Integrative Pathogenicity Assay and Operational Taxonomy-Based Detection of New Forma Specialis of Fusarium oxysporum Causing Datepalm Wilt

Pathogenicity-associated genes are highly host-specific and contribute to host-specific virulence. We tailored the traditional Koch's postulates with integrative omics by hypothesizing that the effector genes associated with host-pathogenicity are determinant markers for virulence, and developed Integrative Pathogenicity (IP) postulates for authenticated pathogenicity testing in plants. To set the criteria, we experimented on datepalm (Phoenix dactylifera) for the vascular wilt pathogen and confirmed the pathogen based on secreted in xylem genes (effectors genes) using genomic and transcriptomic approaches, and found it a reliable solution when pathogenicity is in question. The genic regions ITS, TEF1-α, and RPBII of Fusarium isolates were examined by phylogenetic analysis to unveil the validated operational taxonomy at the species level. The hierarchical tree generated through phylogenetic analysis declared the fungal pathogen as Fusarium oxysporum. Moreover, the Fusarium isolates were investigated at the subspecies level by probing the IGS, TEF1-α, and Pgx4 genic regions to detect the forma specialis of F. oxysporum that causes wilt in datepalm. The phylogram revealed a new forma specialis in F. oxysporum that causes vascular wilt in datepalm.

Trichoderma spp. Genes Involved in the Biocontrol Activity Against Rhizoctonia solani

Rhizoctonia solani is a pathogen that causes considerable harm to plants worldwide. In the absence of hosts, R. solani survives in the soil by forming sclerotia, and management methods, such as cultivar breeding, crop rotations, and fungicide sprays, are insufficient and/or inefficient in controlling R. solani. One of the most challenging problems facing agriculture in the twenty-first century besides with the impact of global warming. Environmentally friendly techniques of crop production and improved agricultural practices are essential for long-term food security. Trichoderma spp. could serve as an excellent example of a model fungus to enhance crop productivity in a sustainable way. Among biocontrol mechanisms, mycoparasitism, competition, and antibiosis are the fundamental mechanisms by which Trichoderma spp. defend against R. solani, thereby preventing or obstructing its proliferation. Additionally, Trichoderma spp. induce a mixed induced systemic resistance (ISR) or systemic acquired resistance (SAR) in plants against R. solani, known as Trichoderma-ISR. Stimulation of every biocontrol mechanism involves Trichoderma spp. genes responsible for encoding secondary metabolites, siderophores, signaling molecules, enzymes for cell wall degradation, and plant growth regulators. Rhizoctonia solani biological control through genes of Trichoderma spp. is summarized in this paper. It also gives information on the Trichoderma-ISR in plants against R. solani. Nonetheless, fast-paced current research on Trichoderma spp. is required to properly utilize their true potential against diseases caused by R. solani.

Antagonistic Activity of Chilean Strains of Pseudomonas protegens Against Fungi Causing Crown and Root Rot of Wheat (Triticum aestivum L.)

Seed treatments with antagonistic bacteria could reduce the severity of crown and root rot diseases in wheat crops. The objective of this study was to evaluate the potential antagonistic activity of a bacterial consortium of three Chilean strains of Pseudomonas protegens against the wheat crown and root rot pathogens Gaeumannomyces graminis var. tritici, Rhizoctonia cerealis, and Fusarium culmorum. Two field experiments were carried out on artificially infested soil during two consecutive seasons (2016-2017 and 2017-2018) in an Andisol soil of southern Chile. Control treatments (not inoculated with fungi) were also included. Each treatment included a seed treatment of spring wheat cv. Pantera-INIA with and without the bacterial consortium. Both phytosanitary damage (incidence and severity) and agronomic components were evaluated. Bacterial populations with the phlD+ gene in the wheat plant rhizosphere during anthesis state (Z6) were also quantified. In both seasons, infection severity decreased by an average of 16.8% in seeds treated with P. protegens consortium, while yield components such as spikes m^-1 and number of grains per spike increased. The use of antagonistic bacteria resulted in a total yield increase only during the first experimental season (P < 0.05). In general, accumulated rainfall influenced the antagonistic effect of the consortium of P. protegens strains, accounting for the differences observed between the two seasons. The results suggest that this P. protegens consortium applied on seeds can promote plant growth and protect wheat crops against crown and root rot pathogens in Southern Chile under field conditions.

First Report of Fusarium sporotrichioides Causing Foliar Spots on Forage Corn in Chile

In southern Chile, forage corn (Zea mays L.) is grown for feeding animals in milk diaries and livestock production. In December 2010, corn plants with small circular spots on leaves were collected from three fields located in Río Negro (Los Lagos region). Symptoms began as small, circular white to brown spots of 5 to 10 mm on different parts of the leaf and necrotic tissue with irregular brown to burgundy margins on the border and tip of the leaf. Estimated visual severity was ~5 to 40% for each leaf from field samples. Twelve small blocks of tissue were taken from the edge of necrotic spots from infected leaves, surface disinfected (2 min in 95% ethanol, 2 min in 0.5% NaOCl, followed by three rinses with sterile distilled water), and then placed on PDA and incubated for 7 days at 24 ± 1°C. Seventy five percent of the sampled tissues developed fungal colonies and a 4-mm³ block of agar that contained the advancing hyphal edge of each colony was transferred to PDA and carnation leaf agar and incubated for 10 days at 24 ± 1°C. Colonies were fast growing with pink-white and dense mycelia; with a carmine red color on the undersurface of the plate and orange sporodochia; polyphialides abundant; microconidia abundant, oval or pear-shaped or spindle-shaped, thin walled, hyaline, often with a papilla at the base, and 5.5 to 12.2 × 2.0 to 3.2 μm. Macroconidia were sickle-shaped, 3 to 5 septate, moderately curved to straight, hyaline, thick walled, and 20.5 to 42.9 × 3.5 to 5.0 μm. Morphology of colonies and conidia matched the description of Fusarium sporotrichioides Sherb. (3). Identity of the fungus was confirmed by molecular characterization of the ITS and 18SrRNA regions (universal primers ITS4/5 and NS1/2, respectively) and the β-tubulin gene (primers Bt1a/Bt1b) of three isolates. BLAST searches of the obtained sequences had between 99 to 100% homology with several isolates of F. sporotrichioides from GenBank (Accession Nos. KC866343 to KC866351). Pathogenicity tests were conducted by dispensing 10 μl of a prepared spore suspension (10⁷ spores/ml) on corn leaves (16 leaves). Negative controls were corn leaves inoculated with sterile distilled water. Inoculated corn leaves were kept at 25 ± 1°C in glass bell jars and monitored for the onset of symptoms for 10 days. The test was conducted twice. Additionally, 20 corn plants of four hybrid lines were inoculated with ~5 ml of a spore suspension (10⁴ macroconidia/ml) 2 months after seeding under field conditions in Valdivia, Los Ríos region, Chile. Seventy five days after sowing, similar lesions to those initially observed on field infected leaves were observed on inoculated leaves but not on water controls. Under field conditions, an extended damage on borders of basal leaves and spots on stems and cobs was observed. The pathogen was reisolated from infected tissues, thereby fulfilling Koch's postulates. F. sporotrichioides is a frequent pathogen in corn silage (1) and cereal crops (3,4), and produces trichothecene mycotoxins that cause toxicosis in animals (2,3). To our knowledge, this is the first report of F. sporotrichioides causing foliar spot on forage corn in Chile and this disease could represent a serious risk of mycotoxin contamination in this crop. References: (1) H. Baath et al. Arch. Tierernahr. 40:397, 1990. (2) A. E. Desjardins et al. Phytopathology 79:170, 1989. (3) J. F. Leslie and B. A. Summerell. Page 256 in: The Fusarium Laboratory Manual. Blackwell Publishing Professional, Hoboken, NJ, 2006. (4) R. H. Vargo et al. Plant Dis. 70:629, 1986.

Population Dynamics Between Fusarium pseudograminearum and Bipolaris sorokiniana in Wheat Stems Using Real-Time qPCR

Fusarium pseudograminearum and Bipolaris sorokiniana are causal agents of Fusarium crown rot and common root rot, respectively, of wheat and cause significant losses worldwide. Understanding the population dynamics between these two pathogens at late stages of wheat development is needed. The effect of F. pseudograminearum and B. sorokiniana inocula applied singly or in mixtures at seeding to spring wheat 'Hank' was measured using seedling stand, grain yield, and pathogen populations in the first internode at heading, milk, and harvest stage of wheat development using real-time quantitative polymerase chain reaction. High and low rates of F. pseudograminearum inoculum reduced B. sorokiniana populations in field trials but B. sorokiniana inoculations did not affect F. pseudograminearum populations. Populations of both pathogens increased from heading until harvest, with F. pseudograminearum colonizing lower internodes earlier than B. sorokiniana. Neither pathogen prevented infection by the other in the first internode of wheat stems. Inoculations increased incidence of infection and co-infection relative to natural settings observed for both pathogens. At the seedling stage, both fungi, individually or combined, reduced the seedling stands when compared with a noninoculated control for the three location-years. Grain yield and F. pseudograminearum populations were inversely correlated, while B. sorokiniana populations were not correlated with yield.

Distribution and Prevalence of Fusarium Crown Rot and Common Root Rot Pathogens of Wheat in Montana

Distribution of Fusarium crown rot (FCR) and common root rot (CRR) pathogens associated with wheat (Triticum aestivum) in 91 fields in Montana were determined during the 2008 and 2009 crop seasons using real-time quantitative polymerase chain reaction (qPCR) and conventional isolation methods. Correlations (P < 0.001) were found between detection methods for both diseases. FCR was detected in 57% of the fields and CRR was detected in 93% of the fields surveyed. Percent incidence based on isolation from individual tillers was Bipolaris sorokiniana (15%), F. culmorum (13%), and F. pseudograminearum (8%). FCR populations were highly variable across the regions and were not detected in any fields from the Gb5 soil types of Judith Basin and Fergus counties. The spatial distributions of FCR and CRR were affected by elevation, soil type, and temperature. High FCR populations were associated with spring wheat crops rather than winter wheat based on qPCR (P < 0.001). FCR and CRR could produce yield losses in a range of 3 to 35%. This study is the first time that qPCR was used to survey these two pathogen groups, and the merits and weakness of qPCR relative to traditional isolation methods are discussed.