Morphological and molecular characterization of the strawberry black leaf spot pathogen referred to as the strawberry pathotype of Alternaria alternata

Evaluation and Control of Alternaria tenuissima Causing Leaf Spots in Blue Honeysuckle in China

Blue honeysuckle is emerging as a popular edible fruit and is also used in folk medicine. However, from June to August 2021, a serious leaf-spot disease affected the yield and quality of blue honeysuckle in Harbin, Heilongjiang Province, China; the species and characteristics of the pathogens responsible for the disease were unknown. In this study, 30 fungal isolates were obtained from infected blue honeysuckle leaves and identified as Alternaria tenuissima based on morphological and molecular characteristics and phylogenetic analyses. To the best of our knowledge, this is one of the first studies to identify A. tenuissima as the causal agent of blue honeysuckle leaf spots in China. Pathogenicity tests of the isolates revealed that most isolates were moderately pathogenic. All blue honeysuckle cultivars tested were found to be susceptible to 30 A. tenuissima isolates. In addition, elder, Dahurian rose fruit, sea buckthorn, rowan, hawthorn, bird cherry, and sorb could be infected by A. tenuissima isolates, while European cranberry bush and Nanking cherry were not infected. A. tenuissima isolates were highly sensitive to prochloraz (EC50 ≤ 0.50 μg ml-1) with 86.21% efficacy at 400 μg ml-1 in the field trials. Therefore, crop rotation and application of chemical fungicides are considered to control the disease-causing leaf spots in blue honeysuckle. These results provide a basis for controlling A. tenuissima in blue honeysuckle in China.

Seven new species of Alternaria (Pleosporales, Pleosporaceae) associated with Chinese fir, based on morphological and molecular evidence

Chinese fir (Cunninghamialanceolata) is a special fast-growing commercial tree species in China and has significant ecological and economic value. However, it experienced damage from leaf blight caused by pathogenic fungi of the genus Alternaria. To determine the diversity of Alternaria species associated with leaf blight of Chinese fir in China, infected leaves were collected from five major cultivation provinces (Fujian, Henan, Hunan, Jiangsu and Shandong provinces). A total of 48 fungal strains of Alternaria were obtained. Comparison of morphology and phylogenetic analyses, based on nine loci (ITS, SSU, LSU, GAPDH, RPB2, TEF1, Alt a1, endoPG and OPA10-2) of the representative isolates as well as the pairwise homoplasy index tests, revealed that the fungal strains belonged to seven undescribed taxa of Alternaria, which are described here and named as Alternariacunninghamiicolasp. nov., A.dongshanqiaoensissp. nov., A.hunanensissp. nov., A.kunyuensissp. nov., А. longqiaoensissp. nov., A.shandongensissp. nov. and A.xinyangensissp. nov. In order to prove Koch's postulates, pathogenicity tests on detached Chinese fir leaves revealed significant pathogenicity amongst these species, of which A.hunanensis is the most pathogenic to Chinese fir. This study represents the first report of A.cunninghamiicola, A.dongshanqiaoensis, A.hunanensis, A.kunyuensis, A.longqiaoensis, A.shandongensis and A.xinyangensis causing leaf blight on Chinese fir. Knowledge obtained in this study enhanced our understanding of Alternaria species causing leaf blight on Chinese fir and was crucial for the disease management and the further studies in the future.

Histone Methylation Is Required for Virulence, Conidiation, and Multi-Stress Resistance of Alternaria alternata

Histone methylation, which is critical for transcriptional regulation and various biological processes in eukaryotes, is a reversible dynamic process regulated by histone methyltransferases (HMTs) and histone demethylases (HDMs). This study determined the function of 5 HMTs (AaDot1, AaHMT1, AaHnrnp, AaSet1, and AaSet2) and 1 HDMs (AaGhd2) in the phytopathogenic fungus Alternaria alternata by analyzing targeted gene deletion mutants. The vegetative growth, conidiation, and pathogenicity of ∆AaSet1 and ∆AaSet2 were severely inhibited indicating that AaSet1 and AaSet2 play critical roles in cell development in A. alternata. Multiple stresses analysis revealed that both AaSet1 and AaSet2 were involved in the adaptation to cell wall interference agents and osmotic stress. Meanwhile, ∆AaSet1 and ∆AaSet2 displayed serious vegetative growth defects in sole carbon source medium, indicating that AaSet1 and AaSet2 play an important role in carbon source utilization. In addition, ∆AaSet2 colony displayed white in color, while the wild-type colony was dark brown, indicating AaSet2 is an essential gene for melanin biosynthesis in A. alternata. AaSet2 was required for the resistance to oxidative stress. On the other hand, all of ∆AaDot1, ∆AaHMT1, and ∆AaGhd2 mutants displayed wild-type phenotype in vegetative growth, multi-stress resistance, pathogenicity, carbon source utilization, and melanin biosynthesis. To explore the regulatory mechanism of AaSet1 and AaSet2, RNA-seq of these mutants and wild-type strain was performed. Phenotypes mentioned above correlated well with the differentially expressed genes in ∆AaSet1 and ∆AaSet2 according to the KEGG and GO enrichment results. Overall, our study provides genetic evidence that defines the central role of HMTs and HDMs in the pathological and biological functions of A. alternata.

Genotypic variation in floral volatiles influences floral microbiome more strongly than interactions with herbivores and mycorrhizae in strawberriesd

The floral microbiome is of significant relevance to plant reproduction and crop productivity. While plant genotype is key to floral microbiome assembly, whether and how genotypic variation in floral traits and plant-level mutualistic and antagonistic interactions at the rhizosphere and phyllosphere influence the microbiome in the anthosphere remain little known. Using a factorial field experiment that manipulated biotic interactions belowground (mycorrhizae treatments) and aboveground (herbivory treatments) in three strawberry genotypes, we assessed how genotypic variation in flower abundance and size and plant-level biotic interactions influence the bidirectional relationships between floral volatile organic compounds (VOCs) and the floral microbiome using structural equation modeling. We found that plant genotype played a stronger role, overall, in shaping the floral microbiome than biotic interactions with mycorrhizae and herbivores. Genotypic variation in flower abundance and size influenced the emission of floral VOCs, especially terpenes (e.g. α- and β-pinene, ocimene isomers) and benzenoids (e.g. p-anisaldehyde, benzaldehyde), which in turn affected floral bacterial and fungal communities. While the effects of biotic interactions on floral traits including VOCs were weak, mycorrhizae treatments (mycorrhizae and herbivory + mycorrhizae) affected the fungal community composition in flowers. These findings improve our understanding of the mechanisms by which plant genotype influences floral microbiome assembly and provide the first evidence that biotic interactions in the rhizosphere and phyllosphere can influence the floral microbiome, and offer important insights into agricultural microbiomes.

Phylogenomic analyses of Alternaria section Alternaria: A high-resolution, genome-wide study of lineage sorting and gene tree discordance

The genus Alternaria contains a diversity of saprobic and pathogenic species that can be found in a wide range of environments. Alternaria is currently divided into 26 subgeneric sections, and the "small-spored" Alternaria section Alternaria includes many species that are economically important agricultural pathogens. Recognizing that a stable framework for systematics and species identification is essential for management and regulation purposes, this section has experienced much taxonomic debate and systematic revision in recent years. Molecular phylogenetic studies have challenged the reliability of using morphological characteristics to differentiate Alternaria species but have also suggested that commonly used molecular markers for fungal phylogenetics may not be sufficiently informative at this taxonomic level. To allow the assessment of molecular variation and evolutionary history at a genome-wide scale, we present an overview and analysis of phylogenomic resources for Alternaria section Alternaria. We review the currently available genomic resources and report five newly sequenced genomes. We then perform multiple comparative genomic analyses, including macrosynteny assessment and inference of phylogenetic relationships using a variety of data sets and analysis methods. Fine-scale, genome-wide phylogenetic reconstruction revealed incomplete lineage sorting and the genomic distribution of gene/species tree discordance. Based on these patterns, we propose a list of candidate genes that may be developed into informative markers that are diagnostic for the main lineages. This overview identifies gaps in knowledge and can guide future genome sequencing efforts for this important group of plant pathogenic fungi.

Incidence of Alternaria Species Associated with Watermelon Leaf Blight in Korea

Alternaria leaf blight is one of the most common diseases in watermelon worldwide. In Korea, however, the Alternaria species causing the watermelon leaf blight have not been investigated thoroughly. A total of 16 Alternaria isolates was recovered from diseased watermelon leaves with leaf blight symptoms, which were collected from 14 fields in Korea. Analysis of internal transcribed spacer (ITS) region, glyceraldehyde-3-phosphate dehydrogenase (GAPDH), and RNA polymerase II second largest subunit (RPB2) were not competent to differentiate the Alternaria isolates. On the contrary, analysis of amplicon size of the histone H3 (HIS3) gene successfully differentiated the isolates into three Alternaria subgroups, and further sequence analysis of them identified three Alternaria spp. Alternaria tenuissima, A. gaisen, and A. alternata. Representative Alternaria isolates from three species induced dark brown leaf spot lesions on detached watermelon leaves, indicating that A. tenuissima, A. gaisen, and A. alternata are all causal agents of Alternaria leaf blight. Our results indicate that the Alternaria species associated watermelon leaf blight in Korea is more complex than reported previously. This is the first report regarding the population structure of Alternaria species causing watermelon leaf blight in Korea.