Discovering a New Fungus Behind Leaf Damage in Fragrant Mint Plants

Mary Jones
2nd February, 2024

Discovering a New Fungus Behind Leaf Damage in Fragrant Mint Plants

Crested Elsholtzia (Elsholtzia ciliata)

Photo adapted from: Igor Balashov / CC BY (Source)
A leaf blight disease affecting Elsholtzia ciliata, a medicinal plant important in traditional Chinese medicine, has been identified as being caused by the fungus Alternaria alternata by researchers at Heilongjiang University of Chinese Medicine[1]. This finding is significant as it represents the first documented instance of this particular disease affecting this plant in China, potentially posing a future economic threat to cultivation. Leaf blight manifests as irregular, dark lesions appearing first at the leaf tips, eventually spreading to encompass the entire leaf area. In September 2022, researchers observed a 15% incidence of this disease within a planting base in Sichuan Province, prompting investigation into the causative agent. The study involved isolating the fungus from infected leaf samples. Standard microbiological techniques were employed, including culturing the fungus on potato dextrose agar (PDA) – a common growth medium – and observing its characteristics under a microscope. Initial observations suggested a close resemblance to Alternaria alternata based on features like conidia shape and size, and conidiophore structure. Conidia are asexual spores, and conidiophores are the structures that bear them. These morphological characteristics are traditionally used in fungal identification. However, relying solely on visual characteristics can be misleading. Recent research has highlighted the complexities within the Alternaria genus[2]. A comprehensive study utilizing whole-genome sequencing and multi-gene analysis revealed that many morphologically similar Alternaria species are, in fact, genetically very close, leading to the synonymization of 35 morphospecies under A. alternata[2]. This underscores the importance of molecular methods for accurate fungal identification. To confirm the identification, the researchers employed molecular techniques, specifically sequencing several regions of the fungal DNA, including the internal transcribed spacer (ITS), glyceraldehyde-3-phosphate dehydrogenase (gpd), Alternaria major allergen (Alt a 1), RNA polymerase second largest subunit gene (RPB2) and translation elongation factor 1-alpha (TEF 1). These gene regions are commonly used in fungal phylogenetics – the study of evolutionary relationships. The resulting DNA sequences were compared to known sequences of A. alternata in a database, and phylogenetic analysis showed a 99% bootstrap value with the type strain CBS 595.93, strongly supporting the identification. The sequences obtained have been deposited in GenBank for future reference. To definitively prove that A. alternata causes the disease, the researchers performed a pathogenicity test. This involved spraying a spore suspension of the fungus onto healthy E. ciliata plants. Control plants were sprayed with water. The plants were then monitored for disease development. The inoculated plants developed the same leaf blight symptoms observed in the field, while the control plants remained healthy. Furthermore, the fungus was successfully re-isolated from the artificially infected plants, fulfilling Koch’s postulates – a set of criteria used to establish a causal relationship between a microorganism and a disease. The findings of this study build upon previous work that has demonstrated the widespread presence and genetic similarity within the Alternaria section Alternaria[2]. The relatively low genetic diversity observed within this section, with genome identities ranging from 96.7-98.2%[2], suggests that A. alternata is a highly adaptable and successful pathogen. This adaptability may explain its ability to infect a new host, E. ciliata, in this instance.

AgricultureEnvironmentPlant Science

References

Main Study

1) First Report of Alternaria alternata Causing Leaf Blight on Elsholtzia ciliata in China.

Published 1st February, 2024

https://doi.org/10.1094/PDIS-09-23-1814-PDN

Related Studies

2) Alternaria section Alternaria: Species, formae speciales or pathotypes?

https://doi.org/10.1016/j.simyco.2015.07.001


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