Crown rot pathogens are associated with higher losses for wheat crop farmers, but information about the distribution and prevalence of these pathogens in Chile is inadequate. Distribution and prevalence of wheat (Triticum aestivum L.) crown rot pathogens were examined in a survey of 48 commercial fields from December 2011 to February 2012 in southern Chile. These fields were located between Collipulli (37º56'00" S; 72°26'39" W) and Purranque (40º50'30" S; 73°22'03" W). Severity of crown rot disease was determined through visual assessment of the first internode of 20 tillers obtained from each field. Incidence of crown rot pathogens per field was determined by plating the 20 tillers on Petri plates with 20% potato dextrose agar amended with lactic acid (aPDA) medium. Resulting fungal colonies from monoxenic culture were identified by morphological or molecular-assisted identification. Severity of crown rot varied between 11.3% and 80% for individual fields. Culture plate analysis showed 72.2% of stems were infected with some fungus. Fusarium avenaceum, F. graminearum, and F. culmorum, pathogens associated with Fusarium crown rot disease were isolated from 13.5% of tillers. Gaeumannomyces graminis, causal agent of take-all disease in cereals, was isolated from 11.1% of culms. Phaeosphaeria sp., an endophyte and possibly a non-pathogenic fungus, was isolated from 13.9% of tillers. Pathogenic fungi such as Rhizoctonia spp. and Microdochium nivale, other saprophyte, and several unidentified non-sporulating fungi were isolated at frequencies lower than 3% of the total. Fusarium crown rot and take-all were the most prevalent and distributed crown rot diseases present in wheat crops in southern Chile.
The 2,4-Diacetylphloroglucinol (2,4-DAPG) and phenazine (PCA)-producing Pseudomonas inhibit wheat pathogens’ development, but the relationship between communities of pathogens and genotypes of these bacteria has been little studied. Relationships between wheat crown fungi associated with the presence of 2,4-DAPG and PCA-producing pseudomonads were evaluated in four commercial wheat crops located in the La Araucanía and Los Lagos Regions of Chile, during two crops seasons. Portions of the base of the first internode of the culm collected during the grain-filling stage were cultured in an artificial medium for fungal isolation, while roots of the same wheat plants and from plants collected previous harvest, and also used to assess yield and plant height, were used for the detection of 2,4-DAPG and PCA-producing Pseudomonas spp. using PCR with specific primers. Genera Phaeosphaeria, Fusarium, Rhizoctonia, and Microdochium were repeatedly isolated (52.6%, 22.1%, 7.8%, and 4.9%, respectively) and the genetic composition of 2,4-DAPG and PCA-producing Pseudomonas spp. varied between fields and sampling periods. Genetic groups A, B, D, K, L, and P associated with the phlD gene were detected. The presence of 2,4-DAPG-producing bacteria benefited crop health, relating their existence with increasing yield and plant height, and the reduction in the incidence and severity of disease caused by pathogenic microorganisms on the first internode of wheat culms.
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.
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