Sen Lian

Glomerella leaf spot (GLS), caused by Glomerella cingulata, poses a serious threat to apple production. Although the QoI fungicide pyraclostrobin has demonstrated high efficacy in controlling GLS, prolonged and intensive use has led to increasingly severe resistance issues. In this study, we report the first detection of pyraclostrobin resistance in field isolates of G. cingulata in Shandong Province, China, associated with G143S mutation in Cytochrome b (Cytb). No significant differences in biological fitness were observed between the pyraclostrobin-resistant (G143S) and sensitive isolates in terms of mycelial growth, sporulation, virulence, and stress tolerance. Molecular docking analysis revealed that the G143S substitution decreased the binding affinity between pyraclostrobin and Cytb. Additionally, the G143S mutation conferred cross-resistance to other QoI fungicides, but no cross-resistance was observed with tebuconazole or bromothalonil. These findings provide critical insights for rational and sustainable management of GLS.

Apple Valsa canker, caused by Valsa mali, is a serious disease affecting apple trees. Saprophytic colonization is a crucial stage in the infection process, with spore germination being a prerequisite for this phase. The effects of nutrients, temperature, moisture, pH, and light on the germination of both conidia and ascospores of V. mali were investigated under controlled environmental conditions. The results showed that the germination of V. mali spores requires supplementary nutrients, and compound nutrients are more conducive to spore germination. In distilled water, neither conidia nor ascospores germinated, resulting in a germination rate of 0%. In a sugar solution, the maximum germination rate remained below 75%. However, germination rates reached 100% in media supplemented with bark extract, fruit extract, or potato dextrose (PD) culture medium. The temperature range for conidial germination was 3.1–34.8°C, with an optimum of 25.6°C. Conidial germination required a duration of wetness or relative humidity exceeding 97% to be sustained for at least 7.6 hours. The suitable pH range for conidial germination was 4–8. In contrast, ascospore germination occurred over a broader temperature range of 0.6–41.7°C, with an optimum of 25.6°C. Ascospore germination required a relative humidity above 90% for at least 6.8 hours and was supported by a wider pH range of 4–10. Light significantly inhibited conidial germination but had no observable effect on ascospore germination. These results can be used to construct a prediction model that estimates the amount and timing of V. mali colonization on branch surfaces based on environmental and nutritional conditions. This prediction model can then inform the timing and methods for implementing apple Valsa canker control programs more effectively.

Glomerella leaf spot (GLS) caused by Glomerella cingulata (Colletotrichum spp.) is a devastating disease of apple (Malus × domestica Borkh.) in China. We evaluated the effects of temperature, moisture, and wetness duration on the production of conidia and ascospores on diseased leaves. The temperature required for producing conidia and ascospores by G. cingulata on diseased leaves ranged from 20 to 30°C, with an optimum of approximately 28°C. The pathogen on diseased leaves produced conidia and ascospores only when the leaves were wet or at a relative humidity of 100%. Conidia could not form until the diseased leaves maintained wetness for at least 24 h, and a modified Weibull model could describe the relationships between the number of conidia formed per square centimeter of diseased leaves and temperature and wetness duration. The production of ascospores on the leaves infected by GLS required 20, 14, and 16 days of leaf wetness at 20, 25, and 30°C, respectively. The sporulation ability of G. cingulata on GLS leaves could last more than 8 weeks. The half-life of sporulation or survival of the pathogen was 2.3 weeks when the diseased leaves were hung outdoors under natural conditions. Collectively, the sporulation of G. cingulata was a vital process affecting the occurrence and epidemic of GLS, and these results will provide valuable guidance for developing a model to predict the disease epidemics and establish effective control strategies.

Autophagy is a fundamental cellular degradation and recycling system that is crucial for maintaining cellular homeostasis, responding to stress, and ensuring the proper functioning of cells. To date, the biological functions of autophagy in the plant-pathogenic fungus Botryosphaeria dothidea remains largely unknown. In this study, we identified and characterized the E2-like enzyme Atg3 in B. dothidea. The autophagic process was blocked in the BdATG3 deletion mutant ΔBdAtg3, and the ΔBdAtg3 mutant showed serious defects in mycelial growth, conidiation, perithecium formation and virulence. In addition, the ΔBdAtg3 mutant exhibited an increased number of nuclei in mycelial compartment. All of the phenotypic changes of the ΔBdAtg3 mutant were restored by gene complementation. These results indicate that the E2-like enzyme Atg3 plays an important role in various developmental processes and pathogenesis of B. dothidea, which provides a potential target for developing novel fungicides.

Apple rust caused by Gymnosporangium yamadae is a significant disease in China’s main apple production areas. We evaluated the effects of temperature, moisture, and ultraviolet (UV) light on the germination, infection, and survival of teliospore horns and basidiospores under artificially controlled environmental conditions. The temperature required for the germination and infection by G. yamadae teliospores and basidiospores ranged from 5°C to 25°C, with an optimum of approximately 17°C. The teliospore horn germinated when soaking in distilled water for 5 min and required at least 2.3 h of development to produce basidiospores under the most favorable conditions. Basidiospores could only germinate in free water and produced germ tubes 0.8 h after being placed in the water. The half-life of the basidiospore was 72.5 h in the dark and only 9.5 h when exposed to intense UV light. The basidiospores inoculated on the host leaves required at least 2.3 h of water exposure to cause rust lesions. A revised Weibull model could describe the relationships between the germination and infection of teliospore horns and basidiospores with temperature and wetness duration. Collectively, these results can serve as a valuable guide for developing a model to predict future apple rust epidemics and establish a method for effective control strategies.

Glomerella leaf spot (GLS) caused by Glomerella cingulata is a newly emerging disease that results in severe defoliation and fruit spots in apples. In China, the compound of pyraclostrobin and tebuconazole was registered to control GLS in 2018 and has achieved excellent control efficiency. In this study, we showed that the high-level resistant isolates of G. cingulata to pyraclostrobin, caused by the point mutation at codon 143 (GGT→GCT, G143A) in cytochrome b gene, has appeared in apple orchards in Shandong province in 2020, and the resistance frequency was 4.8%. Based on the genotype of the resistant isolates, we developed a loop-mediated isothermal amplification (LAMP) assay for detection of the pyraclostrobin resistance. The LAMP assay was demonstrated to have good specificity, sensitivity and repeatability, and exhibited high accuracy to detect pyraclostrobin resistance in the field. This study reported the resistance status of GLS to pyraclostrobin in Shandong province, and developed a molecular tool for the detection of pyraclostrobin resistance, which is of practical significance for the scientific control of GLS.

Background: Apple Glomerella leaf spot (GLS) and apple bitter rot (ABR) are two devastating foliar and fruit diseases on apples. The different symptoms of GLS and ABR could be related to different transcriptome patterns. Thus, the objectives of this study were to compare the transcriptome profiles of Colletotrichum gloeosporioides species complex isolates GC20190701, FL180903, and FL180906, the pathogen of GLS and ABR, and to evaluate the involvement of the genes on pathogenicity. Results: A relatively large difference was discovered between the GLS-isolate GC20190701 and ABR-isolates FL180903, FL180906, and quite many differential expression genes associated with pathogenicity were revealed. The DEGs between the GLS- and ABR-isolate were significantly enriched in GO terms of secondary metabolites, however, the categories of degradation of various cell wall components did not. Many genes associated with secondary metabolism were revealed. A total of 17 Cytochrome P450s (CYP), 11 of which were up-regulated while six were down-regulated, and five up-regulated methyltransferase genes were discovered. The genes associated with the secretion of extracellular enzymes and melanin accumulation were up-regulated. Four genes associated with the degradation of the host cell wall, three genes involved in the degradation of cellulose, and one gene involved in the degradation of xylan were revealed and all up-regulated. In addition, genes involved in melanin syntheses, such as tyrosinase and glucosyltransferase, were highly up-regulated. Conclusions: The penetration ability, pathogenicity of GLS-isolate was greater than that of ABR-isolate, which might indicate that GLS-isolate originated from ABR-isolates by mutation. These results contributed to highlighting the importance to investigate such DEGs between GLS- and ABR-isolate in depth.