Aggressiveness and Mycotoxin Production by Fusarium meridionale Compared with F. graminearum on Maize Ears and Stalks in the Field

Fusarium meridionale and F. graminearum both cause Gibberella ear rot (GER) and Gibberella stalk rot (GSR) of maize in Brazil, but the former is much more common. Recent work with two isolates of each from maize suggested this dominance could be caused by greater aggressiveness and competitiveness of F. meridionale on maize. We evaluated pathogenicity and toxigenicity of 16 isolates of F. graminearum and 24 isolates of F. meridionale recovered from both wheat and maize. Strains were individually inoculated into ears of four maize hybrids in field trials. GER severity varied significantly between isolates within each species. Although ranges overlapped, the average GER severity induced by F. meridionale (25.2%) was two times as high overall as that induced by F. graminearum (12.8%) for isolates obtained from maize but was similar for those isolated from wheat (19.9 and 21.4%, respectively). In contrast, severity of GSR was slightly higher for F. graminearum (22.2%) than for F. meridionale (19.8%), with no effect of the host of origin. Deoxynivalenol and its acetylated form 15ADON were the main mycotoxins produced by F. graminearum (7/16 strains), and nivalenol toxin was produced by F. meridionale (17/24 strains). Six isolates of F. graminearum and three of F. meridionale also produced zearalenone. Results confirmed that F. meridionale from maize is, on average, more aggressive on maize but also suggested greater complexity related to diversity among the isolates within each species and their interactions with different hybrids. Further studies involving other components of the disease cycle are needed to more fully explain observed patterns of host dominance.

Molecular and Alkaloid Characterization of Claviceps purpurea Sensu Lato From Grass Seed Production Areas of the U.S. Pacific Northwest

Ergot, caused by Claviceps purpurea sensu lato, is an economically important seed replacement disease of Kentucky bluegrass (Poa pratensis) and perennial ryegrass (Lolium perenne) seed crops. C. purpurea sensu stricto is considered the primary Claviceps species responsible, but genetic diversity and cryptic species within C. purpurea sensu lato have previously been reported. Fifty-six C. purpurea sensu lato isolates collected from P. pratensis (n = 21) and L. perenne (n = 35) in Oregon and Washington between 2010 and 2014 were characterized via random amplified polymorphic DNA (RAPD), partial internal transcribed spacer (ITS), β-tubulin and elongation factor-1α (EF-1α) sequences, conidial size, and ergot alkaloid chemotype. Based on RAPD analysis, seven isolates from P. pratensis and 33 isolates from L. perenne collected in Oregon corresponded to C. purpurea sensu stricto, and 13 isolates collected from P. pratensis in Washington and Oregon were identified as C. humidiphila. Partial ITS, β-tubulin, and EF-1α sequences identified 10 isolates from P. pratensis as C. humidiphila, and seven isolates from P. pratensis and 33 isolates from L. perenne were identified as C. purpurea sensu stricto. Several isolates generated ambiguous RAPD bands or sequences that prevented identification. Ergot alkaloid chemotype profiling found that ergocornine and its epimer were predominant in sclerotia from P. pratensis, whereas ergotamine and its epimer were most abundant in sclerotia from L. perenne. This study confirms the presence of the C. purpurea sensu lato species complex in the U.S. Pacific Northwest and suggests that more research is needed to characterize and mitigate Claviceps spp. infection of grass seed crops in North America.

EFE-Mediated Ethylene Synthesis Is the Major Pathway in the Citrus Postharvest Pathogen Penicillium digitatum during Fruit Infection

Penicillium digitatum is the main fungal postharvest pathogen of citrus fruit under Mediterranean climate conditions. The role of ethylene in the P. digitatum-citrus fruit interaction is unclear and controversial. We analyzed the involvement of the 2-oxoglutarate-dependent ethylene-forming enzyme (EFE)-encoding gene (efeA) of P. digitatum on the pathogenicity of the fungus. The expression of P. digitatumefeA parallels ethylene production during growth on PDA medium, with maximum levels reached during sporulation. We generated ΔefeA knockout mutants in P. digitatum strain Pd1. These mutants showed no significant defect on mycelial growth or sporulation compared to the parental strain. However, the knockout mutants did not produce ethylene in vitro. Citrus pathogenicity assays showed no differences in virulence between the parental and ΔefeA knockout mutant strains, despite a lack of ethylene production by the knockout mutant throughout the infection process. This result suggests that ethylene plays no role in P. digitatum pathogenicity. Our results clearly show that EFE-mediated ethylene synthesis is the major ethylene synthesis pathway in the citrus postharvest pathogen P. digitatum during both in vitro growth on PDA medium and the infection process, and that this hormone is not necessary for establishing P. digitatum infection in citrus fruit. However, our results also indicate that ethylene produced by P. digitatum during sporulation on the fruit surface may influence the development of secondary fungal infections.

Utility of a Multilevel Modeling Approach to Investigate Differences in Isolation Frequency of Fusarium culmorum in Agricultural Soil Across the Inland Pacific Northwest

The plant pathogen Fusarium culmorum represents an inoculum source capable of contaminating grains with deoxynivalenol in the Inland Northwest region of the United States. A multilevel modeling approach utilizing varying intercepts for different sampling quadrats, fields, and iterations in the dataset was performed to characterize differences in isolation frequency of F. culmorum collected during a 2-year soil survey. Differences in the isolation frequency of F. culmorum varied the most by sampled field followed by quadrat and iteration, respectively. Higher relative elevation within the sampling region of a field limited the amount of F. culmorum recovered. The effect of annual climate variables was investigated using combinations of single-variable and multivariable model equations with linear and polynomial terms. The same data analysis approach was applied to an external dataset of F. culmorum isolation frequencies in grains from fields across eastern Australia. These results represent a case study for investigating variability within datasets containing overdispersed fungal counts and incorporating climate summaries as predictor variables.

Genome Sequence, Assembly and Characterization of Two Metschnikowia fructicola Strains Used as Biocontrol Agents of Postharvest Diseases

The yeast Metschnikowia fructicola was reported as an efficient biological control agent of postharvest diseases of fruits and vegetables, and it is the bases of the commercial formulated product "Shemer." Several mechanisms of action by which M. fructicola inhibits postharvest pathogens were suggested including iron-binding compounds, induction of defense signaling genes, production of fungal cell wall degrading enzymes and relatively high amounts of superoxide anions. We assembled the whole genome sequence of two strains of M. fructicola using PacBio and Illumina shotgun sequencing technologies. Using the PacBio, a high-quality draft genome consisting of 93 contigs, with an estimated genome size of approximately 26 Mb, was obtained. Comparative analysis of M. fructicola proteins with the other three available closely related genomes revealed a shared core of homologous proteins coded by 5,776 genes. Comparing the genomes of the two M. fructicola strains using a SNP calling approach resulted in the identification of 564,302 homologous SNPs with 2,004 predicted high impact mutations. The size of the genome is exceptionally high when compared with those of available closely related organisms, and the high rate of homology among M. fructicola genes points toward a recent whole-genome duplication event as the cause of this large genome. Based on the assembled genome, sequences were annotated with a gene description and gene ontology (GO term) and clustered in functional groups. Analysis of CAZymes family genes revealed 1,145 putative genes, and transcriptomic analysis of CAZyme expression levels in M. fructicola during its interaction with either grapefruit peel tissue or Penicillium digitatum revealed a high level of CAZyme gene expression when the yeast was placed in wounded fruit tissue.

Mapping and Identification of Antifungal Peptides in the Putative Antifungal Protein AfpB from the Filamentous Fungus Penicillium digitatum

Antifungal proteins (AFPs) from Ascomycetes are small cysteine-rich proteins that are abundantly secreted and show antifungal activity against non-producer fungi. A gene coding for a class B AFP (AfpB) was previously identified in the genome of the plant pathogen Penicillium digitatum. However, previous attempts to detect the AfpB protein were not successful despite the high expression of the corresponding afpB gene. In this work, the structure of the putative AfpB was modeled. Based on this model, four synthetic cysteine-containing peptides, PAF109, PAF112, PAF118, and PAF119, were designed and their antimicrobial activity was tested and characterized. PAF109 that corresponds to the γ-core motif present in defensin-like antimicrobial proteins did not show antimicrobial activity. On the contrary, PAF112 and PAF118, which are cationic peptides derived from two surface-exposed loops in AfpB, showed moderate antifungal activity against P. digitatum and other filamentous fungi. It was also confirmed that cyclization through a disulfide bridge prevented peptide degradation. PAF116, which is a peptide analogous to PAF112 but derived from the Penicillium chrysogenum antifungal protein PAF, showed activity against P. digitatum similar to PAF112, but was less active than the native PAF protein. The two AfpB-derived antifungal peptides PAF112 and PAF118 showed positive synergistic interaction when combined against P. digitatum. Furthermore, the synthetic hexapeptide PAF26 previously described in our laboratory also exhibited synergistic interaction with the peptides PAF112, PAF118, and PAF116, as well as with the PAF protein. This study is an important contribution to the mapping of antifungal motifs within the AfpB and other AFPs, and opens up new strategies for the rational design and application of antifungal peptides and proteins.

A Postharvest Fruit Rot in Apple Caused by Phacidiopycnis washingtonensis

During a survey of postharvest diseases in Red Delicious apples (Malus domestica) conducted in 2003, a previously undescribed postharvest fruit rot was discovered in Washington State. The causal agent has been described as a new species, Phacidiopycnis washingtonensis. In this study, we described the symptomatology of this disease, determined its occurrence and prevalence in Washington State, tested pathogenicity, and determined infection courts on fruit of three apple varieties. Decayed fruit were sampled from 26, 72, and 81 grower lots during March to August in 2003, 2004, and 2005, respectively, during packing operations from commercial packinghouses. Symptoms of decayed fruit were recorded and isolations were made from decayed fruit to correlate causal agents with the disease symptoms observed. Fruit of Red Delicious, Golden Delicious, and Fuji apples were inoculated in the orchards with conidial suspensions of the fungus at 1 and 2 weeks before harvest. All fruit were harvested and stored at 0°C, and decay development on the fruit was monitored monthly for up to 9 months. The symptoms were primarily stem-end rot and calyx-end rot. Infection also occurred at lenticels on fruit skin, particularly on Golden Delicious. The decayed area was spongy to firm and appeared light brown to brown. On Red Delicious, brown to black specks at lenticels often appeared at the decayed area as the disease advanced. This disease occurred in 23, 26, and 17% of the total grower lots, accounting for approximately 1, 4, and 3% of the total decay in 2003, 2004, and 2005, respectively. In 2004 and 2005, severe losses of fruit were observed in three grower lots of Red Delicious, and their losses were as high as 24%. After 9 months in storage, 48, 48, and 24% of Red Delicious, Golden Delicious, and Fuji that were inoculated in the orchards, respectively, developed symptoms of this disease, and the fungus was reisolated from decayed fruit. Stem-end rot was common on Red Delicious and Golden Delicious, whereas calyx-end rot was common on Fuji. We propose "speck rot" as the name of this disease.

Volatile Fingerprinting (SPME-GC-FID) to Detect and Discriminate Diseases of Potato Tubers

Volatiles from Russet Burbank potatoes inoculated with Erwinia carotovora subsp. carotovora, E. carotovora subsp. atroseptica, Pythium ultimum, Phytophthora infestans, or Fusarium sambucinum were monitored by sampling the head space 3, 4, and 5 days after inoculation, using a solid phase microextraction (SPME) fiber to trap and gas chromatography with flame ionization detector (GC-FID) to fingerprint volatiles. Noninoculated (NON) potatoes served as the control. Volatile fingerprints varied among diseases. Within a disease, the fingerprints varied with time since inoculation and among blocks. In general, more volatiles were observed on the fourth and fifth day after inoculation than on the third day. The amount of volatile compounds produced (peak area) within a disease group increased with incubation time; however, the variation among blocks was much higher. The amount of volatiles produced, in general, was associated with disease severity. Disease-specific volatiles were observed. The F. sambucinum chromatogram had two unique peaks at retention time (RT) = 14.1 and 17.3 min. P. infestans produced few peaks and the profile was quite similar to NON. In contrast, E. carotovora subsp. carotovora, E. carotovora subsp. atroseptica, and Pythium ultimum produced many peaks, and the P. ultimum was different from the bacteria, in that the chromatogram peaks at RT = 4.04 and 8.76 min were absent. Instead, it produced a distinct peak at RT = 1.71 min. E. carotovora subsp. carotovora and E. carotovora subsp. atroseptica couldn't be discriminated based on unique peaks; however, they varied in concentration of volatiles produced. E. carotovora subsp. carotovora produced more of RT = 2.0 min and less of RT = 2.3 and 2.44 min than E. carotovora subsp. atroseptica. A back-propagation network (using neural networks) was developed to classify volatile profiles into six disease-groups. Cross-validation classification probabilities were NON = 71, E. carotovora subsp. carotovora = 71, E. carotovora subsp. atroseptica = 71, P. ultimum = 67, Phytophthora infestans = 46, and F. sambucinum = 75%.