In the present study, 77 strains of endophytic actinomycetes isolated from cabbage were screened in order to assess their biocontrol potential against Alternaria brassicicola on cabbage seedlings. In the first and second screening trials, cabbage seedlings pretreated with mycelial suspensions of each isolate were spray-inoculated with A. brassicicola. Strain MBCN152-1, which exhibited the best protection in screening trials and had no adverse effects on seedling growth, was selected for the greenhouse trial. In the greenhouse trial, cabbage seedlings, which had been grown in plug trays filled with soil mix containing spores of MBCN152-1 (1×10 8 spores g -1 of soil mix), were spray-inoculated with A. brassicicola and grown in greenhouse conditions. MBCN152-1 reduced disease incidence and significantly increased the number of viable seedlings. The efficacy of MBCN152-1 against damping-off caused by seed-borne A. brassicicola was then evaluated. Cabbage seeds, artificially infested with A. brassicicola, were sown in soil mix containing MBCN152-1 spores. The disease was completely suppressed when infested seeds were sown in a soil mix blended with MBCN152-1 at 1.5×10 7 spores g -1 of soil mix. These results strongly suggest that MBCN152-1 has the potential to control A. brassicicola on cabbage plug seedlings. MBCN152-1 was identified as a Streptomyces humidus-related species based on 16S rDNA sequencing. Scanning electron microscopy showed that the hyphae of MBCN152-1 multiplied on the surface of the seedlings and penetrated their epidermal cells. In conclusion, strain MBCN152-1 is a promising biocontrol agent against A. brassicicola on cabbage plug seedlings.
Susceptible and resistant cultivars of cucumber (cv. Suyo and cv. Shogoin fushinari, respectively) were evaluated for induction of systemic resistance by plant growth-promoting fungal isolates Phoma sp. GS8-1 and nonsporulating fungus GU21-2 against Colletotrichum orbiculare. Reduction in anthracnose disease was assessed at regular intervals of 0, 3, 6, and 9 days after challenge inoculation. Activities of endochitinase, β-1, 3-endoglucanase, peroxidase (PO), polyphenol oxidase, and phenylalanine ammonia lyase (PAL) were assayed at the above intervals, while the activities of exochitinase and β-1,3-exogluconase were assayed at 9 days after challenge inoculation. The data showed that all the enzymes activities increased in plants treated with plant growth-promoting fungi (PGPF) compared with nontreated plants of both cultivars. The activities of endochitinase and β-1,3-endoglucanase were highly increased in PGPF-induced plants than in nontreated plants of both cultivars at different time points. Activities of PO, polyphenol oxidase, and PAL also increased between 3 and 6 days due to inoculation with PGPF and decreased thereafter. The activity of PAL at 9 days in PGPF-induced plants was at the same level as that of nontreated plants of both cultivars. Results of this investigation suggest that PGPF-induced systemic resistance is associated with not only high increases in the activities of chitinase and β-1,3-gluconases but also other plant defense-related enzymes such as PO, PPO, and PAL. Roles of other mechanisms rather than accumulation of defense-related enzymes in PGPF-mediated induced resistance in susceptible cultivars were demonstrated.
Roselle (Hibiscus sabdariffa L.) family Malvaceae is an important crop used in food, cosmetics and pharmaceutics industries. Roselle is cultivated mainly in Upper Egypt (Qena and Aswan governorates) producing 94% of total production. Root rot disease of roselle is one of the most important diseases that attack both seedlings and adult plants causing serious losses in crop productivity and quality. The main objective of the present study is to identify and characterize pathogens associated with root rot and wilt symptoms of roselle in Qena, Upper Egypt and evaluate their pathogenicity under greenhouse and field condition. Fusarium oxysporum, Macrophomina phaseolina, Fusarium solani, Fusarium equiseti and Fusarium semitectum were isolated from the natural root rot diseases in roselle. All isolated fungi were morphologically characterized and varied in their pathogenic potentialities. They could attack roselle plants causing damping-off and root rot/wilt diseases in different pathogenicity tests. The highest pathogenicity was caused by F. oxysporum and M. phaseolina followed by F. solani. The least pathogenic fungi were F. equiseti followed by F. semitectum. It obviously noted that Baladi roselle cultivar was more susceptible to infection with all tested fungi than Sobhia 17 under greenhouse and field conditions. This is the first report of fungal pathogens causing root rot and vascular wilt in roselle in Upper Egypt.
Roselle (Hibiscus sabdariffa L.) is one of the most important medicinal crops in many parts of the world. In this study, the effects of microelements, antioxidants, and bioagents on Fusarium oxysporum, F. solani, and Macrophomina phaseolina, the causal pathogens of root rot and wilt diseases in roselle, were examined under field conditions. Preliminary studies were carried out in vitro in order to select the most effective members to be used in field control trials. Our results showed that microelements (copper and manganese), antioxidants (salicylic acid, ascorbic acid, and EDTA), a fungicide (Dithane M45) and biological control agents (Trichoderma harzianum and Bacillus subtilis) were significantly reduced the linear growth of the causal pathogens. Additionally, application of the previous microelements, antioxidants, a fungicide and biological control agents significantly reduced disease incidence of root rot and wilt diseases under field conditions. Copper, salicylic acid, and T. harzianum showed the best results in this respect. In conclusion, microelements, antioxidants, and biocontrol agents could be used as alternative strategies to fungicides for controlling root rot and wilt diseases in roselle.
Some rhizosphere fungal isolates belonging to Phoma, Penicillium, Trichoderma and Fusarium were evaluated to control Fusarium and Rhizopus blights and bakanae diseases of rice seedlings. Among the isolates, Phoma spp. GS8-2, GS6-4 and GS10-2 showed 74-89% reduction of severity of Fusarium blight in rice seedlings. In case of Rhizopus blight, Phoma spp. GS8-2, GS6-4 and GS12-2 and Trichoderma harzianum GT3-2 reduced disease severity to 81-93%. Phoma sp. GS12-2 was the most effective isolate in controlling bakanae disease (90% reduction in disease severity). The effect of the tested isolates on the growth of rice seedlings varied significantly in terms of fresh and dry weight in rice trays. However, Phoma sp. GS12-2 effectively increased fresh and dry weight of shoots to 40 and 30%, respectively, compared with untreated control plants. In contrast, Phoma sp. GS6-4, T. harzianum GT3-2 and Penicillium sp. GP17-1 showed inhibitory effect to shoot growth of rice seedlings.
Binucleate Rhizoctonia (BNR) W7 was evaluated for the efficacy to control damping-off disease in cucumber seedlings. Results revealed that Lobate appresoria and dome-shaped infection cushion were produced by virulent Rhizoctonia solani C9 AG4 on the surface of the cucumber seedling hypocotyls, causing damping-off, 12 h after inoculation. A significant positive correlation was observed between disease severity rating and number of infection cushions (r = 0.94) and appresoria (r = 0.97) on the hypocotyls of seedlings inoculated with virulent R. solani C9 alone. Inoculation of a nonpathogenic species of BNR prior to virulent R. solani significantly reduced the number of appresoria on hypocotyls surface and inhibited the formation of infection cushions that accompanied by highly significant (P ≤ 0.01) reduction in disease severity in comparison with seedling inoculated with R. solani C9 alone. Formation of mucilaginous material upon recognition of the nonpathogenic isolate of BNR followed by hyphal lysis of BNR W7 6 h after inoculation was responsible for reduction in appresorium formation and complete inhibition of infection cushion formation. However, the number of infection structures produced on cucumber was not significantly different for hypocotyls of seedlings inoculated with R. solani C9 with and without the nonpathogenic BNR W7.
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