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Southern blight, caused by Sclerotium rolfsii Sacc., is a significant disease of apple in China and many other counties. Sclerotium germination and hyphal growth are two critical processes for epidemic development. Controlled experiments were conducted to study the effects of environmental conditions on sclerotium germination and hyphal growth of the pathogen. Temperature for sclerotium germination and hyphal growth of S. rolfsii ranges from 15 to 35oC with the optimum at 31.5oC. The soil water content needs to exceed 30%, and once over this threshold, soil water content did not affect sclerotium germination and hyphal growth. The rate of sclerotium germination was high in sawdust, and hyphae grew fastest in the mixture of sawdust and soil. Sclerotium germination and hyphal growth were inhibited in nursery substrate. Hyphal growth was affected by pH but to a far lesser extent sclerotium germination. Soaking in water inhibited sclerotium germination.

Grape white rot is a destructive fungal disease occurring worldwide. Recently, Coniella vitis was identified as the predominant pathogen causing this disease in China. As the periderms of grape shoots are severely degraded by C. vitis, it was speculated that cell wall-degrading enzymes (CWDEs) might play a key role in the pathogenesis of this disease. Therefore, this study aimed to examine the hydrolytic activity of the CWDEs of C. vitis. The results showed that xylanase (Xy) and xyloglucanase (XEG) had high levels of hydrolytic activity both in vitro and in vivo. Furthermore, a high-virulence fungal strain exhibited higher levels of Xy and XEG activities compared with a low-virulence strain. The genome of the fungus was found to harbor two XEG-coding genes CvGH74A and CvGH74B, which belonged to the glycoside hydrolase (GH)74 family. The expression level of CvGH74A was found to be high during pathogen infection. CvGH74A gene deletion mutants were generated using the split-marker method. The deletion of CvGH74A decreased both the hydrolytic activities of XEG and Xy and also the ability of the fungus to infect the grape leaves. No differences in the hyphal growth, morphology of colonies, or conidiation were found between the ΔCvGH74A mutant strains and the wild-type strain. Together, these results suggested that CvGH74A acted as an important virulence factor, and its enzymatic activity might regulate the virulence of the pathogen. This study was novel in reporting that GH74 XEG acted as a virulence factor in C. vitis.

FgGCN5, a GCN5 homolog in Fusarium graminearum, plays a critical role in hyphal vegetative growth, asexual and sexual reproduction, deoxynivalenol (DON) biosynthesis and plant infection. For nuclear localized GCN5, four conserved sequence motifs (I–IV) are presented in the catalytic domain and a bromodomain in the carboxy-terminus. As a lysine acetyltransferase, conserved negatively charged residues are present to neutralize the protons from lysine substrates. However, the role of conserved motifs/domains and residues in FgGCN5 are unclear. Here, we generated deletion mutant strains for each the conserved motifs/domains and a glutamate residue 130 (E130) replacement mutant. Deletion of each conserved motif in the catalytic domain and replacement of E130 site resulted in manifold defects in hyphae growth, asexual and sexual development, DON biosynthesis, and plant infection. Phenotypic defects in the mutant strains were similar to deletion mutants. The deletion of the bromodomain led a significant reduction in DON production and virulence, with no effects on hyphae growth, asexual or sexual reproduction. FgGCN5 was further found to localize to the nucleus in conidia and hyphae cells. In conclusion, FgGCN5 encodes a nuclear localized acetyltransferase. The conserved motifs in the catalytic domain and E130 are essential for correct functions of the gene. The conserved bromodomain is important for DON production and pathogen virulence. This was the first report to identify the functions of conserved motifs/domains in FgGCN5, which will contribute to our understanding of the mechanism(s) by which FgGCN5 regulates F. graminearum.

Apple Marssonina leaf blotch (AMLB

Bletilla striata, known as Chinese ground orchid, is economically important because of its medicinal and ornamental properties around the world, especially in China. The wild population of B. striata is decreasing, it was listed in appendix II (species that may be threatened or extinct) of the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) (CITES 2013). Therefore, farm-based cultivation of B. striata is emerging to meet the demands of the industry. In June 2016, disease symptoms similar to orchid southern blight (Punja 1985) were observed in a B. striata cultivation field (N30°21′27.37″, E112°19′58.37″) in Jingzhou, Hubei Province, China. In a follow-up survey, the disease was also found in nearby counties and provinces. Disease incidence reached 25% in the most severely affected field. Symptoms first appeared at the junction of the stem base and the soil, as necrotic dark brown stem lesions. Additional symptoms included leaf yellowing and wilting, root rot, and death during warm and humid weather. White mycelium and tan to brown sclerotia were found on the stems and roots of plants in the field and plants incubated in the lab. To isolate the pathogen, tissue from plant samples collected from a nursery (Orchid Biotechnology, Jingzhou, China) was surface sterilized with 2% sodium hypochlorite (NaOCl) for 3 min and then rinsed and incubated on potato dextrose agar medium with 50 μg/ml of ampicillin and rifampin. After 7 days of incubation at 28°C, sclerotial primordia developed followed by sclerotia approximately 2 mm in diameter after 10 days. No spores were observed. Total genomic DNA of isolate BH-2 was extracted with the cetyltrimethylammonium bromide method and then amplified and sequenced with fungal ribosomal primers ITS1/ITS4 (White et al. 1990). The length of the amplicon was 683 bp (GenBank accession no. KY606616), 99% similarity to Athelia rolfsii (teleomorph) isolate ATCC201126 (accession no. AF499018). Phylogenetic analysis of the internal transcribed spacer sequences with the neighbor-joining method in MEGA6 (Pennsylvania State University, University Park, PA) showed BH-2 was clustered with six Sclerotium rolfsii isolates selected from NCBI database (accession nos. AF499018, JN081867, JN241565, GQ3585118, JF966208, and KJ002764). Therefore, isolate BH-2 was identified as S. rolfsii. To produce inoculum for Koch’s postulates, autoclaved oat grain was inoculated with BH-2 then incubated at 30°C for 10 days. Infected oat grains were placed near the roots of 90-day-old healthy seedlings (n = 3), and sterilized oat grain was used as a control (n = 3). All seedlings were placed in sealed plastic bags with air valves and grown in a greenhouse (25 ± 2°C, relative humidity 65%). Water-soaked streaks and white hyphae were observed after 2 to 3 days. Symptoms typical of southern blight, necrotic stems, and leaf lesions were observed after 9 days. Reisolation was performed with surface-sterilized tissue from symptomatic seedlings, and isolates were confirmed as S. rolfsii based on morphological and molecular characters. To the best of our knowledge, this is the first report of S. rolfsii causing southern blight of Bletilla spp. in China.

Ipomoea purpurea is widely used as an ornamental plant and in traditional Chinese medicine in China. Seed is used in treating edema and anuria, dispelling phlegm. In June 2016, severe leaf spot disease on I. purpurea with 30% disease incidence was observed in the Laoshan Mountain of Qingdao, China. Symptoms on leaves initially appeared as light to dark brown, necrotic spots with yellow borders or no borders. The spots gradually enlarged in size, becoming irregular. Tissue samples were taken from the margins of lesions, cut into small pieces, surface-disinfected with 70% ethanol for 30 seconds, rinsed with sterile water three times, placed on potato dextrose agar (PDA), and incubated at 25°C in the dark. A total of 10 isolates were obtained from 13 symptomatic leaf samples. Thereafter, 10 single spore isolates were obtained as described (Dong 2009) and transferred onto a potato carrot agar (PCA) medium (Mirkova 2003). After six days of growth on PCA at 25°C in the dark, colonies had round margins and aeria...