Dopamine enhances the resistance of apple to Valsa mali infection

Apple Valsa canker is considered one of the most serious apple diseases. Dopamine is a catecholamine with key physiological functions in plants. Tyrosine decarboxylase (TYDC) is not only involved in the synthesis of dopamine in plants but may also play an important role in the resistance of plants to pathogen infection. In this study, we show that 100 μM exogenous dopamine application and MdTYDC (Malus domestica TYDC) overexpression (OE) enhances the resistance of apple to V. mali (Valsa mali) infection, likely because the increased dopamine content reduces the accumulation of H2O2 and increases the accumulation of phenolic compounds and salicylic acid (SA) in dopamine-treated and OE apple plants. The activity of chitinase and β-1, 3-glucanase and the expression of SA-related genes were induced more strongly by V. mali in dopamine-treated and OE apples. The dopamine content was significantly higher in dopamine-treated and OE apples than in their respective controls under both normal and inoculated conditions (P < 0.05). Overall, these findings indicate that the application of exogenous dopamine and the overexpression of MdTYDC may enhance the resistance of apples to V. mali infection by altering the dopamine content, which improves antioxidant capacity, promotes the accumulation of phenolic compounds and SA, and enhances the activity of disease resistance-related proteins.

Surface Enhanced Raman Scattering (SERS) Spectroscopy Combined With Chemical Imaging Analysis For The Early Detection of Apple Valsa Canker

Background: Apple Valsa Canker (AVC) with early incubation characteristics is a severe apple tree disease. Therefore, early detection of the infected trees is necessary to prevent the rapid development of the disease. Surface enhanced Raman Scattering (SERS) spectroscopy is a promising technique that simplifies detection procedures and reduces detection time. Meanwhile, SERS enhance signals at low laser powers and suppress biological fluorescence. In this study, the early detection of the AVC disease was carried out by combining SERS spectroscopy with the chemometrics methods and machine learning algorithms, and then chemical distribution imaging was successfully applied to the analysis of disease dynamics.Results: Firstly, the microstructure, UV-Vis spectrum, and Raman spectrum of SERS metallic nano-substrates were proved to investigate the enhancement effects of the synthesized AgNPs. Secondly, the multiple spectral baseline correction (MSBC), the asymmetric least squares (AsLS), and the adaptive iterative reweighted penalized least squares (air-PLS) were adopted to eliminate the disturbances of the baseline offset. The correlation analysis method was employed to identify the best baseline correction algorithm, which was the air-PLS algorithm herein. Meanwhile, principal component analysis (PCA) was used to perform clustering analysis based on the healthy, early disease, and late disease sample datasets, demonstrating obvious clustering effects. After that, optimal spectral variables were selected to build machine learning models to detect AVC disease, incorporating the BP-ANN, ELM, RForest, and LS-SVM algorithms. The accuracy of these models was above 90%, showing excellent discriminant performance. Finally, SERS chemical imaging provided the spatiotemporal dynamic characteristics of changes in the cellulose and lignin of the phloem disease-health junction under AVC stress. The results suggested that cellulose and lignin in the cell walls of infected tissues reduced significantly.Conclusions: SERS spectroscopy combining with chemical imaging analysis for early detection of the AVC disease was considered feasible and promising. This study provided a practical method for the rapid diagnosis of apple orchard diseases.

Biological Control of Pear Valsa Canker Caused by Valsa pyri Using Penicillium citrinum

Valsa canker caused by Valsa pyri is one of the most destructive diseases of commercial pear. For the present analysis, 29 different endophytic fungal strains were isolated from the branches of a healthy pear tree. In dual culture assays, strain ZZ1 exhibited robust antifungal activity against all tested pathogens including Valsa pyri. Microscopic analyses suggested that following co-culture with ZZ1, the hyphae of V. pyri were ragged, thin, and ruptured. ZZ1 also induced significant decreases in lesion length and disease incidence on detached pear branches inoculated with V. pyri. ZZ1 isolate-derived culture filtrates also exhibited antifungal activity against V. pyri, decreasing mycelial growth and conidium germination and inhibiting V. pyri-associated lesion development on pear branches. These results suggest that the ZZ1 isolate has the potential for use as a biological control agent against V. pyri. The strain was further identified as Penicillium citrinum based on its morphological characteristics and molecular analyses. Overall, these data highlight a potentially valuable new biocontrol resource for combating pear Valsa canker.

Full-Length Transcriptome-Wide Characteristic and Functional Identification of WRKY Family in Malus sieversii during the Valsa Canker Disease Response

WRKY transcription factors are one of the largest families in plants, playing important roles in regulating plant immunity. Malus sievesii has abundant genetic diversity and can offer various and high-quality gene resources. In this study, 112 putative MsWRKY proteins were identified from a full-length transcriptome of M. sieversii during the Valsa canker disease (caused by Valsa mali). The MsWRKY proteins were phylogenetically divided into three groups (I–III). Motif compositions of the MsWRKY proteins were clustered and fifteen conserved motifs were observed. Expression pattern analysis showed that thirty-four MsWRKY transcripts strongly responded to the V. mali infection, demonstrating that MsWRKY transcripts might play different roles during the response. Functional identifications were subsequently conducted with transient expressions, demonstrating that MsWRKY16, MsWRKY21, MsWRKY70, MsWRKY74 and MsWRKY85 positively regulated the resistant response. Besides, the MsWRKY21, MsWRKY70 and MsWRKY85 were dramatically induced by salicylic acid (SA), methyl-jasmonate acid (MeJA) and 1-aminocyclopropane-1-carboxylate (ACC), indicating that they play important roles in the regulatory resistance of V. mali infection. This work provides a comprehensive understanding of the WRKY family in M. sieversii and will build a foundation for future research of the potential disease resistances MsWRKY transcripts.

Infection of Two Heterologous Mycoviruses Reduces the Virulence of Valsa mali, a Fungal Agent of Apple Valsa Canker Disease

Mycovirus infection has been widely shown to attenuate the virulence of phytopathogenic fungi. Valsa mali is an agriculturally important fungus that causes Valsa canker disease in apple trees. In this study, two unrelated mycoviruses [Cryphonectria hypovirus 1 (CHV1, genus Hypovirus, and single-stranded RNA) and Mycoreovirus 1 (MyRV1, genus Mycoreovirus, double-stranded RNA)] that originated from Cryphonectria parasitica (chestnut blight fungus) were singly or doubly introduced into V. mali via protoplast fusion. CHV1 and MyRV1 stably infected V. mali and caused a reduction in fungal vegetative growth and virulence. Co-infection of both viruses further reduced the virulence of V. mali but compromised the stability of CHV1 infection and horizontal transmission through hyphal anastomosis. Infections of MyRV1 and, to a lesser extent, CHV1 up-regulated the transcript expression of RNA silencing-related genes in V. mali. The accumulation of CHV1 (but not MyRV1) was elevated by the knockdown of dcl2, a key gene of the RNA silencing pathway. Similarly, the accumulation of CHV1 and the efficiency of the horizontal transmission of CHV1 during co-infection was restored by the knockdown of dcl2. Thus, CHV1 and MyRV1 are potential biological control agents for apple Valsa canker disease, but co-infection of both viruses has a negative effect on CHV1 infection in V. mali due to the activation of antiviral RNA silencing by MyRV1 infection.

The influence of lower temperature induction of Valsa mali on the infection of apple trees

Apple valsa canker (AVC), caused by Valsa mali (Vm), is one of the most important diseases of apple trees in China. AVC occurred severely along with cold winter or cold spring. However, the effect of lower temperature on Vm is poorly understood. This study evaluated the influence of lower temperature pre-treatment of Vm on the infection of apple twigs and leaves. The results showed that exposing of Vm at lower temperature (between -10°C and 10°C) for more than 18 h significantly increased the disease severity of apple leaves and twigs, with a higher LAR, lesion length and DI than that at 25°C. In addition, cold treatment ranging from -5°C to 10°C promoted the colony growth. Meanwhile, the relative expression of four cell wall degrading enzymes (CWDEs) related genes pre-treated at -5°C and 5°C were significantly higher than that at 25°C. The results indicated that the virulence of Vm mycelium is sensitivity for lower temperature change. After sensing lower temperature changes, Vm can adjust its infection of apple trees by regulating the expression of pathogenicity gene and growth rate. Spring is the season with very frequent temperature changes, and Vm is highly invasive in this season. Therefore, more attention should be paid in spring to protecting apple trees from infection by Vm, such as reduction of pruning wounds formation in spring, and application of protective agents to the pruning wounds in time.

Fungal species associated with apple Valsa canker in East Asia

Since its discovery more than 110 years ago, Valsa canker has emerged as a devastating disease of apple in East Asia. However, our understanding of this disease, particularly the identity of the causative agents, has been in a state of confusion. Here we provide a synopsis for the current understanding of Valsa canker and the taxonomy of its causal agents. We highlight the major changes concerning the identity of pathogens and the conflicting viewpoints in moving to “One Fungus = One Name” system for this group of fungal species. We compiled a list of 21 Cytospora species associated with Malus hosts worldwide and curated 12 of them with rDNA-ITS sequences. The inadequacy of rDNA-ITS in discriminating Cytospora species suggests that additional molecular markers, more intraspecific samples and robust methods are required to achieve reliable species recognition.