The biocontrol potential of antagonistic bacteria and yeasts isolated from tomato phyllosphere and fructoplane was evaluated against fruit rot of tomato caused by Alternaria alternata. A total of 48 bacterial and 24 yeast isolates were obtained from field-grown tomato leaves/fruits collected from Barka, Oman. The antagonistic activities of these isolates were evaluated against A. alternata using in vitro dual culture assay. Among the 72 bacterial and yeast isolates tested, one yeast (Meyerozyma guilliermondii) and two bacteria (Enterobacter roggenkampii and Pseudomonas aeruginosa) showed inhibitory activity against A. alternata. Examination of hyphal morphology of A. alternata at the edge of inhibition zone using scanning electron microscopy revealed abnormalities such as twisting, shriveling, distortion and collapsing of hyphae. Postharvest treatment of tomato fruits with E. roggenkampii, P. aeruginosa and M. guilliermondii reduced the fruit rot lesion size by 60%, 57% and 50%, respectively, compared to untreated control. The antifungal metabolites produced by the antagonistic yeast M. guilliermondii were fractionated using thin-layer chromatography. Characterization of the bioactive fractions using gas chromatography-mass spectrometry revealed the presence of different types of alkanes, predominantly tricosane and pentacosane, suggesting that volatile compounds, especially alkanes, produced by M. guilliermondii might also be involved in the suppression of A. alternata.

This study was conducted to investigate the antagonistic activity of endophytic and rhizosphere fungi isolated from a medicinal plant, Sesuvium portulacastrum, against Pythium aphanidermatum, the cause of damping off of cucumber. A total of 40 endophytic and 19 rhizosphere fungi were isolated from S. portulacastrum. Three endophytic isolates and two rhizosphere isolates gave >50% suppression of P. aphanidermatum in the in vitro dual-culture tests. Scanning electron microscopic studies at the inhibition zone showed hyphae wall damage and abnormal mycelial growth of the genus Pythium. Molecular analysis identified the antagonistic endophytes as Aspergillus insulicola (isolate A435), A. insulicola (A419), and Aspergillus melleus (A412) and the rhizosphere antagonists as Aspergillus terreus (A213) and Aspergillus luchuensis (A116). Except for A116, the culture filtrates of the other antagonists significantly increased the electrolyte leakage from Pythium mycelia, whereas ethyl acetate extracts of A435, A412, and A213 showed significant growth suppression. All five antagonists were able to produce varying amounts of cellulase and β-glucanase enzymes. However, A435, A412, and A213 showed significantly higher cellulase activity, whereas A435 and A116 showed the highest β-glucanase activity. Controlled glasshouse growth experiments showed that isolates A435 and A116 resulted in up to 70% control of damping off, whereas isolates A412 and A213 showed 30 to 40% damping-off control. The antagonists A435, A116, and A213 also contributed to increased cucumber shoot length as well as shoot and root dry mass. The synergetic effects of metabolites and hydrolytic enzymes could be the reason for the variation between isolates in the antagonistic activity and cucumber growth promotion. This study reports for the first time A. insulicola, A. melleus, and A. luchuensis as potential biocontrol agents against P. aphanidermatum-induced damping off of cucumber.

The genus Micropsalliota is a group of saprotrophic mushrooms, characterized by small to medium-sized basidiomata with fibrillose-squamulose or glabrous pilei, ellipsoid to amygdaliform or cymbiform basidiospores and cheilocystidia that vary in shape from clavate to ventricose or lageniform with more or less a capitate apex. In this study, we described a new species of Micropsalliota from southern Oman. Species description is based on morphological features of basidiomata and phylogenetic analyses of nuc ribosomal DNA internal transcribed spacer (ITS) and D1–D2 domains of the 28S region. The new species Micropsalliota ventricocystidiata is characterized by a medium-sized basidiomata, with plano-convex pileus, covered with reddish-brown squamules and basidiospores are ellipsoid to amygdaliform with acute apex in side view and oblong in face view, measuring 7.5–8.5 × 4.5–5 µm. Morphological comparison of M. ventricocystidiata with respect to other species of the genus is provided. We discussed that species of the genus can be divided into two groups based on their pileus morphology and basidiospores size.

Soil biofumigation is a widely used farming practice for soil-borne disease management in vegetable crops. Soil biofumigation is the practice of the incorporation of fresh plant materials from the members of the Brassicaceae family into the soil which release antimicrobial volatile organic compounds (VOCs), namely glucosinolates and their hydrolysis products. However, the VOCs may affect non-target beneficial soil biota including microbial biocontrol agents (MBAs) due to their broad-spectrum toxicity. In this study, native antagonistic rhizobacteria were isolated from cabbage plants grown in biofumigated fields and their potential in the management of Pythium aphanidermatum cucumber damping-off was evaluated with and also without biofumigation using cabbage leaf residue. A total of 20 morphologically distinct, culturable bacterial strains were recovered from the rhizosphere soil of cabbage cultivated in a biofumigated field, with the largest fraction of the isolates belonging to the genus Bacillus. The other bacterial genera isolated were Pseudomonas, Serratia, Stenotrophomonas, Microbacterium and Priestia. Of the 20 bacterial isolates, Pseudomonas aeruginosa B1-SQU, Pseudomonas indica B2-SQU, Serratia marcescens B3-SQU and Pseudomonas brenneri B4-SQU exhibited inhibitory activity against P. aphanidermatum in a co-culture assay. The deformation and shrinkage of P. aphanidermatum hyphae due to diffusible antimicrobial compounds from these bacterial strains were witnessed through scanning electron microscopy. A biofilm was formed by these bacterial strains in vitro and they exhibited compatibility with each other; however, they had no significant plant growth promotion effect on cucumber. These bacterial strains significantly reduced damping-off in cucumber under greenhouse conditions when applied to soil singly, but failed to show a significant effect when combined with biofumigation treatment, though the bacterial population in the soil was maintained at higher levels. Soil biofumigation alone was also not effective in suppressing the damping-off of cucumber. Gas chromatography–mass spectrometry analysis revealed that 1-butanol, n-butyl butyrate and butyl acetate were the predominant VOCs in the soil amended with cabbage leaf residue. The results suggest that soil biofumigation with cabbage leaf residue has no significant effect on P. aphanidermatum under high inoculum levels; furthermore, it affects the efficiency of the bacterial antagonists in controlling damping-off in cucumber.

This study was conducted to investigate the presence of native antagonistic bacteria from compost and to evaluate their ability to suppress Pythium and Fusarium-induced damping-off of cucumber. Seven bacterial strains isolated from compost were screened for their in vitro antagonism against Pythium aphanidermatum and Fusarium solani. Out of these, two strains designated as ISO1 and ISO2 showed significant level of antagonism. Scanning electron microscopic observation of the mycelium of P. aphanidermatum and F. solani at the inhibition zones showed morphological changes in the hyphae including deformation, shrinkage and loss of turgidity. Based on 16S rDNA gene sequence analysis, both bacterial strains (ISO1 and ISO2) were identified as Pseudomonas aeruginosa. The cell-free culture filtrates of P. aeruginosa ISO1 and ISO2 significantly reduced the mycelial growth of P. aphanidermatum and F. solani under in vitro conditions. Compatibility analysis of the bacterial strains using cross-streak assay showed that they are compatible to each other. In addition, both strains were found effective in controlling P. aphanidermatum-induced damping-off of cucumber. Soil application of P. aeruginosa ISO1 and ISO2 reduced the yellowing and stunting of cucumber seedlings caused by F. solani. This study demonstrates the effectiveness of P. aeruginosa strains ISO1 and ISO2 isolated from compost in the management of damping-off of cucumber. It also shows that compost can harbour bacterial strains with antagonistic activity against P. aphanidermatum and F. solani.