Chemical and Microbial Characteristics of Blackening Disease in Lotus (Nelumbo nucifera Gaertn.) Caused by Hirschmanniella diversa Sher

Hazuki Kurashita,Motonori Takagi,Masashi Hatamoto,Takeshi Sato,Kyohei Kuroda,Maki Goto,Takashi Narihiro,Tetsuro Kariya,Takashi Yamaguchi,Shun Tomita,Shinya Maki

doi:10.3390/agronomy11122517

Hazuki Kurashita, Motonori Takagi + Show 9 more

Open Access

https://doi.org/10.3390/agronomy11122517

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Abstract

The lotus (Nelumbo nucifera Gaertn.) is widely cultivated in Asia, but a blackening disease in the lotus tuber, called “kurokawa-senchu-byo”, is a serious problem caused by the Hirschmanniella diversa Sher plant-parasitic nematode. To effectively control the disease, we must elucidate the blackening mechanisms; therefore, in this study, we performed a soil chemical analysis and an evaluation of the disease level in the lotus cultivation fields, identified the chemical components of the black spots on the lotus surface, and performed a 16S rRNA gene-based microbial community analysis of the black spots. Using linear regression analysis, a positive linear relationship with a strong correlation between the damage index values and fertilizer components such as P2O5 was observed. As a result of scanning electron microscopy and energy-dispersive X-ray spectroscopy analysis, phosphorus (P) and iron (Fe) were found to be concentrated in the black spots of the lotus tubers. Furthermore, we found that the concentrations of P and Fe in the black spots were 1.5- and 2.7-fold higher, respectively, than those found in the healthy parts of the lotus tubers. A 16S rRNA gene analysis revealed that dissimilatory Fe(III)-reducing bacteria (DIRB) were predominant in the black spots, suggesting that these bacteria are important to the formation of P and Fe compounds in the black spots.

Highlights

The lotus (Nelumbo nucifera Gaertn.) is widely cultivated in Asia, and the cultivation areas are estimated to be 133,300 ha in China and 4000 ha in Japan [1,2]
The analyses revealed a positive linear relationship with a strong correlation between the damage index values and P2O5 (R2 = 0.63), CEC (0.55), EC (0.63), CaO (0.55), TN (0.60), and TC (0.64), (Figure 1A–F), whereas there were weak or no correlations with the NH4-N, K2O, MgO, and C/N ratios
The specific gravity (SG), which has a negative linear relationship, indicates that softer soils have greater damage index values than harder soils; changes in SG properties may affect the mobility of H. diversa in soil

Summary

Introduction

The lotus (Nelumbo nucifera Gaertn.) is widely cultivated in Asia, and the cultivation areas are estimated to be 133,300 ha in China and 4000 ha in Japan [1,2]. The disease causes blackening and deformation of the lotus tubers [1,4], and a resulting annual economic loss is estimated at USD 1 million in the Tokushima Prefecture (the second-largest lotus production area in Japan). It is known that the disease has been caused by two plant-parasitic nematodes, Hirschmanniella diversa Sher and H. imamuri Sher [5]. A recent study revealed that H. diversa is a more critical causative agent of the disease than H. imamuri, based on quantitative PCR approaches [1,6]. H. diversa is known to exhibit root-parasitic nematodes that may have a wide range of plant hosts since it has been detected in several types of weeds [7]. Several stages of the nematodes (e.g., adults, juveniles, and eggs) were found in the lotus roots, suggesting that H. diversa completes most of its lifecycle in the roots [8]

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