Characterization of the Maize Stalk Rot Pathogens Fusarium subglutinans and F. temperatum and the Effect of Fungicides on Their Mycelial Growth and Colony Formation

Assessment of Genetic Diversity and the Population Structure of Species from the Fusarium fujikuroi Species Complex Causing Fusarium Stalk Rot of Maize

Fusarium stalk rot (FSR), caused by the Fusarium species complex, is an economic threat to maize cultivation all over the world. We investigated the population structure and genetic diversity of Fusarium species obtained from five major maize-growing regions in India. The Tef-1α locus was used for phylogenetic analysis of geographically distinct isolates of Fusarium verticillioides, F. andiyazi, F. proliferatum, F. nygamai, and F. acutatum causing FSR. A phylogenetic tree showed monophyletic, polyphyletic, and paraphyletic groupings reflecting the complex evolutionary history and genetic diversity within the genus. Monophyletic groupings depicting strong bootstrap support were shown to have a single common ancestor and genetic coherence with limited genetic divergence among sequences. Polyphyletic groupings also presented significant genetic differentiation within the F. verticillioides sequences from diverse ecological niches. Nucleotide diversity of moderate level 0.02471 reflected genetic variations within populations that were attributed to factors such as mutation, genetic drift, or varying selection pressures. The Fst value of 0.98205 is particularly indicative of high genetic differentiation, implying that most of the genetic variance is due to differences between populations rather than within them. F. verticillioides, with 57 sequences, showed low genetic diversity with three segregating sites and a low haplotype diversity of 0.19486, suggesting the founder effect, where a reduced population expands from a limited genetic pool. The total data estimates across all populations for haplotype analysis showed 72 sequences, 44 segregating sites, and 9 haplotypes with a haplotype diversity of 0.48513. The evolutionary dynamics showed genetic differentiations among Fusarium species causing FSR. AMOVA indicated high within-population variations, depicting a substantial genetic diversity within individual populations. The results offer a comprehensive framework for discussing the implications of genetic diversity in pathogen management and the evolutionary dynamics of the Fusarium species causing FSR in maize in the Indian subcontinent.

Stalk rot species diversity and molecular phylogeny associated with diseased maize in India

Stalk rot disease is a major constraint in maize production and till date reported to be caused by two to three species of phytopathogenic fungi but, in our present study, we disclose the first report of stalk rot is caused by complex species of phytopathogens, which belongs to five different genera. Therefore, to substantiate these findings, a total of 105 diseased samples of maize were collected from 21 different locations in six different geographical locations of India from which 48 isolates were used for the research study. Morphological features such as pigmentation, colony color, type of mycelium and pattern of mycelium was examined using macro and microscopic methods. A total of 11 different spp. of pathogens belonging to the five different genera: Fusarium verticillioides (56.25%), F. equiseti (14.5%), F. andiyazi (6.25%), F. solani (2.08%), F. proliferatum (2.08%), F. incarnatum (2.08%), Lasidioplodia theobrame (6.25%), Exserohilum rostrtum (4.16%), Nigrospora spp. (4.16%). and Schizophyllum commune (2.08%) were identified by different housekeeping genes (ITS, TEF-1α, RPB2 and Actin). Fusarium verticillioides, F. equiseti and F. andiyazi were major pathogens involved in stalk rot. This is the first report on F. proliferatum, F. solani, F. incarnatum, Lasidioplodia theobrame, Exserohilum rostrtum, Nigrospora spp. and Schizophyllum commune causing stalk rot of maize and their distribution in the different states of India. Studies on population dynamics of PFSR will enhance the understanding of pathogen behavior, virulence, or its association with different pathogens across India, which will facilitate the development of resistant maize genotypes against the PFSR.

Diversity and Pathogenicity of Fusarium Species Associated with Stalk and Crown Rot in Maize in Northern Italy

The genus Fusarium includes several agronomically important and toxin-producing species that are distributed worldwide and can cause a wide range of diseases. Crown and stalk rot and grain infections are among the most severe symptoms that Fusarium spp. can cause in maize. Disease development usually occurs during germination, but it may also affect the later phases of plant growth. The purpose of this study was to investigate the diversity and pathogenicity of 41 isolates recovered from symptomatic seedlings collected in Northern Italy and seeds of five different geographical origins in 2019 and 2020. The pathogenicity was tested and confirmed in 23 isolates causing rotting in maize seedlings, with disease indexes from 20% to 90%. A multilocus phylogeny analysis based on four genomic loci (tef1-α, rpb2, calm and tub2) was performed on 23 representative isolates. Representative isolates were identified as species belonging to three species complexes (SC), including Fusarium verticillioides and F. annulatum in the F. fujikuroi SC. Fusarium commune was identified in the F. nisikadoi SC, and three different lineages were found in the Fusarium oxysporum SC. This study reports F. annulatum and two lineages of the Fusarium oxysporum SC as maize pathogens for the first time in Italy.

Effectiveness of introgression of resistance loci for Gibberella ear rot from two European flint landraces into adapted elite maize (Zea mays L.)

European flint landraces are a major class of maize possessing favorable alleles for improving host resistance to Gibberella ear rot (GER) disease which reduces yield and contaminates the grains with mycotoxins. However, the incorporation of these landraces into breeding programs requires a clear understanding of the effectiveness of their introgression into elite materials. We evaluated 15 pre-selected doubled haploid (DH) lines from two European flint landraces, "Kemater Landmais Gelb" (KE) and "Petkuser Ferdinand Rot" (PE), together with two adapted elite flint lines and seven standard lines for GER severity as the main trait, and several adaptation traits (plant height, days to silking, seed-set, plant vigor) across four environments. From this evaluation, three KE DH lines and one PE DH line, with the lowest GER severity, were selected and used as donor parents that were crossed with the two adapted and GER susceptible flint lines (Flint1 and Flint2) to develop six bi-parental DH populations with 34-145 DH lines each. Each DH population was evaluated across two locations. Correlations between GER severity, which was the target trait, and adaptation traits were weak (-0.02 to 0.19). GER severity of lines from PE landrace was on average 2-fold higher than lines from KE landrace, indicating a clear superiority of the KE landrace lines. Mean GER severity of the DH populations was 39.4-61.0% lower than the adapted elite flint lines. All KE-derived DH populations were on average more resistant (27.0-36.7%) than the PE-derived population (51.0%). Highly resistant lines (1.3-5.2%) were found in all of the populations, suggesting that the DH populations can be successfully integrated into elite breeding programs. The findings demonstrate that selected KE landrace lines used as donors were effective in improving GER resistance of the adapted elite inbreds.

Current Studies on Bakanae Disease in Rice: Host Range, Molecular Identification, and Disease Management

The seed borne disease such as bakanae is difficult to control. Crop yield loss caused by bakanae depending on the regions and varieties grown, ranging from 3.0% to 95.4%. Bakanae is an important disease of rice worldwide and the pathogen was identified as Fusarium fujikuroi Nirenberg (teleomorph: Gibberella fujikuroi Sawada). Currently, four Fusaria (F. fujikuroi, F. proliferatum, F. verticillioides and F. andiyazi) belonging to F. fujikuroi species complex are generally known as the pathogens of bakanae. The infection occurs through both seed and soil-borne transmission. When infection occurs during the heading stage, rice seeds become contaminated. Molecular detection of pathogens of bakanae is important because identification based on morphological and biological characters could lead to incorrect species designation and time-consuming. Seed disinfection has been studied for a long time in Korea for the management of the bakanae disease of rice. As seed disinfectants have been studied to control bakanae, resistance studies to chemicals have been also conducted. Presently biological control and resistant varieties are not widely used. The detection of this pathogen is critical for seed certification and for preventing field infections. In South Korea, bakanae is designated as a regulated pathogen. To provide highly qualified rice seeds to farms, Korea Seed & Variety Service (KSVS) has been producing and distributing certified rice seeds for producing healthy rice in fields. Therefore, the objective of the study is to summarize the recent progress in molecular identification, fungicide resistance, and the management strategy of bakanae.

Morphological characterization, pathogenicity screening, and molecular identification of Fusarium spp. isolates causing post-flowering stalk rot in maize

Post flowering stalk rot (PFSR) of maize caused by the Fusarium species complex is a serious threat to maize production worldwide. The identification of Fusarium species causing PFSR based on morphology traditionally relies on a small set of phenomic characteristics with only minor morphological variations among distinct Fusarium species. Seventy-one isolates were collected from 40 sites in five agro-climatic zones of India to assess the diversity of Fusarium spp. associated with maize crops showing symptoms of PFSR in the field. To investigate the pathogenicity of Fusarium spp. causing PFSR sixty isolates were toothpick inoculated between the first and second node at 55 days after sowing during the tassel formation stage of the crop in Kharif (Rainy season), and Rabi (Winter season) season field trials. Ten most virulent Fusarium isolates, based on the highest observed disease index, were identified by homology and phylogenetic analyses of partial sequences of the translation elongation factor 1 α (Tef-1α). Based on morphological traits such as mycelial growth patterns and mycelial pigmentation, Fusarium isolates were divided into nine clusters. The isolates were judged to be virulent based on their ability to decrease seedling vigour in in-vivo situations and high disease severity in field experiments. Pathogenicity test during the Kharif season showed 12 isolates with virulent disease symptoms with a mean severity ranging between 50 to 67 percent disease index (PDI) whereas in Rabi season, only five isolates were considered virulent, and the mean severity ranged between 52 to 67 PDI. Based on pathological characterization and molecular identification, 10 strains of Fusarium species namely, Fusarium acutatum (2/10), Fusarium verticillioides (Syn. Gibberella fujikuroi var. moniliformis) (7/10), Fusarium andiyazi (2/10) recorded the highest diseases index. All these species are part of the Fusarium fujikuroi species complex (FFSC). The distribution of virulent isolates is specific to a geographical location with a hot humid climate. Increased knowledge regarding the variability of Fusarium spp. responsible for PFSR of maize occurring across wide geographical locations of India will enable more informed decisions to be made to support the management of the disease, including screening for resistance in maize-inbred lines.

Genetic diversity of toxigenic Fusarium verticillioides associated with maize grains, India

In the present investigation, prevalence, genetic diversity, and mycotoxin producing potential of Fusarium species associated with maize grain samples were studied from different geographical regions of India. The highest prevalence of Fusarium verticillioides was recorded as 88.52%, followed by F. coffeatum, F. foetens, and F. euwallaceae, 6.55%, 3.27%, and 1.63%, respectively. We isolated 54 strains of F. verticillioides, and their genetic diversity was studied by inter simple sequence repeats (ISSR). The ISSR fingerprints (AG) 8C and (AG) 8G showed 252 and 368 microsatellite sites in the genome of F. verticillioides and resulted in 99-100% repeatability and reproducibility. The Simpson (SID) and Shannon (H) indices (0.78 and 2.36) suggest that F. verticillioides strains exhibit moderate to high diversity. Molecular detection of fumonisin B1 (FB1) biosynthetic genes (FUM1 and FUM13) involved in FB1 production in F. verticillioides was confirmed by polymerase chain reaction (PCR). Furthermore, 91% of the strains were positive for FB1 production, which was affirmed by liquid chromatography with tandem mass spectrometry (LC-MS-MS). In-vitro appurtenance of F. verticillioides spores exhibited a high to moderate effect on the growth and development of the maize. The current finding demonstrated that most F. verticillioides strains showed a wide range of genetic diversity with varied toxigenic and pathogenic potentials. In conclusion, for the first time, F. coffeatum, F. foetens, and F. euwallaceae species were reported from maize grain samples in India. They were positive for FB1 and negatively affecting grain quality, which is a major concern in food safety.

Maize stalk rot caused by Fusarium graminearum alters soil microbial composition and is directly inhibited by Bacillus siamensis isolated from rhizosphere soil

Maize stalk rot caused by Fusarium graminearum can reduce the yield of maize and efficiency of mechanized harvesting. Besides, deoxynivalenol and zearalenone toxins produced by F. graminearum can also affect domestic animals and human health. As chemical fungicides are expensive and exert negative effects on the environment, the use of biological control agents has become attractive in recent years. In the present study, we collected rhizosphere soil with severe stalk rot disease (ZDD), the rhizosphere soil with disease-free near by the ZDD (ZDH), and measured rhizosphere microbial diversity and microbial taxonomic composition by amplicon sequencing targeting either bacteria or fungi. The results showed that Fusarium stalk rot caused by the Fusarium species among which F. graminearum is frequent and can reduce the abundance and alpha diversity of rhizosphere microbial community, and shift the beta diversity of microorganisms. Furthermore, a bacterial strain, Bacillus siamensis GL-02, isolated from ZDD, was found to significantly affect growth of F. graminearum. In vitro and in vivo assays demonstrated that B. siamensis GL-02 had good capability to inhibit F. graminearum. These results revealed that B. siamensis GL-02 could be a potential biocontrol agent for the control of maize stalk rot.

2,4-Diacetylphloroglucinol producing Pseudomonas fluorescens JM-1 for management of ear rot disease caused by Fusarium moniliforme in Zea mays L

Maize (Zea mays L.) is a major cereal crop grown in a large number of countries. Loss in maize yield due to biotic stresses including fungal phytopathogens is a matter of immense concern. Control measures applied for eradication of fungal phytopathogens in maize are not up to the mark and more often involve harsh chemical(s)/pesticide(s) that cause deleterious effects both in humans and soil biota. Greener alternatives, such as the use of rhizosphere microbes in the form of bioinoculants, have proven to be very successful in terms of enhancing crop yield and suppressing fungal phytopathogens. In the present study, fluorescent pseudomonads were isolated from the maize rhizosphere and monitored for their plant growth-promoting (PGP) and biocontrol activities against Fusarium moniliforme. Based on various PGP traits and biocontrol potential, isolate JM-1 was found to be most effective and as per 16S rRNA gene sequencing analysis was identified as Pseudomonas fluorescens. Further experiments showed that the biocontrol potential of JM-1 against ear rot fungus involved the production of antifungal compound 2,4-diacetylphloroglucinol (DAPG). When examined for antagonistic interaction under scanning electron microscopy (SEM), structural abnormality, hyphal lysis, and deformity in fungal mycelium were observed. In the pot experiment, application of talc-based JM-1 containing bioformulation (in pot trials) showed significant enhancement in maize growth parameters (including the seed number and weight) in comparison to control even in presence of the phytopathogen. Ear fresh weight, dry weight, number of seeds per plant, and 100-grain weight were found to increase significantly by 34, 34, 52, and 18% respectively, in comparison to control. P. fluorescens JM-1 can therefore be used as a bioinoculant for ear rot disease control and sustainably enhancing maize yield.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-022-03201-7.

Toxicity and action mechanisms of silver nanoparticles against the mycotoxin-producing fungus Fusarium graminearum

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AgNPs possess high activity towards fungicide-resistant strains.

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AgNPs exert great activity against mycotoxin-producing fungus F. graminearum.

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AgNPs induce the expression of two azole resistance-related ABC genes.

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AgNPs lead to accumulation of toxisome and notorious mycotoxin DON by provoking ROS.

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AgNPs combined with DON-reducing fungicides are recommended for FHB control.

Introduction

Fusarium graminearum is a most destructive fungal pathogen that causes Fusarium head blight (FHB) disease in cereal crops, resulting in severe yield loss and mycotoxin contamination in food and feed. Silver nanoparticles (AgNPs) are extensively applied in multiple fields due to their strong antimicrobial activity and are considered alternatives to fungicides. However, the antifungal mechanisms and the effects of AgNPs on mycotoxin production have not been well characterized.

Objectives

This study aimed to investigate the antifungal activity and mechanisms of AgNPs against both fungicide-resistant and fungicide-sensitive F. graminearum strains, determine their effects on mycotoxin deoxynivalenol (DON) production, and evaluate the potential of AgNPs for FHB management in the field.

Methods

Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and fluorescence microscopy were used to examine the fungal morphological changes caused by AgNPs. In addition, RNA-Seq, qRT-PCR, and western blotting were conducted to detect gene transcription and DON levels.

Results

AgNPs with a diameter of 2 nm exhibited effective antifungal activity against both fungicide-sensitive and fungicide-resistant strains of F. graminearum. Further studies showed that AgNP application could impair the development, cell structure, cellular energy utilization, and metabolism pathways of this fungus. RNA-Seq analysis and sensitivity determination revealed that AgNP treatment significantly induced the expression of azole-related ATP-binding cassette (ABC) transporters without compromising the control efficacy of azoles in F. graminearum. AgNP treatment stimulated the generation of reactive oxygen species (ROS), subsequently induced transcription of DON biosynthesis genes, toxisome formation, and mycotoxin production.

Conclusion

This study revealed the underlying mechanisms of AgNPs against F. graminearum, determined their effects on DON production, and evaluated the potential of AgNPs for controlling fungicide-resistant F. graminearum strains. Together, our findings suggest that combinations of AgNPs with DON-reducing fungicides could be used for the management of FHB in the future.

Occurrence, Pathogenicity, and Mycotoxin Production of Fusarium temperatum in Relation to Other Fusarium Species on Maize in Germany

Fusarium subglutinans is a plant pathogenic fungus infecting cereal grain crops. In 2011, the species was divided in Fusarium temperatumsp. nov. and F. subglutinans sensu stricto. In order to determine the occurrence and significance of F. temperatum and F. subglutinans on maize, a monitoring of maize ears and stalks was carried out in Germany in 2017 and 2018. Species identification was conducted by analysis of the translation elongation factor 1α (TEF-1α) gene. Ninety-four isolates of F. temperatum and eight isolates of F. subglutinans were obtained during two years of monitoring from 60 sampling sites in nine federal states of Germany. Inoculation of maize ears revealed a superior aggressiveness for F. temperatum, followed by Fusarium graminearum, Fusarium verticillioides, and F. subglutinans. On maize stalks, F. graminearum was the most aggressive species while F. temperatum and F. subglutinans caused only small lesions. The optimal temperature for infection of maize ears with F. temperatum was 24 °C and 21 °C for F. subglutinans. All strains of F. temperatum and F. subglutinans were pathogenic on wheat and capable to cause moderate to severe head blight symptoms. The assessment of mycotoxin production of 60 strains of F. temperatum cultivated on rice revealed that all strains produced beauvericin, moniliformin, fusaric acid, and fusaproliferin. The results demonstrate a higher prevalence and aggressiveness of F. temperatum compared to F. subglutinans in German maize cultivation areas.

Effect of fungicides commonly used for Fusarium head blight management on growth and fumonisin production by Fusarium proliferatum

Fumonisin occurrence was reported in wheat grains and F. proliferatum has been suggested to be the main contributor to its presence in wheat. Thus, a survey was performed in order to study the impact of four commercial fungicides used in Argentina for controlling Fusarium head blight disease (epoxiconazole+metconazole, tebuconazole, pyraclostrobin+epoxiconazole, and prothioconazole) on growth and fumonisin production of two F. proliferatum strains in relation to water activity (a_W; 0.99, 0.97, 0.95) and temperature (15°C and 25°C). Most fungicides reduced growth rates when compared to the control (reduction increased as fungicide concentration increased), and reduced fumonisin production when they were used at high doses; however, most fungicides enhanced fumonisin production at sublethal doses, with the exception of prothioconazole. Thus, fungicides used for FHB management could enhance fumonisin production by F. proliferatum strains present in wheat grains.

Fusarium head blight in wheat: contemporary status and molecular approaches

Fusarium head blight (FHB) disease that occurs in wheat is caused by Fusarium graminearum and is a major risk to wheat yield. Although several research efforts focusing on FHB have been conducted in the past several decades, conditions have become more critical due to the increase in its virulent forms. In such a scenario, conferring complete resistance in plants seems to be difficult for handling this issue. The phenotyping for FHB and finding a solution for it at the genetic level comprises a long-term process as FHB infection is largely affected by environmental conditions. Modern molecular strategies have played a crucial role in revealing the host-pathogen interaction in FHB. The integration of molecular biology-based methods such as genome-wide association studies and marker-based genomic selection has provided potential cultivars for breeding programs. In this review, we aim at outlining the contemporary status of the studies conducted on FHB in wheat. The influence of FHB in wheat on animals and human health is also discussed. In addition, a summary of the advancement in the molecular technologies for identifying and developing the FHB-resistant wheat genetic resources is provided. It also suggests the future measures that are required to reduce the world's vulnerability to FHB which was one of the main goals of the US Wheat and Barley Scab Initiative.

Impact of Environmental Conditions and Agronomic Practices on the Prevalence of Fusarium Species Associated with Ear- and Stalk Rot in Maize

Fusarium species are common pathogens on maize and reduce the product quality through contamination with mycotoxins thus jeopardizing safety of both animal feed and human food products. Monitoring of Fusarium infected maize ears and stalks was conducted in Germany to determine the range of Fusarium species present in the field and to assess the impact of tillage, crop rotation, and weather conditions on the frequency of Fusarium species. From 2016 till 2018, a total of 387 infected ears and 190 stalk segments from 58 locations in Germany were collected. For each sample location, site-specific agronomic data on tillage and previous crops as well as meteorological data on precipitation, air temperature, and relative humidity during the vegetation period were recorded. The most frequent Fusarium species detected in maize ears were Fusariumgraminearum, F.verticillioides and F.temperatum, whereas, F.graminearum, F.equiseti, F.culmorum, and F.temperatum were the species prevailing on maize stalks. Differences in the local species composition were found to be primarily associated with weather variations between the years and the microclimate at the different locations. The results indicate that mean temperature and precipitation in July, during flowering, has the strongest impact on the local range of Fusarium spp. on ears, whereas the incidence of Fusarium species on stalks is mostly affected by weather conditions during September. Ploughing significantly reduced the infection with F.graminearum and F.temperatum, while crop rotation exerted only minor effects.

Characterization of Antagonistic Bacillus methylotrophicus Isolated From Rhizosphere and Its Biocontrol Effects on Maize Stalk Rot

Stalk rot is one of the most serious and widespread diseases in maize, and effective control measures are currently lacking. Therefore, this study aimed to develop a new biological agent to manage this disease. An antagonistic bacterial strain, TA-1, was isolated from rhizosphere soil and identified as Bacillus methylotrophicus based on morphological and biochemical characterization and 16S ribosomal RNA and gyrB gene sequence analyses. TA-1 exhibited a strong antifungal effect on the growth of Fusarium graminearum mycelium, with 86.3% inhibition at a concentration of 10⁸ CFU per ml. Transmission electron microscopy showed that TA-1 could disrupt the cellular structure of the fungus, induce necrosis, and degrade the cell wall. Greenhouse and field trials were performed to evaluate the biocontrol efficacy of TA-1 on maize stalk rot, and the results of greenhouse experiment revealed that the bacterium significantly reduced disease incidence and disease index. Seeds treated with a 10⁸ CFU ml^-1 cell suspension had the highest disease suppression at 86.8%. Results of field trials show that seed bacterization with TA-1 could not only reduce maize stalk rot incidence but also increase maize height, stem diameter, and grain yield. The lipopeptide antibiotics were isolated from the culture supernatants of TA-1 and identified as surfactins and iturins. Consequently, B. methylotrophicus TA-1 is a potential biocontrol agent against maize stalk rot.

Potential of Novel Sequence Type of Burkholderia cenocepacia for Biological Control of Root Rot of Maize (Zea mays L.) Caused by Fusarium temperatum

In this study, two Burkholderia strains, strain KNU17BI2 and strain KNU17BI3, were isolated from maize rhizospheric soil, South Korea. The 16S rRNA gene and multilocus sequence analysis and typing (MLSA-MLST) were used for the identification of the studied strains. Strain KNU17BI2, which belonged to Burkholderia cenocepacia, was of a novel sequence type (ST) designated ST-1538, while strain KNU17BI3 had a similar allelic profile with the seven loci of Burkholderia contaminans strain LMG 23361. The strains were evaluated in vitro for their specific plant growth promoting (PGP) traits, such as zinc solubilization, phosphate solubilization, ammonia production, 1-aminocyclopropane-1-carboxylate (ACC) deaminase activity, indole acetic acid (IAA) production, siderophore, and hydrolytic enzyme activity. Interestingly, the strains exhibited a positive effect on all of the tested parameters. The strains also showed broad-spectrum antifungal activity against economically important phytopathogens in the dual culture assay. Furthermore, the strains were evaluated under greenhouse conditions for their in vivo effect to promote plant growth and to suppress the root rot of maize that is caused by Fusarium temperatum on four Korean maize cultivars. The results of the greenhouse study revealed that both of the strains were promising to significantly suppress fusarium root rot and enhance plant growth promotion on the four maize cultivars. This study, for the first time, reported in vitro antifungal potential of B. cenocepacia of novel ST against economically important plant pathogens viz., F. temperatum, Fusarium graminearum, Fusarium moniliforme, Fusarium oxysporum f.sp. melonis, Fusarium subglutinans, Phytophthora drechsleri, and Stemphylium lycopersici. This is also the first report of zinc solubilization by B. cenocepacia. Moreover, the present research work reports, for the first time, about the potential of B. cenocepacia and B. contaminans to control the root rot of maize that is caused by F. temperatum. Therefore, we recommend further studies to precisely identify the bioactive chemical compounds behind such activities that would be novel sources of natural products for biological control and plant growth promotion of different crops.

Simple Detection of Cochliobolus Fungal Pathogens in Maize

Northern corn leaf spot and southern corn leaf blight caused by Cochliobolus carbonum (anamorph, Bipolaris zeicola) and Cochliobolus heterostrophus (anamorph, Bipolaris maydis), respectively, are common maize diseases in Korea. Accurate detection of plant pathogens is necessary for effective disease management. Based on the polyketide synthase gene (PKS) of Cochliobolus carbonum and the nonribosomal peptide synthetase gene (NRPS) of Cochliobolus heterostrophus, primer pairs were designed for PCR to simultaneously detect the two fungal pathogens and were specific and sensitive enough to be used for duplex PCR analysis. This duplex PCR-based method was found to be effective for diagnosing simultaneous infections from the two Cochliobolus species that display similar morphological and mycological characteristics. With this method, it is possible to prevent infections in maize by detecting infected seeds or maize and discarding them. Besides saving time and effort, early diagnosis can help to prevent infections, establish comprehensive management systems, and secure healthy seeds.

Species composition of and fumonisin production by the Fusarium fujikuroi species complex isolated from Korean cereals

To assess the risk of fumonisin contamination in Korean cereals, we isolated colonies of the Fusarium fujikuroi species complex (FFSC) from barley, maize, rice and soybean samples from 2011 to 2015. A total of 878 FFSC strains were isolated mostly from maize and rice, and species identity of the isolates were determined using the DNA sequence of the translation elongation factor 1-α (TEF-1α) and RNA polymerase II (RPB2) genes. Fusaria recovered from Korean cereals included F. fujikuroi (317 isolates and a frequency of 36%), F. proliferatum (212 isolates and 24.1%), F. verticillioides (170 isolates and 19.4%), F. concentricum (86 strains and 9.8%), F. andiyazi (56 isolates and 6.4%), F. subglutinans (28 isolates and 3.2%), F. thapsinum (5 isolates and 0.6%), and F. circinatum (2 isolates and 0.2%). The rice samples were dominated by F. fujikuroi (47.4%), F. proliferatum (27.3%), and F. concentricum (15.1%), whereas maize samples were dominated by F. verticillioides (33.9%), F. fujikuroi (25.3%), and F. proliferatum (21.1%). A phylogenetic analysis of 70 representative isolates demonstrated that each species was resolved as genealogically exclusive in the ML tree. Fumonisin production potential was evaluated using a PCR assay for the fumonisin biosynthesis gene, FUM1 in all of the isolates. Most of the isolates tested (94%) were positive for FUM1. All of the isolates assigned to F. fujikuroi, F. proliferatum, F. verticillioides and F. thapsinum were positive for FUM1 irrespective of their host origin. Seventy-seven representative isolates positive for FUM1 were examined for fumonisin production in rice medium. The majority of F. proliferatum (26/27, 96.3%), F. verticillioides (16/17, 94.1%) and F. fujikuroi (19/25, 76.0%) produced both FB₁ and FB₂. Notably, 16 of 19 fumonisin-producing F. fujikuroi produced >1000μg/g of fumonisins (FB₁+FB₂) in rice medium, which is higher than that in previous reports. These results suggest that F. fujikuroi can produce high levels of fumonisins similar to F. verticillioides and F. proliferatum.

Fusarium diseases of maize associated with mycotoxin contamination of agricultural products intended to be used for food and feed

Infections of maize with phytopathogenic and toxinogenic Fusarium spp. may occur throughout the cultivation period. This can cause different types of diseases in vegetative and generative organs of the plant. Along with these infections, mycotoxins are often produced and accumulated in affected tissues, which could pose a significant risk on human and animal health when entering the food and feed chain. Most important fungal species infecting European maize belong to the Fusarium sections Discolour and Liseola, the first being more prevalent in cooler and humid climate regions than the second predominating in warmer and dryer areas. Coexistence of several Fusarium spp. pathogens in growing maize under field conditions is the usual case and may lead to multi-contamination with mycotoxins like trichothecenes, zearalenone and fumonisins. The pathways how the fungi gain access to the target organs of the plant are extensively described in relation to specific symptoms of typical rot diseases regarding ears, kernels, rudimentary ears, roots, stem, leaves, seed and seedlings. Both Gibberella and Fusarium ear rots are of major importance in affecting the toxinogenic quality of grain or ear-based products as well as forage maize used for human or animal nutrition. Although rudimentary ears may contain high amounts of Fusarium toxins, the contribution to the contamination of forage maize is minor due to their small proportion on the whole plant dry matter yield. The impact of foliar diseases on forage maize contamination is regarded to be low, as Fusarium infections are restricted to some parts on the leaf sheaths and husks. Mycotoxins produced in rotted basal part of the stem may contribute to forage maize contamination, but usually remain in the stubbles after harvest. As the probability of a more severe disease progression is increasing with a prolonged cultivation period, maize should be harvested at the appropriate maturity stage to keep Fusarium toxin contamination as low as possible. Ongoing surveillance and research is needed to recognise changes in the spectrum of dominating Fusarium pathogens involved in mycotoxin contamination of maize to ensure safety in the food and feed chain.

Fusarium Species and Their Associated Mycotoxins

The genus Fusarium includes numerous toxigenic species that are pathogenic to plants or humans, and are able to colonize a wide range of environments on earth. The genus comprises around 70 well-known species, identified by using a polyphasic approach, and as many as 300 putative species, according to phylogenetic species concepts; many putative species do not yet have formal names. Fusarium is one of the most economically important fungal genera because of yield loss due to plant pathogenic activity; mycotoxin contamination of food and feed products which often render them unaccep for marketing; and health impacts to humans and livestock, due to consumption of mycotoxins. Among the most important mycotoxins produced by species of Fusarium are the trichothecenes and the fumonisins. Fumonisins cause fatal livestock diseases and are considered potentially carcinogenic mycotoxins for humans, while trichothecenes are potent inhibitors of protein synthesis. This chapter summarizes the main aspects of morphology, pathology, and toxigenicity of the main Fusarium species that colonize different agricultural crops and environments worldwide, and cause mycotoxin contamination of food and feed.

Effectiveness of Different Classes of Fungicides on Botrytis cinerea Causing Gray Mold on Fruit and Vegetables

Botrytis cinerea is a necrotrophic pathogen causing a major problem in the export and post-harvest of strawberries. Inappropriate use of fungicides leads to resistance among fungal pathogens. Therefore, it is necessary to evaluate the sensitivity of B. cinerea to various classes of fungicide and to determine the effectiveness of different concentrations of commonly used fungicides. We thus evaluated the effectiveness of six classes of fungicide in inhibiting the growth and development of this pathogen, namely, fludioxonil, iprodione, pyrimethanil, tebuconazole, fenpyrazamine, and boscalid. Fludioxonil was the most effective (EC50 < 0.1 μg/ml), and pyrimethanil was the least effective (EC50 = 50 μg/ml), at inhibiting the mycelial growth of B. cinerea. Fenpyrazamine and pyrimethanil showed relatively low effectiveness in inhibiting the germination and conidial production of B. cinerea. Our results are useful for the management of B. cinerea and as a basis for monitoring the sensitivity of B. cinerea strains to fungicides.

Combined Effects of Soil Biotic and Abiotic Factors, Influenced by Sewage Sludge Incorporation, on the Incidence of Corn Stalk Rot

The objectives of this study were to evaluate the combined effects of soil biotic and abiotic factors on the incidence of Fusarium corn stalk rot, during four annual incorporations of two types of sewage sludge into soil in a 5-years field assay under tropical conditions and to predict the effects of these variables on the disease. For each type of sewage sludge, the following treatments were included: control with mineral fertilization recommended for corn; control without fertilization; sewage sludge based on the nitrogen concentration that provided the same amount of nitrogen as in the mineral fertilizer treatment; and sewage sludge that provided two, four and eight times the nitrogen concentration recommended for corn. Increasing dosages of both types of sewage sludge incorporated into soil resulted in increased corn stalk rot incidence, being negatively correlated with corn yield. A global analysis highlighted the effect of the year of the experiment, followed by the sewage sludge dosages. The type of sewage sludge did not affect the disease incidence. A multiple logistic model using a stepwise procedure was fitted based on the selection of a model that included the three explanatory parameters for disease incidence: electrical conductivity, magnesium and Fusarium population. In the selected model, the probability of higher disease incidence increased with an increase of these three explanatory parameters. When the explanatory parameters were compared, electrical conductivity presented a dominant effect and was the main variable to predict the probability distribution curves of Fusarium corn stalk rot, after sewage sludge application into the soil.

Analysis of Fungicide Sensitivity and Genetic Diversity among Colletotrichum Species in Sweet Persimmon

Anthracnose, caused by Colletotrichum gloeosporioides (C. gloeosporioides; Teleomorph: Glomerella cingulata), is the most destructive disease that affects sweet persimmon production worldwide. However, the biology, ecology, and genetic variations of C. gloeosporioides remain largely unknown. Therefore, in this study, the development of fungicide resistance and genetic diversity among an anthracnose pathogen population with different geographical origins and the exposure of this population to different cultivation strategies were investigated. A total of 150 pathogen isolates were tested in fungicide sensitivity assays. Five of the tested fungicides suppressed mycelial pathogen growth effectively. However, there were significant differences in the sensitivities exhibited by the pathogen isolates examined. Interestingly, the isolates obtained from practical management orchards versus organic cultivation orchards showed no differences in sensitivity to the same fungicide. PCR-restriction fragment length polymorphism (RFLP) analyses were performed to detect internal transcribed spacer regions and the β-tubulin and glutamine synthetase genes of the pathogens examined. Both the glutamine synthetase and β-tubulin genes contained a complex set of polymorphisms. Based on these results, the pathogens isolated from organic cultivation orchards were found to have more diversity than the isolates obtained from the practical management orchards.