Reemerging Rice Orange Leaf Phytoplasma with Varying Symptoms Expressions and Its Transmission by a New Leafhopper Vector-Nephotettix virescens Distant.

Gilda B Jonson,Jerlie M Matres,Toshiharu Tanaka,Socheath Ong,Il-Ryong Choi,Sotaro Chiba

doi:10.3390/pathogens9120990

Abstract

Rice orange leaf phytoplasma (ROLP) belongs to the “Candidatus Phytoplasma asteris” 16SrI-B subgroup, which is solely transmitted by the zigzag-striped leafhopper (Recilia dorsalis Motchulsky) and the green leafhopper (Nephotettix cincticeps Uhler) (Hemiptera: Cicadellidae). Recently, rice plants showing orange leaf discoloration have become ubiquitous in several paddies of two provinces in the Philippines. In total of 98 symptomatic rice plants, 82% (Laguna) and 95% (Mindanao) were ROLP-positive by nested PCR detection. These plants showed more varying symptoms than previously reported. The vector insect R. dorsalis was scarcely present but green paddy leafhopper, N. virescens Distant (Hemiptera: Cicadellidae), was commonly observed in the paddies, thus the ability of N. virescens to transmit ROLP was thoroughly investigated. Newly emerged adult N. virescens, which fed on ROLD-source rice plants, were used to inoculate a susceptible rice seedling and was serially transferred into a new healthy seedling. Resultant positive transmission rates varied from 5.1% to 17.8%. The transmission ability of the insects was generally decreased over time. These findings suggest that N. virescens is an alternative vector of ROLP in the Philippines. Altogether, this study highlighted the increasing importance of ROLD-reemergence in Southeast and East Asia and proved the need for careful management of this alternative vector insect.

Highlights

Rice orange leaf disease (ROLD) was first reported in the Philippines in 1960 [1]
All the ROLD-suspected rice plants, in Mindanao showed a wide range of symptoms, which include a rice plant with severe stunting, reduced tillers, and orange-colored leaves (Figure 2b); a plant with golden to deep orange leaf discoloration and an inward leaf rolling, which are typical orange leaf disease symptoms (Figure 2c); and a plant with severe stunting, reduced tillers, and drying of leaves (Figure 2d)
(a) Typical rice orange leaf disease (ROLD) symptoms appeared on a rice plant at an early tillering stage in a field in Laguna province. (b) and (c) A rice plant

Summary

Introduction

Rice orange leaf disease (ROLD) was first reported in the Philippines in 1960 [1]. The causal agent—rice orange leaf phytoplasma (ROLP)—was identified in 1987 based on insect transmission tests and by electron microscopy observations [2]. Another phytoplasma species, which causes a disease in rice called rice yellow dwarf phytoplasma (RYDP) has been identified [3]. Phytoplasma belongs to a family of bacteria that is unculturable in vitro. Phytoplasma belongs to the Pathogens 2020, 9, 990; doi:10.3390/pathogens9120990 www.mdpi.com/journal/pathogens. Pathogens 2020, 9, 990 class Mollicutes, and has been classified within the genus “Candidatus Phytoplasma” based on 16S rDNA sequence analysis [4]. The ROLP isolates from the Philippines [8]

Methods

Results

Conclusion