Fungal Diseases of Rice

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Source: asianscientists

Introduction to Rice

Rice, Oryza sativa is one of the main staple crops in the whole world especially in Asia. It provides carbohydrate and calories daily to about half of the world population. Besides, in some countries, this annual plant acts also as an economic crop and serves as a source of foreign income. Round the year billions of tons of rice are consumed and billion others are exported worldwide.

Here are some quick fun facts about rice.

  • Rice is the grain from species Oryza sativa (Asian rice) or Oryza glaberrima (African rice).
  • There are over 40,000 varieties of rice. The three favorites are Basmati, Thai Jasmine and Italian Arborio.
  • Two Japanese car brands were named after rice: Toyota, meaning “Bountiful Rice Field,” and Honda, meaning “Main Rice Field”.
  • It takes 5000 liters of water to produce 1 kilo of rice.

Here are some of the common and important diseases of rice that farmers deal with nationwide. But before delving into them, we need to know about some disease prone parts of the plant so that it becomes easy for us to know where to look and what to find in paddy to identify diseases.

Some fundamental terms about the parts of paddy

  • Leaf sheath: A structure at the base of a leaf’s petiole that partly surrounds or protects the stem or another organ that it subtends.

    A spikelet of paddy
  • Panicle: A loose branching cluster of flowers.
  • Glumes: Each of two membranous bracts surrounding the spikelet of a paddy.
  • Spikelet: The basic unit of a paddy flower, consisting of two glumes or outer bracts at the base and one or more florets above.

Diseases described here are:

  1. Blast disease of rice
  2. Brown spot of rice
  3. Stem rot of rice
  4. Sheath blight of rice

Blast disease of rice

This particular disease is considered as the most significant disease of paddy as it has extensive distribution and destructiveness all over the rice growing countries. Affected by this disease, the plant may get destructed at any stage between the seedling and the tillering periods.

Several rice blast epidemics have occurred in different parts of the world, resulting in yield losses in these areas ranging from 50 to 90% of the expected crop.

Conidia with conidiophore

Pathogen

  • Pyricularia oryzae
  • Synonym: Magnaporthe oryzae (Teleomorph stage) (Has not found in nature but produced in laboratory)
  • Class: Deuteromycetes

Characteristics of pathogen

  • Simple, gray conidiophores.
  • Bear terminal, pear-shaped, mostly two-septate conidia.

Infected portions

  • All parts of the paddy plant except the root. The fungus can infect the plant at any growth stage.

Disease causing abiotic factors

  • High amounts of rainfall and high levels of nitrogen fertilizer.

Symptoms

Lesions with whitish center and dark brown borders Source: https://www.dpi.nsw.gov.au

a. Initial lesions (tissue that suffered damage) with dark green borders.

b. Older lesions are spindle or diamond shaped with whitish to gray central part and red to brownish or necrotic border.

c. The spots enlarge as the disease progresses. And they may coalesce, and kill entire leaves.

Photo gallery

Conidia of the rice blast fungus Magnaporthe (Pyricularia) sp. [Photographs courtesy of (A and C) J. Breithaupt, FAO
Rice blast symptoms on (A) rice leaves, (B) rice stalks, and (C) neck rot or blast symptoms leading
to white heads. (D) Severe blasting of rice panicles in the field.

 


Brown spot of rice

It is one of the most common and most destructive diseases of rice. And it was behind the infamous Bengal famine of 1943 when approximately 2 million people died from starvation. One interesting fact about this disease is that it was considered to be used against Japan by USA during the World War II.

Pathogen

  • Drechslera oryzae
  • Synonym: Cochliobolus miyabeanus, Bipolaris oryzae (anamorph)

Infected portions:

  • Leaf, leaf sheaths, panicles, glumes, spikelets etc. The infection may also attack the seedlings.

Symptoms

a. Small, circular brown lesions.

b. Fully developed lesions have a light brown to grey centers surrounded by a reddish brown margin.

c. Entire glumes may be covered by several small spots or one large spot.

Good to know

  • The fungus causes damage primarily by attacking the
    leaves during the seedling stage.

Photo gallery

Brown spot of rice caused by Cochliobolus
miyabeanus. (Photograph courtesy of Plant Pathology Department,
University of Florida.)

Stem rot of rice

Pathogen

  • Sclerotium oryzae

Infected portions

  • Stem, leaf sheath

Symptoms

a. Visible, numerous tiny white or black sclerotia in stem.

b. At the early stage of disease development, irregular black lesions are found on the outer leaf sheaths near the water level.

 


Sheath blight of rice

Pathogen

  • Rhizoctonia solani

Infected portions

  • Leaves

Disease causing abiotic factors

  • High temperature (28−32°C)
  • High levels of nitrogen fertilizer, and
  • Relative humidity of crop canopy from 85−100%.

Symptoms

Usually observed from tillering (shoot growing after the initial parent shoot is grown) to milk stage.

a. Oval or ellipsoidal greenish gray lesions, usually 1-3 cm long, on the leaf sheath, initially just above the soil or water level in the case of conventionally flooded rice.

b. Under favorable conditions, these initial lesions multiply and expand to the upper part of the sheaths, the leaves, and then spread to neighboring tillers.

c. Lesions on the leaves usually have irregular pattern, often with gray-white centers and brown margins as they grow older.

Sheath blight, Source: Rice Knowledge Bank

 

Outcome

  • Leaves dry out rapidly and die then. As a result, the leaf area of the canopy gets significantly reduced.

Good to know

  • Plants are more vulnerable to this disease during the rainy season.