Globisporangium concavum sp. nov. and three oomycete species causing rhizome rot of lotus (Nelumbo nucifera) in Japan

In the quiet, damp corners of Japan's rice paddies, a perennial aquatic plant known as lotus thrives, its thick, edible roots harvested for food. These roots, called rhizomes, are a staple in supermarkets, often sold fresh and stored in cool conditions to maintain their crisp texture. However, like many harvested crops, they are vulnerable to invisible invaders. Among the most common of these are water molds, a group of organisms that look and behave like fungi but belong to a distinct biological family. While true fungi have cell walls made of chitin, water molds build theirs from cellulose, the same tough material found in plant cell walls. These microscopic organisms are notorious for causing rot in crops, especially in wet environments, and they can strike even after a plant has been dug up and placed in storage. Understanding exactly which species are attacking a specific crop is vital for farmers and food producers, as different species may require different management strategies to prevent loss.

Researchers at Gifu University recently turned their attention to lotus rhizomes that had begun to rot while sitting in refrigerated storage. Between 2014 and 2017, they collected samples from supermarkets where the roots had developed a soft, black decay accompanied by a fuzzy white growth. By carefully isolating the organisms responsible for this rot, the team identified five distinct strains of water molds. All five proved capable of causing disease when introduced to healthy lotus tissue, but only if the tissue had been wounded, such as by a cut or a puncture. This finding highlights a specific vulnerability: the pathogens cannot easily penetrate the intact skin of the root but thrive once the barrier is broken, a condition that often occurs during harvesting and handling.

The investigation revealed a mix of known and unknown culprits. Three of the isolated strains were identified as species already known to science: one was Globisporangium oryzicola, a species previously known to infect rice seedlings; two were Phytopythium helicoides, a mold already linked to lotus rot in Japan and China; and one was Pythium dissotocum, a common soil-dwelling pathogen. The discovery of these three species on lotus confirms that the disease is caused by a diverse group of organisms that can jump between different crops grown in similar wet environments. However, the most significant discovery was the fourth strain, which did not match any known species. This organism, designated as strain GK16Ren2, represented a new species of water mold, which the researchers named Globisporangium concavum.

To confirm that this new organism was indeed unique, the scientists examined its microscopic features and compared its genetic code to that of its relatives. Under a microscope, the new species produced spherical structures that held its reproductive cells, known as oospores. What set it apart was the shape of the wall surrounding these structures. While most related species have smooth walls, the walls of Globisporangium concavum were noticeably indented or concave, giving them a hollowed-out appearance. Additionally, this new species produced an abundance of swollen, lemon-shaped bumps along its thread-like body, a feature that was rare in its closest known relative. Genetic analysis of its DNA further confirmed that it occupied a distinct branch on the family tree of water molds, separate from all other known species.

The researchers also tested how well these organisms could grow at different temperatures to understand their behavior in storage. The new species, Globisporangium concavum, could grow at temperatures as low as 3 degrees Celsius, which is colder than the minimum temperature for its relative Globisporangium oryzicola. This ability to grow in the cold explains why the rot appeared in refrigerated storage, where temperatures are typically kept low to preserve the lotus roots. In fact, the disease was more severe at these low temperatures than at room temperature, suggesting that the cold storage intended to keep the lotus fresh might inadvertently favor the growth of these specific pathogens.

This study provides the first confirmed report of Globisporangium oryzicola, Globisporangium concavum, and Pythium dissotocum as causes of lotus rhizome rot. The identification of a new species, Globisporangium concavum, adds a critical piece to the puzzle of post-harvest diseases affecting this important crop. By pinpointing the exact organisms responsible and understanding their specific traits, such as their need for wounds to infect and their ability to grow in the cold, the findings offer a clearer picture of the biological challenges facing lotus production. The work underscores that even in controlled storage environments, a diverse array of microscopic life can threaten food security, and that the discovery of new species remains an ongoing process in the study of plant health.