False smut of rice: An emerging threat
Authors: Prashant Mishra, Hamsa S, Rohini Sreevathsa and Rhitu Rai
National Research Centre on Plant Biotechnology, Pusa Campus, New Delhi-110012

INTRODUCTION AND ECONOMIC IMPORTANCE:

Rice feeds major part of world with India taking its predominant share, especially Eastern and Southern India. Immense enhancement in rice production has come through irrigation, high yield rice varieties and utilization of fertilizers. Although, these changes have led to increase in yield, however it has simultaneously also favoured emergence of pathogens responsible for new plant diseases or enhanced the severity of existing disease.

False smut also known as pseudo-smut or green smut is one such disease which has stemmed from long term and wide use of hybrid rice varieties fed with heavy application of nitrogen fertilizer. It was first reported from Tirunelveli district of Tamil Nadu state in 1870s. It was initially named as Lakshmi disease and was considered as a symbol of bumper harvest. Therefore, owing to its sporadic nature, earlier it was considered as minor disease, however, recently it has become endemic in the eastern India region. High yielding rice varieties like CO43, CR1009, ADT38, ADT39 and BPT5204 are known to be inflicted with this disease.

The occurrence of this disease reduces the quality and quantity of rice grain. It causes chaffiness and sterility of spikelets near to smut balls which leads to loss in yield. Rate of yield loss has been estimated from 0.2% to 49% in many states of country, depending upon the rice variety and disease intensity. Besides reducing grain yield, it affect grain quality also as it produces toxins like ustilotoxin and antimitotic cyclic peptides from its chlamydospores, which are poisonous to both human and animals. It also reduces the rate of seed germination.

PATHOGEN AND ITS LIFE CYCLE:

The causal agent of this disease is Ustilaginoidea virens, a biotroph fungus, member of tribe Ustilaginoideae of the family Clavicipitaceae belonging to Ascomycete. Excessive use of nitrogen fertilizer, high relative humidity, rain fall and low temperature during flowering favours the growth of fungus. It undergoes both sexual (sclerotia) and asexual (chalymdospores) stages in life cycles. Recently, Villosiclava virens has been suggested as new name for teleomorph
(sexual reproductive stage) of this fungus. Sclerotia and chlamydospores are found on the surface and inside of smut balls, respectively. Sclerotia eventually fall on ground leaving chalymdospores. Sclerotia in or on the soil over winter germinate to fruit-body producing ascospores which serves as primary inoculum while air borne chalymdospores act as secondary inoculum.

DISEASE SYMPTOM:

The characteristic symptom of rice false smut is the formation of powdery ball-like colonies in spikelets which are visible only in flowering stage one to two weeks after infection. Fungus transforms individual grain of the panicle initially to yellowish smut ball, which changes to yellowish orange, green, olive green and finally to greenish black during maturation stage. U. virens infect rice coleoptiles and roots at the seedling stage and the stamen filaments at early booting stage. However, it is not clear if U. virens mycelia that colonize in seedlings can extend to the spikelets and form the ball-like symptom. At booting stage, U. virens invades rice through small gap at the apex of spikelet but does not penetrate host cell walls. Its hyphae enter floral organs primarily at upper parts of stamen filaments between the ovary and lodicules but is not known to infect ovaries. Primarily, the hyphae extend into central vascular tissues without penetrating cell walls, thus this pathogen is considered as biotrophic.

ALTERNATE HOSTS:

In addition to rice, maize and several weeds common in rice fields act as alternative hosts for U.virens. Digitaria marginata, a common rice, Panicum trypheron, a common grass around paddy, Echinochloa crusgalli and Imperata cylindrica, two common weeds are few examples of U virens inhabitation beyond rice.

MANAGEMENT OF DISEASE:

Several common management practices like planning of sowing date, using uninfected seeds, spraying of chlorothalonil or Propiconazole during flowering, cleaning of bunds and fields reduces the severity and incidence of disease. Artificial infection method into the growing spikelets has been developed to evaluate rice germplasm for resistance/tolerance to pathogen. The lack of known resistance genes/germplasm and skewed information about molecular host responses to U virens attack or specific pathogenicity genes from U virens hinders the development of strategies to tackle this disease. Draft genome of U virens is now available. However, knowledge of the biosynthetic pathways/genes of ustiloxins, specific genes from host as well as pathogen, facilitating the disease is largely incomplete. With growing necessity to unravel the mechanisms involved in U virens- rice interaction, researchers will discover ways to successfully manage this disease, threatening to be an epidemic in future.



About Author / Additional Info:
I am a research scientist pursuing work on rice diseases Bacterial leaf blight and false smut.