Role of the XylA gene, encoding a cell wall degrading enzyme, during common wheat, durum wheat and barley colonization by Fusarium graminearum.

Fusarium graminearum is the main causal agent of fusarium head blight (FHB) of wheat and barley. This filamentous fungus is able to produce hydrolytic enzymes, such as xylanases, that cause cell wall degradation, permitting host colonization. This study investigated the role of the F. graminearum XylA (FGSG_10999) gene during infection, using a knockout mutant in strain CS3005. Assays were carried out on common wheat, durum wheat and barley to compare virulence of a XylA knockout to that of wild type strain. These assays were conducted on wheat and barley seedling roots, seedling stem bases and heads. Furthermore, additional in vitro experiments were conducted to investigate the role of XylA gene in the utilisation of D-xylose, the main component of cereals cell wall. In planta assays showed the importance of XylA gene for F. graminearum virulence towards its main hosts. A positive correlation between symptom incidence and fungal biomass development was also observed for both the wild type and the knockout strains. Finally, gene expression studies performed in a liquid medium enriched with D-xylose, a known xylanase inducer in other fungi, showed that the absence of the gene in the FGSG_10999 locus was not compensated by two other F. graminearum xylanase encoding genes analysed (loci FGSG_06445 and FGSG_11478).

Causal agents of Fusarium head blight of durum wheat (Triticum durum Desf.) in central Italy and their in vitro biosynthesis of secondary metabolites.

Durum wheat samples harvested in central Italy (Umbria) were analyzed to: evaluate the occurrence of the fungal community in the grains, molecularly identify the Fusarium spp. which are part of the Fusarium head blight (FHB) complex and characterize the in vitro secondary metabolite profiles of a subset of Fusarium strains. The Fusarium genus was one of the main components of the durum wheat fungal community. The FHB complex was composed of eight species: Fusarium avenaceum (61%), F. graminearum (22%), F. poae (9%), F. culmorum (4%), F. proliferatum (2%), F. sporotrichioides (1%), F. sambucinum (0.5%) and F. langsethiae (0.5%). F. graminearum population was mainly composed of the 15-acetyldeoxynivalenol chemotype, while, F. culmorum population was composed of the 3-acetyldeoxynivalenol chemotype. In vitro characterization of secondary metabolite biosynthesis was conducted for a wide spectrum of substances, showing the mycotoxigenic potential of the species complex. F. avenaceum strains were characterized by high enniantin and moniliformin production. F. graminearum strains were in prevalence deoxynivalenol producers. F. poae strains were characterized by a high biosynthesis of beauvericin like the F. sporotrichioides strain which was also found to be a high T-2/HT-2 toxins producer. Production of aurofusarin, butenolide, gibepyrone D, fusarin C, apicidin was also reported for the analyzed strains.

Biosynthesis of beauvericin and enniatins in vitro by wheat Fusarium species and natural grain contamination in an area of central Italy.

Contamination of wheat grain by beauvericin (BEA) and enniatins (ENs) is a global emerging mycotoxicological food problem. In this study, strains of Fusarium avenaceum (FA), Fusarium poae (FP), Fusarium equiseti and Fusarium sporotrichioides, all potential BEA and EN producers, isolated from 162 grain samples of durum and soft wheat harvested in 2009 and 2010 collected in an area of central Italy, were preliminarily screened for the presence of the esyn1 gene, encoding the multifunctional enzyme enniatin-synthetase for the detection of potential hexadepsipeptide-producing isolates. All positive isolates were tested for their ability to biosynthesize BEA and ENs in vitro. In addition, all wheat samples were investigated for the natural presence of BEA and ENs (ENA, ENA1, ENB, ENB1). All FA and FP strains resulted to be positive for the presence of the esyn1 gene. All FA strains showed the ability to biosynthesize ENs in vitro but not BEA. Conversely, all FP strains resulted to be BEA producers and some of them co-biosynthesized ENs. A remarkable presence of "emerging" mycotoxins was found in the grains, particularly ENs. Co-contamination by BEA and ENs also occurred. This study gives an important contribution to assess the risk posed by mycotoxigenic fungi and their mycotoxins in food.

Occurrence of Black Dot of Potato Caused by Colletotrichum coccodes in Central Italy.

Between 1997 and 2000, black dot of potato (Solanum tuberosum L.), caused by the polyphagous soilborne fungus Colletotrichum coccodes (Wallr.) Hughes, was observed each summer in fields located in Umbria (central Italy). Disease incidence ranged from 50 to 100%, and early potato cultivars were generally more susceptible than late-maturing ones. Disease symptoms were first observed during August as a yellowing and wilting of foliage in the tops of plants, followed by rotting of the roots and stems, which led to the premature death of 50 to 70% of plants. Setose1 sclerotia (300 to 500 mm in diameter) and acervuli of the fungus were found on roots and stems of infected plants. Acervuli produced hyaline, aseptate, cylindrical conidia (16 to 22 × 2.5 to 4.5 µm) formed on unicellular cylindrical phialidic conidiophores. The fungus was isolated from diseased stems and roots on potato dextrose agar (PDA) at pH 6.5. Pathogenicity of the fungus was confirmed by fulfilling Koch's postulates using 3- to 4-week-old potato plants of a local cultivar. A superficial 5-mm vertical cut was made with a scalpel into the base of potato stems (2 cm beneath the soil surface), and 5-mm-diameter plugs of PDA alone (control plants) or PDA plus fungal growth were placed over the cuts. The wounds were sealed with wet cotton swabs that were held in place with Parafilm. Symptoms that resembled those in the field were observed on inoculated plants 6 to 8 weeks postinoculation. Symptoms did not appear on the control plants. The same fungus was reisolated from the diseased plants. Based on morphological characteristics of sclerotia, acervuli, and conidia, as well as pathogenicity tests, the fungus was identified as C. coccodes. To our knowledge, this is the first report of C. coccodes as the causal agent of black dot of potato in central Italy. We did not observe foliar outbreaks of the disease, which were reported from the United States (2). In both 1921 (1) and 1951 (3), the fungus was reported to cause severe outbreaks of the disease in northern Italy. Since then, its presence in Italy has been rarely recorded in potato (4). The occurrence of extremely dry and hot weather conditions during the summers of 1997 to 2000, which are favorable for disease development, made the disease particularly severe. We cannot exclude the possibility that the disease may have been present in central Italy before our observations, as it can be misdiagnosed and its symptoms can be masked by the symptoms of other diseases. The significance of black dot in central Italy needs to be reappraised in terms of both yield loss and tuber quality. References: (1) C. Arnaudi. Atti Ist. Bot. Univ. Pavia. Ser. 3, 1:71, 1924. (2) A. W. Barkdoll and J. R. Davis. Plant Dis. 76:131, 1992. (3) G. Goidanich. Inf. Fitopatol. 1:5, 1951. (4) S. Vitale et al. J. Plant Pathol. 80:265, 1998.