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.

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.

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.

Development and application of a LAMP assay for the detection of the latent apple tree pathogen Valsa mali

Valsa mali, the causal agent of apple Valsa canker (AVC), produces cankers, resulting in the death of infected tissues and eventually the entire tree. Due to the long latent period of the disease, it is necessary to develop a rapid, sensitive and reliable field-based assay to effectively diagnose AVC when the plant is still symptomless. Loop-mediated isothermal amplification (LAMP) is a novel detection method that synthesizes large amount of DNA and produces the visible byproduct (magnesium pyrophosphate) without conventional thermal cycling. A set of six LAMP primers were designed to target a species specific region of the EF-1α sequence which can be completed at 61 ˚C in 60 min. A positive result is indicated by color change after adding the intercalating dye SYBR Green I. The specificity of the LAMP was validated using DNA from 45 representative isolates of V. mali as well as closely related species V. malicola, V. leucostoma and V. sordida. The sensitivity of the LAMP was determined to be 1 ng/μL of DNA or as few as 10 spores. Since the assay does not require expensive equipment or specialized techniques, the LAMP-based diagnostic method can be applied under field conditions to more precisely and efficiently access disease pressure in apple orchards.

Saccharothrix yanglingensis Strain Hhs.015 Is a Promising Biocontrol Agent on Apple Valsa Canker

Apple Valsa canker (AVC), caused by Valsa mali, results in a serious and persistent disease problem for apple production in China and is difficult to control by chemical and agricultural measures. In this study, we determined the inhibitory effects of an endophytic actinomycete Saccharothrix yanglingensis strain Hhs.015 on V. mali under laboratory and the field conditions. Fermentation broth (FB) of Hhs.015 significantly inhibited conidial germination and mycelial growth, causing malformed and dysfunctional fungal structures. Detached apple (Malus domestica ‘Fuji’) twigs smeared with FB before V. mali inoculation significantly reduced lesion development, especially sporulation. In the orchard trials conducted from 2010 to 2013, trunks and branches smeared with FB three times significantly reduced the number of new lesions. In addition, wounds after lesion surgery coated with FB also enhanced callus formation and significantly improved the cure rate. The results showed that S. yanglingensis Hhs.015 is effective against AVC in orchards. Thus, S. yanglingensis Hhs.015 can be further developed as an effective biocontrol agent for more sustainable management of AVC.