Rice Sheath Rot: An Emerging Ubiquitous Destructive Disease Complex

Abstract

Around one century ago, a rice disease characterized mainly by rotting of sheaths was reported in Taiwan. The causal agent was identified as Acrocylindrium oryzae, later known as Sarocladium oryzae. Since then it has become clear that various other organisms can cause similar disease symptoms, including Fusarium sp. and fluorescent pseudomonads. These organisms have in common that they produce a range of phytotoxins that induce necrosis in plants. The same agents also cause grain discoloration, chaffiness, and sterility and are all seed-transmitted. Rice sheath rot disease symptoms are found in all rice-growing areas of the world. The disease is now getting momentum and is considered as an important emerging rice production threat. The disease can lead to variable yield losses, which can be as high as 85%. This review aims at improving our understanding of the disease etiology of rice sheath rot and mainly deals with the three most reported rice sheath rot pathogens: S. oryzae, the Fusarium fujikuroi complex, and Pseudomonas fuscovaginae. Causal agents, pathogenicity determinants, interactions among the various pathogens, epidemiology, geographical distribution, and control options will be discussed.

Keywords: Fusarium fujikuroi complex; Pseudomonas fuscovaginae; Sarocladium oryzae; fumonisins; grain discoloration; phytotoxins; rice; sheath rot.

FIGURE 1

Morphology of two different Sarocladium oryzae isolates from Rwanda on PDA medium after 14 days of growth at 28°C. Top is reverse view, bottom is front view.

FIGURE 2

Microscopy of S. oryzae grown on PDA medium. All structures were stained with lactophenol blue. (A) Conidia; (B) Conidiogenous cell; (C) Aerial conidiophores.

FIGURE 3

Rice sheath rot symptoms caused by S. oryzae (photos M. Höfte).

FIGURE 4

Disease cycle of sheath rot caused by S. oryzae, Fusarium sp. or Pseudomonas fuscovaginae. Sheath rot pathogens are seed-transmitted, resulting in infected seedlings (1). Infected seedlings can die (2) resulting in infected plant debris (3) or survive. P fuscovaginae can colonize the whole plant as an endophyte or survive epiphytically and infect the inflorescences at booting stage. The seedling transmission of the fungal pathogens is less well understood (4). Secondary infections result from conidia or bacterial cells released from infected plants (5). Conidia or bacterial cells are spread by wind or rain to healthy plants. Plants at booting stage are especially susceptible to infection. In the case of S. oryzae, insects and mites can also spread conidia and facilitate infection by creating wounds (6). Rot occurs on the sheath enclosing the young panicles; grains on affected tillers become chaffy and discolored. Grains infected with Fusarium sp. can become contaminated with mycotoxins (7). Pathogens can spread to new field via contaminated grains (8). After harvest, infected plant debris will remain in the field (9) serving as inoculum for the next growth cycle (10).

FIGURE 5

Structures of toxins produced by rice sheath rot causing agents. Helvolic acid and cerulenin are produced by S. oryzae; Fumonisin B1 is produced by Fusarium sp., Fuscopeptin B and syringotoxin B are produced by P. fuscovaginae.

FIGURE 6

Symptoms caused by P. fuscovaginae and morphology on King’s Medium B plates after 48 h of growth at 28°C (top is reverse side, bottom is front side).