Etiology and Epidemiological Conditions Promoting Fusarium Root Rot in Sweetpotato

Antifungal activity of copper oxide nanoparticles derived from Zizyphus spina leaf extract against Fusarium root rot disease in tomato plants

Incorporating green chemistry concepts into nanotechnology is an important focus area in nanoscience. The demand for green metal oxide nanoparticle production has grown in recent years. The beneficial effects of using nanoparticles in agriculture have already been established. Here, we highlight some potential antifungal properties of Zizyphus spina leaf extract-derived copper oxide nanoparticles (CuO-Zs-NPs), produced with a spherical shape and defined a 13-30 nm particle size. Three different dosages of CuO-Zs-NPs were utilized and showed promising antifungal efficacy in vitro and in vivo against the selected fungal strain of F. solani causes tomato root rot disease, which was molecularly identified with accession number (OP824846). In vivo  results indicated that, for all CuO-Zs-NPs concentrations, a significant reduction in Fusarium root rot disease occurred between 72.0 to 88.6% compared to 80.5% disease severity in the infected control. Although treatments with either the chemical fungicide (Kocide 2000) showed a better disease reduction and incidence with (18.33% and 6.67%) values, respectively, than CuO-Zs-NPs at conc. 50 mg/l, however CuO-Zs-NPs at 250 mg/l conc. showed the highest disease reduction (9.17 ± 2.89%) and lowest disease incidence (4.17 ± 3.80%). On the other hand, CuO-Zs-NPs at varied values elevated the beneficial effects of tomato seedling vigor at the initial stages and plant growth development compared to either treatment with the commercial fungicide or Trichoderma Biocide. Additionally, CuO-Zs-NPs treatments introduced beneficial results for tomato seedling development, with a significant increase in chlorophyll pigments and enzymatic activity for CuO-Zs-NPs treatments. Additionally, treatment with low concentrations of CuO-Zs-NPs led to a rise in the number of mature pollen grains compared to the immature ones.  however the data showed that CuO-Zs-NPs have a unique antifungal mechanism against F. solani, they  subsequently imply that CuO-Zs-NPs might be a useful environmentally friendly controlling agent for the Fusarium root rot disease that affects tomato plants.

Genome-wide association study to identify novel loci and genes for Fusarium root rot resistance in sweet potato using genotyping-by-sequencing

Fusarium root rot, caused by Fusarium solani, is a major post-harvest disease in sweet potatoes (Ipomoea batatas (L.) Lam.). An effective strategy for controlling this disease is the development of resistant varieties. In this study, a genome-wide association study (GWAS) was conducted on 96 sweet potato genotypes to identify novel candidate loci and dissect the genetic basis of Fusarium root rot resistance. Genotyping was performed using genotyping-by-sequencing (GBS), and 44,255 SNPs were identified after filtering. The genotypes (n = 96) were evaluated through resistance tests in 2021 and 2022, separately and combined. The GWAS identified two significant SNP markers (LG3_22903756 and LG4_2449919) on chromosomes 3 and 4 associated with Fusarium root rot resistance, respectively. Lesion length showed significant differences between homozygous A and G alleles of LG3_22903756, which can potentially be used to develop molecular markers for selecting accessions resistant to Fusarium root rot. Expression analysis of 11 putative genes flanking the significant SNPs revealed the alteration in the expression of nine genes, indicating their possible involvement in Fusarium root rot resistance. The results of this study will aid in the marker-assisted selection and functional analysis of candidate genes for Fusarium root rot resistance in sweet potatoes.

Inhibitory effect of cinnamaldehyde on Fusarium solani and its application in postharvest preservation of sweet potato

Root rot caused by Fusarium solani is one of major postharvest diseases limiting sweet potato production. Antifungal effect and possible mode of action of cinnamaldehyde (CA) against F. solani were investigated. CA concentration of 0.075 g/L inhibited conidial viability of F. solani. CA vapor of 0.3 g/L in air completely controlled the F. solani development in sweet potatoes during storage for 10 days at 28 °C, and protected soluble sugar and starch in the flesh from depletion by the fungus. Further results demonstrated that CA induced reduction in mitochondrial membrane potential (Δψ_m), ROS accumulation, and cell apoptosis characterized by DNA fragmentation in F. solani. Moreover, CA facilitated decomposition of mitochondria-specific cardiolipin (CL) into its catabolites by the catalytic action of phospholipases. Altogether, the results revealed a specific antifungal mechanism of CA against F. solani, and suggest that CA holds promise as a preservative for postharvest preservation of sweet potato.

Antifungal Activity of Perillaldehyde on Fusarium solani and Its Control Effect on Postharvest Decay of Sweet Potatoes

Root rot caused by Fusarium solani is one of the major postharvest diseases limiting sweet potato production. Here, antifungal activity and the action mode of perillaldehyde (PAE) against F. solani were investigated. A PAE concentration of 0.15 mL/L in air (mL/L air) markedly inhibited the mycelial growth, spore reproduction and spore viability of F. solani. A PAE vapor of 0.25 mL/L in air could control the F. solani development in sweet potatoes during storage for 9 days at 28 °C. Moreover, the results of a flow cytometer demonstrated that PAE drove an increase in cell membrane permeability, reduction of mitochondrial membrane potential (MMP) and accumulation of reactive oxygen species (ROS) in F. solani spores. Subsequently, a fluorescence microscopy assay demonstrated that PAE caused serious damage to the cell nuclei in F. solani by inducing chromatin condensation. Further, the spread plate method showed that the spore survival rate was negatively correlated with the level of ROS and nuclear damage, of which the results indicated that PAE-driven ROS accumulation plays a critical role in contributing to cell death in F. solani. In all, the results revealed a specific antifungal mechanism of PAE against F. solani, and suggest that PAE could be a useful fumigant for controlling the postharvest diseases of sweet potatoes.

Metabolomic and Transcriptomic Analyses of Quality Deterioration in Fusarium solani-Infected Sweet Potato (Ipomoea batatas (L.) Lam cv Xinxiang) Storage Roots

Fusarium solani-induced quality deterioration in stored sweet potato is poorly characterized and understood. This study examined the effects of F. solani infection in Xinxiang sweet potato roots during storage. The results showed that while there were no external symptoms following F. solani infection, upon cutting the roots, the cut surface of the infected root rapidly turned black, whereas the untreated control roots remained unaffected. The metabolites and transcriptive differences between F. solani-infected and control sweet potato roots were investigated with high-performance liquid chromatography, metabolomic analysis, and an Illumina Novaseq platform. The results showed that levels of the toxic ipomeamarone accumulated as high as 2.36 mg/kg DW in tissue after F. solani inoculation and 6 days storage at 28 °C, where the control tissue sample did not accumulate any ipomeamarone. Metabolomic analysis showed that isochlorogenic acid and l-tyrosine significantly increased in the infected tissue and associated with the darkening cut surface of the infected sweet potato. In transcriptomic analysis, a total of 13, 14, and 6 key genes in ipomeamarone, isochlorogenic acid, and l-tyrosine biosynthesis pathways, respectively, were identified. A conceptual model elucidating the physiological and molecular mechanism of F. solani-induced quality deterioration in sweet potato is proposed.

Incidence Rates of Root Rot in Sweetpotato Caused by Cultivation Soil and Soil Microorganisms During Storage Periods

Sweetpotatoes require a storage period for year-round use and improved sweetness by starch degradation. However, long-term storage can cause root rot, and a large amount of sweetpotatoes can be discarded. Root rot is typically caused by pathogenic soil-borne Fusarium spp., and the development of root rot induced by the characteristics of cultivating soil in stored sweetpotato has not yet been identified. In this study, the effect of Fusarium spp. and microbial community in the cultivated soil on the root rot of sweetpotatoes was to be elucidated. Wounded sweetpotato were treated in soil cultures inoculated with F. solani or F. oxysporum for 2 days, and showed symptoms of root rot after 2 months of storage. The three study fields (Naju, Yeongam A, and B) were subjected to the same curing and storage treatments after harvest, and the incidence of root rot was 1.7- to 1.8-fold different after 3 months of storage. Across the three fields, concentrations of Fusarium spp. and of microbial communities differed according to the cultivation soil and period. In particular, Naju, which had the lowest incidence of root rot, had the lowest concentration of Fusarium spp. before harvest, and the smallest change in diversity of the microbial community during the cultivation period. However, tuberous roots harvested from the fields showed no significant differences in antioxidant activity or lesion size with the treatment of 10⁶ conidia/ml F. solani. By solidifying the importance of cultivating soil and related microorganisms in the advancement of root rot of sweetpotato, our results may aid in preventing the decrease in the yield of cultivated sweetpotatoes through root rot control.

Fungi Associated with Postharvest Diseases of Sweet Potato Storage Roots and In Vitro Antagonistic Assay of Trichoderma harzianum against the Diseases

Sweet potato is the 11th most important food crop in the world and an excellent source of nutrition. Postharvest diseases were monitored in sweet potato storage roots collected from the local markets in Korea during 2021. Several diseases including Fusarium surface and root rot, charcoal rot, dry rot, and soft rot were observed in the postharvest sweet potatoes. A total of 68 fungal isolates were obtained from the diseased samples, and the isolates were grouped into 8 different fungal colony types. Based on multilocus phylogeny and morphological analysis of 17 representative isolates, the isolates were identified as Fusarium oxysporum, F. ipomoeae, F. solani, Penicillium citrinum, P. rotoruae, Aspergillus wentii, Mucor variicolumellatus (Mu. circinelloides species complex), and Macrophomina phaseolina. F. oxysporum was the predominant pathogen as this is the most common pathogen of sweet potato storage roots causing the surface rot disease, and M. phaseolina caused the most severe disease among the pathogens. Dual culture antagonistic assays were evaluated using Trichoderma harzianum strains CMML20–26 and CMML20–27. The results revealed that the two strains showed strong antifungal activity in different ranges against all tested pathogens. This study provides an understanding of diverse postharvest diseases in sweet potatoes and suggests potential biocontrol agents to manage the diseases. In addition, this is the first report of sweet potato storage root rot diseases caused by A. wentii, and P. rotoruae worldwide.

Sweetpotato Root Development Influences Susceptibility to Black Rot Caused by the Fungal Pathogen Ceratocystis fimbriata

Black rot of sweetpotato, caused by Ceratocystis fimbriata, is an important reemerging disease threatening sweetpotato production in the United States. This study assessed disease susceptibility of the storage root surface, storage root cambium, and slips (vine cuttings) of 48 sweetpotato cultivars, advanced breeding lines, and wild relative accessions. We also characterized the effect of storage root development on susceptibility to C. fimbriata. None of the cultivars examined at the storage root level were resistant, with most cultivars exhibiting similar levels of susceptibility. In storage roots, Jewel and Covington were the least susceptible and significantly different from White Bonita, the most susceptible cultivar. In the slip, significant differences in disease incidence were observed for above- and below-ground plant structures among cultivars, advanced breeding lines, and wild relative accessions. Burgundy and Ipomoea littoralis displayed less below-ground disease incidence compared with NASPOT 8, Sunnyside, and LSU-417, the most susceptible cultivars. Correlation of black rot susceptibility between storage roots and slips was not significant, suggesting that slip assays are not useful to predict resistance in storage roots. Immature, early-developing storage roots were comparatively more susceptible than older, fully developed storage roots. The high significant correlation between the storage root cross-section area and the cross-sectional lesion ratio suggests the presence of an unfavorable environment for C. fimbriata as the storage root develops. Incorporating applications of effective fungicides at transplanting and during early-storage root development when sweetpotato tissues are most susceptible to black rot infection may improve disease management efforts.

Fungal keratitis: Mechanisms of infection and management strategies

Fungal corneal ulcers are an uncommon, yet challenging, cause of vision loss. In the United States, geographic location appears to dictate not only the incidence of fungal ulcers, but also the fungal genera most encountered. These patterns of infection can be linked to environmental factors and individual characteristics of fungal organisms. Successful management of fungal ulcers is dependent on an early diagnosis. New diagnostic modalities like confocal microscopy and polymerase chain reaction are being increasingly used to detect and identify infectious organisms. Several novel therapies, including crosslinking and light therapy, are currently being tested as alternatives to conventional antifungal medications. We explore the biology of Candida, Fusarium, and Aspergillus, the three most common genera of fungi causing corneal ulcers in the United States and discuss current treatment regimens for the management of fungal keratitis.

Real-Time PCR Assay for the Diagnosis and Quantification of Co-infections by Diaporthe batatas and Diaporthe destruens in Sweet Potato

Foot rot disease caused by Diaporthe destruens (formerly Plenodomus destruens) has become a major concern for the production of sweet potato [Ipomoea batatas (L.) Lam.] in Japan. A related fungus Diaporthe batatas, which causes dry rot disease of sweet potato, is native and is widespread in fields in Japan. The similar characteristics of these two pathogens pose a challenge for conventional disease diagnosis. Currently, there are no effective molecular measures for identifying and distinguishing D. destruens and D. batatas. Here, we demonstrate a real-time PCR assay that distinguishes and quantifies D. batatas and D. destruens from co-infected sweet potato. The assay was performed with various simulated DNA combinations of D. batatas and D. destruens ranging from 1:1 to 1:100000. The assay was also used with the ratios of D. batatas: D. destruens: sweet potato DNA ranging from 1:1:1 to 1:1:100000. These assays produced a specific amplification product for each of the pathogens, and quantified the fungal biomass over the entire range tested without detecting false positives. The assay was validated by using infected sweet potato collected from various fields; it showed sufficient sensitivity and specificity to quantify and distinguish D. batatas and D. destruens from these field samples. Thus, our real-time PCR assay would be a useful tool for diagnosis of D. batatas and D. destruens and is expected to provide the foundation for the design of integrated disease management strategies for foot rot disease in sweet potato.

Isolation and Identification of Trichoderma asperellum, the Novel Causal Agent of Green Mold Disease in Sweetpotato

Postharvest disease is an important limiting factor for sweetpotato production. Recently, a new green mold disease was found in sweetpotato storage roots. To investigate the mechanism underlying the pathogenesis of the disease, the pathogen was isolated and identified based on morphological and molecular features, and its characteristics were further analyzed by pathogenic and antagonistic evaluations. The results showed that the isolated pathogen (CRI-Ta1) was identified as Trichoderma asperellum based on the similar growth and morphological features with Trichoderma spp., 99% homology of internal transcribed spacer (ITS) sequence, and membership to the same phylogenetic group with the model strain of T. asperellum (CBS 433.97). The pathogenic analysis revealed that CRI-Ta1 could cause green mold disease through wound infection on the storage roots and the strains reisolated from infected storage roots could cause disease in different sweetpotato varieties, which was fulfilled in Koch's postulate. Moreover, CRI-Ta1 could also infect other common crop species, including chestnut, carrot, apple, pear, and others. It indicated that CRI-Ta1 was the pathogen to the storage roots of sweetpotato and had a wide host range. Additionally, in vitro antagonistic evaluation showed that CRI-Ta1 effectively inhibited the growth of common sweetpotato pathogens, including Fusarium solani and Rhizopus nigricans. However, further research is needed on the potential of CRI-Ta1 to control sweetpotato diseases in vivo. Collectively, our findings provided valuable insights into the characteristics of the T. asperellum CRI-Ta1 in sweetpotato and would be helpful to the prevention and control of sweetpotato green mold disease.

Effects of Water Temperature, Inoculum Concentration and Age, and Sanitizers on Infection of Ceratocystis fimbriata, Causal Agent of Black Rot in Sweetpotato

Black rot, caused by Ceratocystis fimbriata, is a devastating postharvest disease of sweetpotato that recently re-emerged in 2014. Although the disease is known to develop in storage and during export to overseas markets, little is known as to how pathogen dispersal occurs. This study was designed to investigate dump tank water as a means of dispersal through four different types of water treatments: inoculum concentration (0, 5, 5 × 10¹, 5 × 10², and 5 × 10³ spores/ml), inoculum age (0, 24, 48, 96, and 144 h), water temperature (10°C, 23°C, 35°C, and 45°C), and presence of a water sanitizer (DryTec, SaniDate, FruitGard, and Selectrocide). Wounded and nonwounded sweetpotato storage roots were soaked in each water treatment for 20 min, stored at 29°C for a 14-day period, and rated for disease incidence every other day. Disease was observed in sweetpotato storage roots in all water treatments tested, except in the negative controls. Disease incidence decreased with both inoculum concentration and inoculum age, yet values of 16.26% and up to 50% were observed for roots exposed to 5 spores/ml and 144-h water treatments, respectively. Sanitizer products that contained a form of chlorine as the active ingredient significantly reduced disease incidence in storage roots when compared with control roots and roots exposed to a hydrogen-peroxide based product. Finally, no significant differences in final incidence were detected in wounded sweetpotato storage roots exposed to water treatments of any temperature, but a significant reduction in disease progression was observed in the 45°C treatment. These findings indicate that if packing line dump tanks are improperly managed, they can aid C. fimbriata dispersal through the build-up of inoculum as infected roots are unknowingly washed after storage. Chlorine-based sanitizers can reduce infection when applied after root washing and not in the presence of high organic matter typically found in dump tanks.

Occurrence of Sweetpotato (Ipomoea batatas) Wilt and Surface Rot Disease and Determining Resistance of Selected Varieties to the Pathogen in Korea

Fusarium wilt and Fusarium surface rot caused by Fusarium oxysporum Schltdl are the major diseases of sweetpotato (Ipomoea batatas) and was surveyed in different locations (Cheongju, Heanam, Iksan, Icheon, Kimje, Nonsan, Yeoungam, and Yeoju) in Korea from 2015 to 2017 in the field, after harvesting and in storehouse. The wilt incidence in the early stage represented 17.9%, 5.9%, and 8.3% in 2015, 2016, and 2017, respectively. Samples were collected, and the causal organism was isolated on potato dextrose agar (PDA). Ten pure cultures were stored at the Sweetpotato Research Laboratory, Bioenergy Crop Research Institute, Muan, Korea. Morphological analysis, along with molecular phylogeny of the sequences of internal-transcribed spacer (ITS) and elongation factor 1-α (EF-1α) genes and their combined phylogenetic analysis, confirmed the isolates as the Fusarium oxysporum Schltdl. Pathogenicity tests were conducted on sweetpotato stems, and storage roots by artificially inoculation methods, and the most virulent isolate was selected as SPL18019. A rapid screening method on 21 selected varieties for resistant variety selection was applied on stems. The Pungwanmi was found resistant to Fusarium wilt, whereas Annobeni was the most susceptible. On the other hand, six varieties were used to test surface rot resistance, and Yulmi and Yesumi were resistant and susceptible, respectively, to Fusarium surface rot.

Assessing the Role of Temperature, Inoculum Density, and Wounding on Disease Progression of the Fungal Pathogen Ceratocystis fimbriata Causing Black Rot in Sweetpotato

In 2014, Ceratocystis fimbriata, causal agent of black rot in sweetpotato, reemerged and inflicted large financial losses on growers in the United States. Black rot continues to damage sweetpotatoes and has become a priority to the industry since then. In contrast, little is known about the biology of C. fimbriata and the epidemiology of sweetpotato black rot. In this study, effects of environmental factors such as inoculum density, RH, and temperature on sweetpotato black rot were determined. Cured sweetpotatoes were wounded with a toothpick to simulate puncture wounds, inoculated with different spore suspensions (inoculum density) (10⁴, 10⁵, or 10⁶ spores/ml), and incubated under different RH (85.53, 94.09, or 97.01%) and temperature (13, 18, 23, 29, or 35°C) for 21 days. In a separate experiment, five root wounding types (cuts, punctures, abrasions, end breaks, and macerating bruises) were compared. All wounded roots were subsequently soaked in a 10³ spores/ml suspension and incubated at 100% RH and 23°C for 21 days. This study found 29 and 23°C to be the optimal temperature for black rot disease development and sporulation, respectively. No pathogen growth was observed at 13 and 35°C. Increased inoculum density significantly (P < 0.0001) increased disease incidence, but increasing RH had an effect only on sporulation area. All wound types resulted in increased disease incidence and sporulation as early as 7 days postinoculation. Our results highlight the importance of characterizing factors that affect disease development for achieving successful disease management strategies. Findings from this study will be used to improve disease management for sweetpotato black rot by suggesting tighter regulation of curing and storage conditions and better postharvest handling of sweetpotato roots to avoid unnecessary wounding.

In Vitro and In Vivo Inhibitory Effects of Gaseous Chlorine Dioxide Against Diaporthe batatas Isolated from Stored Sweetpotato

Chlorine dioxide (ClO₂) can be used as an alternative disinfectant for controlling fungal contamination during postharvest storage. In this study, we tested the in vitro and in vivo inhibitory effects of gaseous ClO₂ against Diaporthe batatas SP-d1, the causal agent of sweetpotato dry rot. In in vitro tests, spore suspensions of SP-d1 spread on acidified potato dextrose agar were treated with various ClO₂ concentrations (1-20 ppm) for 0-60 min. Fungal growth was significantly inhibited at 1 ppm of ClO₂ treatment for 30 min, and completely inhibited at 20 ppm. In in vivo tests, spore suspensions were drop-inoculated onto sweetpotato slices, followed by ClO₂ treatment with different concentrations and durations. Lesion diameters were not significantly different between the tested ClO₂ concentrations; however, lesion diameters significantly decreased upon increasing the exposure time. Similarly, fungal populations decreased at the tested ClO₂ concentrations over time. However, the sliced tissue itself hardened after 60-min ClO₂ treatments, especially at 20 ppm of ClO₂. When sweetpotato roots were dip-inoculated in spore suspensions for 10 min prior to treatment with 20 and 40 ppm of ClO₂ for 0-60 min, fungal populations decreased with increasing ClO₂ concentrations. Taken together, these results showed that gaseous ClO₂ could significantly inhibit D. batatas growth and dry rot development in sweetpotato. Overall, gaseous ClO₂ could be used to control this fungal disease during the postharvest storage of sweetpotato.

Genetic Diversity, Fungicide Sensitivity, and Host Resistance to Ceratocystis fimbriata Infecting Sweetpotato in North Carolina

Black rot of sweetpotato, caused by Ceratocystis fimbriata, has recently reemerged as a significant threat to sweetpotato production in North Carolina and other states across the United States. This disease has historically been controlled largely through cultural management strategies and, in some cases, fungicide application. The sudden and destructive reemergence of this disease in 2015 created the need for rapidly evaluating disease control strategies. Genetic diversity of current C. fimbriata isolates infecting sweetpotato in North Carolina was assessed using ITS, TEF, and MAT-2 sequences. All 50 tested isolates were confirmed to be of a single mating type, MAT-2, based on PCR amplification. Alignment of ITS, TEF, and MAT-2 sequences revealed all isolates were identical at each locus. Fourteen common sweetpotato cultivars and advanced breeding lines were screened for black rot resistance using two isolates. None of the cultivars were completely resistant to the disease and most were equally susceptible. 'Stokes Purple' and 'Covington' were the least susceptible, but significantly (P < 0.05) differed only from 'Bellevue', the most susceptible cultivar. Sensitivity of 50 C. fimbriata isolates to difenoconazole, fludioxonil, thiabendazole, dicloran, azoxystrobin, pyraclostrobin, fenamidone, and fluazinam was evaluated in vitro. Difenoconazole, thiabendazole, and fluazinam were most effective in reducing mycelia growth. Postharvest fungicide application on black rot-infected roots provided similar results. Low efficacy of dicloran, as well as a range of EC₅₀ values among isolates, suggests potential resistance to this commonly applied fungicide. Results obtained in this study provide current and useful information so that improved recommendations can be made to reduce losses in sweetpotato to black rot.