very high levels of disease protection in mango seedlings subsequently inoculated with the pathogen. This was evident by the dramatic reduction in the estimated disease severity indices of the mango dieback of individual biocontrol agent (BCA) applications compared with the pathogen alone, confirming their potential in the management of mango dieback disease. L. theobromae-infected mango seedlings treated with S. samsunensis showed significantly reduced number of defoliated leaves and conidia counts of L. theobromae by 2-and 4-fold, respectively, in comparison to the other two BCA applications. This indicates that the synergistic antifungal effects of S. samsunensis using multiple modes of action retarded the in planta invasion of L. theobromae. This is the first report of BCA effects against L. theobromae on mango seedlings by microbial antagonists. It is also the first report of actinobacteria naturally existing in the soils of the UAE or elsewhere that show the ability to suppress the mango dieback disease.

Many fungal diseases affect date palm causing considerable losses in date production
worldwide. We found that the fungicide Cidely
R Top inhibited the mycelial growth of the
soil-borne pathogenic fungus Thielaviopsis punctulata, the causal agent of black scorch
disease of date palm, both in vitro and in vivo. Because the use of biocontrol agents
(BCAs) can minimize the impact of pathogen control on economic and environmental
concerns related to chemical control, we aimed at testing local actinomycete strains
isolated from the rhizosphere soil of healthy date palm cultivated in the United Arab
Emirates (UAE) against T. punctulata. The selected isolate can thus be used as a
potential agent for integrated disease management programs. In general, the BCA
showed antagonism in vitro and in greenhouse experiments against this pathogen. The
most promising actinomycete isolate screened showed the highest efficacy against the
black scorch disease when applied before or at the same time of inoculation with T.
punctulata, compared with BCA or fungicide application after inoculation. The nucleotide
sequence and phylogenetic analyses using the 16S ribosomal RNA gene with other
Streptomyces spp. in addition to morphological and cultural characteristics revealed
that the isolated UAE strain belongs to Streptomyces globosus UAE1. The antagonistic
activity of S. globosus against T. punctulata, was associated with the production by this
strain of diffusible antifungal metabolites i.e., metabolites that can inhibit mycelial growth
of the pathogen. This was evident in the responses of the vegetative growth of pure
cultures of the pathogen when exposed to the culture filtrates of the BCA. Altogether,
the pathogenicity tests, disease severity indices and mode of action tests confirmed that
the BCA was not only capable of suppressing black scorch disease symptoms, but also
could prevent the spread of the pathogen, as a potential practical method to improve
disease management in the palm plantations. This is the first report of an actinomycete,
naturally occurring in the UAE with the potential for use as a BCA in the management of
the black scorch disease of date palms in the region.