Phenology-Based Management of Alternaria Fruit Rot in Pink Lady Apples

Characterization of Fungal Species Isolated from Cankered Apple Barks Demonstrates the Alternaria alternata Causing Apple Canker Disease

Apple canker disease, also named as apple Valsa canker, is one of the most destructive diseases for apples (Malus domestica Borkh.). Cytospora/Valsa spp. are the dominant causal agent of this disease, but many studies have revealed that fungi from some other genus can also cause typical apple canker symptoms. In this study, we performed fungal pathogen isolation from cankered 'Fuji' apple barks. Six representative morphologically different fungi (Strain 1-6) were further subjected to ITS sequencing and evolutionary analysis. Molecular identification results revealed that Strains 1-6 are Cytospora mali, Fusarium cf. solani, Alternaria alternata, C. mali, Diplodia seriata and F. proliferatum, respectively. All these fungi have been reported to be causal agents of apple diseases. By inoculating fungal plugs onto trunks of 'Fuji' apple trees, the pathogenicity of the six fungi were accessed. Only the inoculations of the two C. mali strains (Strain 1 and Strain 4) and the A. alternata strain (Strain 3) resulted in typical apple canker symptoms in trunks. It is worth noting that Strain 1 caused much more severe canker symptoms and higher pathogenicity incidence than the other two fungi. A. alternata has been identified as a pathogen causing diseases on apple fruits and leaves. By further assessing its pathogenicity on apple fruits and leaves, we verified that it can also cause typical fruit rot and leaf spot symptoms. To the best of our knowledge, this is the first report on apple canker disease caused by A. alternata in China. Our present study can provide a theoretical foundation for the prevention and control of apple canker disease.

Understanding Environmental and Physiological Factors Affecting the Biology of Diaporthe ilicicola, the Fungus Causing Latent Fruit Rot in Winterberry

Fruit rot in winterberry is associated with a complex of fungal pathogens. Among them, Diaporthe ilicicola plays a unique role by infecting flowers at bloom, resulting in symptom development in mature fruit. This research aimed to identify at what stage of maturation Ilex fruit can develop disease symptoms and correlate changes in fruit physiology (sugar and phenolic content) and environment (temperature and light intensity) with disease incidence. Correlation data informed in vitro studies testing the ability of putative factors to alter growth of D. ilicicola and select opportunistic fungi within the fruit rot complex: Alternaria alternata, Colletotrichum fioriniae, and Epicoccum nigrum. Results indicated that Ilex fruit do not develop symptoms until 81 to 108 days after inoculation. Temperature and fruit phenolic content were negatively correlated with disease incidence, while fruit sugar concentration and light intensity were positively correlated. In vitro assays revealed that sugar concentration had no effect on the growth of D. ilicicola, but increased light intensity increased hyphal growth and pycnidium formation. Additionally, phenolics extracted from fruit inhibited spore germination in A. alternata, induced secondary conidiation in C. fioriniae, and late season phenolic extracts increased hyphal melanization and pycnidial formation in D. ilicicola. Finally, drops in field temperatures, when replicated in vitro, resulted in a decrease in hyphal growth and spore germination for all fungi. These results suggest that changes in Ilex fruit phenolics during maturation and the increased exposure to light following defoliation may play a role in symptom development by altering D. ilicicola growth within the fruit.

Comparative transcriptomics analysis reveals MdGRAS53 contributes to disease resistance against Alternaria blotch of apple

Alternaria blotch disease, caused by Alternaria alternata apple pathotype (AAAP), is one of the most prevalent diseases in apple production. To identify AAAP resistance-related genes and provide a theoretical basis for Alternaria blotch disease resistance breeding, we used two apple cultivars, 'Jonathan', a variety resistant to AAAP infection, and 'Starking Delicious', a variety susceptible to AAAP infection, as materials to perform transcriptome sequencing of apple leaves 72 h after AAAP infection. A Venn diagram showed that a total of 5229 DEGs of 'Jonathan' and 4326 DEGs of 'Starking Delicious' were identified. GO analysis showed that these DEGs were clustered into 25 GO terms, primarily "metabolic process" and "catalytic activity." Functional classification analyses of the DEGs indicated that "MAPK signaling pathway-plant pathway" is the most significant metabolic pathway among the top 15 KEGG pathways, followed by the "plant hormone signal transduction" pathway. There are more DEGs in 'Jonathan' that are significantly classified GO terms and KEGG pathways than in 'Starking Delicious'. Specifically, 13 DEGs were identified as involved in the GA-GID1-DELLA module, and the expression of MdGRAS53, a homologous gene of DELLA, was significantly upregulated in 'Jonathan' compared with 'Starking Delicious'. Phenotype analysis revealed that exogenous hormone GA3 suppressed apple resistance to AAAP infection and reduced the expression of MdGRAS53. The opposite result was observed for exogenous spraying of paclobutrazol (PAC), an inhibitor of gibberellin synthesis. Overexpression of MdGRAS53 in apple leaves by transient transformation decreased lesion area and the number of spores in leaves infected with AAAP, while silencing MdGRAS53 showed the opposite result. Meanwhile, SA/JA signaling pathway-related genes were upregulated significantly in MdGRAS53-overexpressed leaves and downregulated significantly in MdGRAS53-silenced leaves. The findings suggest that the GA-GID1-DELLA module is involved in apple resistance to AAAP, and MdGRAS53, a DELLA homologous gene, may play a positive role in this resistance by modulating cooperative JA- and SA-dependent pathways.

β-Aminobutyric Acid Induced Resistance against Alternaria Fruit Rot in Apple Fruits

Fruit body rot and calyx rot caused by Alternaria alternata f. sp. mali is an important disease of apple worldwide. The disease has recently become severe in cv. Pink Lady apple in Israel to an extent that has never been reported elsewhere in the world. No alternative control measures of the disease except fungicides are known. Here, we show for the first time that dl-β-aminobutyric acid (BABA) induces resistance against Alternaria fruit rot (AFR) in apple fruits in the laboratory and in the orchard. AFR was inhibited in fruits treated with BABA of 1000 μg/mL. BABA did not inhibit spore germination or mycelial growth of the pathogen in vitro (up to 2000 μg/mL). It was most inhibitory when applied 4 days prior to inoculation of detached fruits. BABA inhibited AFR also curatively when applied at 24 h post inoculation. Five other isomers of aminobutyric acid failed to protect the fruits from rot formation. Three field trials in commercial apple orchards proved that BABA was as protective against AFR as the commercial standard fungicidal mixture of azoxystrobin and difenoconazole. This research suggests that BABA may serve as a resistance inducer in apple against AFR. It can be used as an adequate alternative to the currently used fungicides or integrated in disease management programs to reduce fungicide load and buildup of fungicide resistance.