soft rot disease
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ABSTRACT Orchids (Phalaenopsis) are ornamental plants that are cultivated commercially and in great demand in the market. Soft-rot disease (SRD) caused by the necrotrophic pathogen Dickeya dadantii is a cause of considerable economic loss to cultivators of many orchid species. Our previous experiment identified a limited number of species that were resistant to D. dadantii. This study aimed to validate the resistance level of four Phalaenopsis species in a detached leaf inoculation protocol to identify the resistance mechanism(s) involved. Soft-rot symptom diameter was measured from 6 to 18 hours post-inoculation (HPI) with D. dadantii. Disease assessment confirmed that P. amboinensis is a resistant species, P. pantherina is a susceptible species, and P. amabilis and P. schilleriana are very susceptible species. There was no difference in the lignin content between the resistant and very susceptible species. Detailed observation of resistant and very susceptible species, P. amboinensis vs. P. amabilis, revealed higher phenylalanine ammonia-lyase (PAL) and peroxidase (POD) in P. amabilis than in P. amboinensis. In contrast, there was higher salicylic acid (SA) content in P. amboinensis than in P. amabilis. These results suggest that POD and PAL activities may not be effective in defense against soft-rot disease, while SA plays an important role in the resistance of P. amboinensis to D. dadantii. Low PAL activity in P. amboinensis implies that the SA contents from the isochorismate pathway may be involved in the mechanism of P. amboinensis resistance to D. dadantii. Therefore, endogenous SA content may be a good indicator for screening resistant species in Phalaenopsis.

Soft rot caused by Pectobacterium spp. is responsible for significant losses in vegetable production worldwide. Methods for the effective control of this disease are limited and are primarily based on good agricultural practices. The use of phages as biocontrol agents appears to be a promising alternative to combat phytopathogens. In this study, we investigated the efficacy of lytic phages against soft rot caused by Pectobacterium carotovorum subsp. carotovorum. Designated as PPc_A3, PPc_D1, and PPc_J3, three bacteriophage isolates, which were recovered from symptomatic tissues and environmental samples, were observed to effectively lyse P. carotovorum subsp. carotovorum. PPc_A3 belongs to the Podoviridae family, while phages PPc_D1 and PPc_J3 belong to the Myoviridae family based on the morphological features of the virions as observed using transmission electron microscopy. The optimal multiplicity of infection (MOI) differed greatly among the three phages. All survived incubations at 30°C, 40°C and 50°C and at pH ranging from 3.0 to 9.0, but were all inactivated at 60°C and at pH 12. Both monophage and cocktail preparations were effective in inhibiting the growth of P. carotovorum subsp. carotovorum in in vitro challenge tests. In the semi-in planta assays, monophage treatments resulted in significant reduction of tissue maceration in potato slices, while treatment with cocktail preparations completely inhibited the development of soft rot disease. Overall, these results demonstrate the efficacy of cocktail formulations of phages PPc_A3, PPc_D1, and PPc_J3 for the biocontrol of soft rot disease caused by P. carotovorum subsp. carotovorum.

About 45% of the world’s fruit and vegetables are wasted, resulting in postharvest losses and contributing to economic losses ranging from $10 billion to $100 billion worldwide. Soft rot disease caused by Rhizopus stolonifer leads to postharvest storage losses of sweet potatoes. Nanoscience stands as a new tool in our arsenal against these mounting challenges that will restrict efforts to achieve and maintain global food security. In this study, three nanomaterials (NMs) namely C60, CuO, and TiO2 were evaluated for their potential application in the restriction of Rhizopus soft rot disease in two cultivars of sweet potato (Y25, J26). CuO NM exhibited a better antifungal effect than C60 and TiO2 NMs. The contents of three important hormones, indolepropionic acid (IPA), gibberellic acid 3 (GA-3), and indole-3-acetic acid (IAA) in the infected J26 sweet potato treated with 50 mg/L CuO NM were significantly higher than those of the control by 14.5%, 10.8%, and 24.1%. CuO and C60 NMs promoted antioxidants in both cultivars of sweet potato. Overall, CuO NM at 50 mg/L exhibited the best antifungal properties, followed by TiO2 NM and C60 NM, and these results were further confirmed through scanning electron microscope (SEM) analysis. The use of CuO NMs as an antifungal agent in the prevention of Rhizopus stolonifer infections in sweet potatoes could greatly reduce postharvest storage and delivery losses.

Erwinia is a genus of Enterobacteriacea containing mostly pathogens, which cause soft rot disease in many ornamental plants and crops, including Asparagus officinalis. Chemical treatments to control Erwinia have lost their attractiveness because of the development of resistant strains and the negative impacts on the environment and human health. Therefore, the study of biological controls of soft rot disease has gained great importance. There are several types of microorganisms that show activity against Erwinia spp. such as Pseudomonas fluorescence, Bacillus subtilis, and Streptomyces spp. Among them, Streptomyces spp. are found to be the most effective control agents. In this study, 64 isolates of Streptomyces were screened for their antibacterial activity against Erwinia spp. The results indicated that 18 isolates showed an antagonistic reaction against Erwinia spp. Among them, isolate D5.1 showed the highest inhibition activity. In addition, the morphological and antibacterial activities of isolate D5.1 grown in different conditions were also characterized.