Plant Pathology is defined as the study of the organisms and environmental conditions that causes disease in plants; the mechanisms by which this occurs, the interactions between these causal agents and the plant (effects on plant growth, yield and quality), and the methods of managing or controlling plant disease.

RICE DISEASE CONTROL:

Rice diseases continue to be important in limiting profitable rice production in Guyana and other parts of the world, and is considered to be one of the most important biotic and abiotic factor that affect rice cultivation and its productivity. Rice diseases can result in losses of up to 100% in a crop. There are two types of diseases that affect plants:

  1. Biotic or infectious diseases (due to another micro-organism such as: nematode, fungi, bacteria, virus, mycoplasmas etc.).
  2. Abiotic or noninfectious diseases (due to environmental conditions, deficiencies etc.).

For an infectious (biotic) disease to occur there are three conditions that must be present and come together to form what is referred to as the “Disease Triangle”. A combination of all three sides of the triangle results in the highest level of diseased plants. If one of the sides is not present, then disease will be less severe or nonexistent.

 

Figure 1: Disease triangle.

In Guyana, there are four major rice diseases that affect the rice cultivation, these are Rice Blast (Pyricularia grisea, P. oryzae, (Perfect stage -Magnaporthea oryzae)); Brown spot (Bipolaris oryzae (synonyms Dreschlera oryzae and Helminthosporium oryzae)); Sheath Blight (Rhizoctonia solani) and Sheath rot (Sarocladium oryzae). There are other that can be consider to be very minor such as False smut (Ustilaginoidea virens) etc and has been observed in sporadic cases.

Rice Blast Disease:

Rice blast (Magnaporthea oryzae) has caused extensive damages over the years and is, without doubt, the most important of the rice diseases.  The farmers and researchers have been faced with the challenge of combating this dreaded disease for years.  Today, the GRDB’s research programme has managed to produce several blast resistant varieties.  However, controlling rice blast necessitates continuous research and experiments to develop stronger blast resistant strains because the fungus continues to mutate and form newer strains that overcome the resistance in the new varieties.

Causal Organism:                                                                                                     A Fungus – Perfect stage: Magnaporthe oryzae

Imperfect stage (anamorph): Pyricularia grisea (P. oryzae)

Figure 2: Conidia and conidiogenous cell of M. oryzae

Figure 2: Conidia and conidiogenous cell of M. oryzae

 

Symptoms: 

Leaf Blast: The initial infections occurs on leaves usually around seedling – tillering and appear as diamond, football, or spindle shape lesion with pointed ends. Lesions start as small water soaked areas on young leaves and enlarge into diamond shape with a blue gray cast which are the fungal spores. Lesions often dry out and turn tan with a brown border. Lesion shape and size can vary.

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Favourable conditions for blast infection: 

The following conditions are optimum for blast infection:

  • Aerobic (dry land) conditions or in wetlands (flooded) where intermittent drought stress occurs.
  • High rates of nitrogen.
  • High planting density.
  • Long duration of leaf wetness due to drizzle or dew.
  • No wind at nights
  • Night temperatures between 17°C and 23 °
  • Optimum temperature of 25 °C to 28 °C for fungus sporulation.

Management of the disease:

  1. The use of resistant varieties is the most effective method of control of the blast disease. The resistant varieties currently available are BR 444, BR 240, F7-10, G98 22-4, G98 24- 1, G98-196, G98-135, G98-30-3, GRDB 9, GRDB 10, GRDB 11, GRDB 12 and GRDB 14.
  2. Cultural practices for blast control:
    • Sow in the recommended period to avoid weather conditions that would be highly favorable blast e.g. Rustic (blast susceptible variety) can be sown in the 1st crop but avoided in the 2nd
    • Proper field sanitation must be practiced because spores can survive on rice straw and seeds. Burn field after harvesting.
    • Do not have an excessively high planting density. High plant density results in high humidity and temperatures that are favorable for blast infection and development.
    • Avoid application of excessive amounts of nitrogen as this would promote the growth and spread of the blast fungus. Use split applications of nitrogen where possible.
    • Keep field flooded at a depth of 4” to 6”. Drier fields are more liable to blast attack.
    • Keep alternative blast hosts under control e.g. birdseed grass, red rice and off-types of the rice varieties.
  1. The current recommended fungicides for blast control in Guyana are Fugi-one, Manzate and Carbendazim.

Brown Spot Disease:  

Causal organism: Bipolaris oryzae (synonyms Cochilobolus miyabeans,  Dreschlera oryzae and Helminthosporium oryzae

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Symptoms: 

This disease may manifest as seedling blight or as a foliar and glume disease of mature plants.

In seedlings the fungus produces small, circular brown lesions which may girdle the coleoptile and cause distortion of the leaves.

The fungus may also cause black discoloration of the roots.

Infected seedlings are stunted or killed.

In older plants, the lesions on the leaves are light brown to gray in the center and have a reddish brown margin.

Lesions vary between 1 mm-14 mm long, depending on cultivar.

On resistant cultivars the fungus produces tiny dark specks.

Lesions may coalesce in severe infections, killing large areas of affected leaves.

The fungus can infect the glume and the grains.

Brown spot reduces the number of grains per panicle and the kernel weight.

CONDITIONS FAVOR BROWN SPOT INFECTION:

  • Presence of infected seeds, volunteer rice, rice debris, and several weeds.
  • Poorly drained or nutrient deficient soils.
  • Abnormal soils, which are deficient in nutrient elements.
  • Temperature ranging from 25oC-30o
  • Water stress and high humidity (86-100%).
  • Leaves must be wet for 8-24 hours for infection to occur.
  • Maximum tillering up to the ripening stages of the crop.

 

Management of the Disease:

  • Provide well balanced nutrients for the soil. It has been reported that it is mainly an indicator of nutritional or physiological disorder, than a pathological one.

ADVICE: Routinely sample and test soil for nutrients and apply the recommended fertilizers, especially K; and Use the recommended rate and timing for N fertilization.

  • Avoid water stress conditions.
  • Use resistant cultivars in cases where the soil amendment does not work to correct the problem. The use of resistant varieties is the most economical means of control.
  • The fungus can survive on seeds for up to 4 years. Seed treatment with captan, carbendazim, or mancozeb has been found to reduce seedling infection.

 

Sheath Blight disease:

  • It is the second most important rice disease in the world, after Rice Blast ( oryzae).
  • This disease affects rice cultivation under the irrigated or wetland system of production.  With the advent of blast resistant varieties in Guyana Sheath Blight is threatening to become a major problem.
  • Previously, because fungicides were used to control blast, Sheath Blight and other fungal diseases were also controlled.
  • The disease is soil borne and can develop within a week.
  • The pathogen can be spread rapidly through irrigation water and by the movement of soil during land preparation and cultivation.
  • The disease damages plants by the formation of lesions and production of empty grains.
  • The lesions interrupt the flow of water and nutrients to the leaf which may die.

Causal Organism: Rhizoctonia solani

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Symptoms 

Symptoms generally appear at late tillering or early internode elongation growth stages. One may encounter symptoms on young plants.

Initial symptoms consist of circular, oblong or ellipsoid green-gray, water soaked spots, about 1 cm long that occur on the leaf sheaths near the waterline.

Lesions enlarge to 2 cm-3 cm long by 1 cm wide.

The centers of the lesions become pale green or white and are surrounded by an irregular purple-brown border.

Lesions on upper portion of plants may coalesce to encompass the entire leaf sheath and stem.

Heavily infected plants produce poorly filled grains, especially in the lower portion of the panicle.

Management of the Disease:

  1. Practise proper sanitation because the pathogen survives between crops in plant debris.
  2. Make use of the cultivars. No immunity has been found but there are some cultivars that exhibit some level of resistance.
  3. Do not over fertilize with nitrogen as this increases plant susceptibility to the fungus.
  4. Avoid short cultivars with broad leaves and high tillering which produce a close canopy (favorable for sheath blight), but use tall cultivars with fewer tillers.
  5. Use fungicides, if necessary, to control sheath blight.

 

Sheath Rot disease:

  • Sheath rot disease usually occurs on the flag leaf sheath (boot) that encloses the panicle.
  • The disease is generally of minor importance and scattered within fields, but occasionally areas within fields may develop sheath rot at a level that affects yield.
  • Losses ranges from 20% to 85%.
  • In Guyana however, the losses are not huge.
  • Sheath Rot was first described in 1992 by Sawada in Taiwan.

 

Causal organism: Sarocladium oryzae (Acrocylindrium oryzae)

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Symptoms:

Lesions appear on the upper leaf sheath, especially the flag leaf sheath.

Lesions are initially oblong, 5 mm – 15 mm in length with gray brown centers surrounded by a dark reddish brown margin.

As the disease progresses, lesions enlarge and coalesce and may cover the rest of the leaf sheath.

In cases of severe infection, the panicle may fail to emerge completely or not at all.

Panicles that do not emerge tend to rot and florets turn red-brownish to dark brown.

Sterile, shrivelled or partially filled grains is also indicate infection of the panicle.

Management of the Disease:

  1. Use resistant varieties if available.
  2. Use fungicides, which can reduce some of the damage.

 

False Smut (Green Smut) 

The disease is characterized by large orange to brown-green fruiting structures on one or more grains of the mature panicle. When the orange covering ruptures, a mass of greenish-black spores is exposed.

Causal organism: Ustilaginoide virens

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Symptoms: 

The rice kernels are replaced by globose, velvety spore balls up to 1 cm in diameter (occasionally up to 5 cm), which burst out from between the glumes.

Immature spore balls appear orange and are covered by membrane.

In some cases, the ovary is destroyed, whereas the style, stigma and anthers remain intact and are incorporated into the spore ball.

At the center of the spore ball, one or more irregular, hard, black sclerotia, 5-13 x 2-5 mm, develops.

In general, only a few grains of a panicle are affected.

Some researchers have reported that the grains adjacent to smut balls are sterile.

Management of the Disease:

  1. No control measures are generally warranted for false smut.
  2. Fungicides applied at the heading have been reported to be successful in controlling the disease.
  3. Cultivars have been reported to be resistant to the fungus.