Rice Blast: Meaning, Symptoms and Disease Predication | Plant Disease

In this article we will discuss about:- 1. Introduction to Rice Blast 2. Economic Importance of Rice Blast 3. Symptoms 4. Models 5. Disease Predication 6. Forecast 7. Factors.

Contents:

1. Introduction to Rice Blast
2. Economic Importance of Rice Blast
3. Symptoms of Rice Blast
4. Models of Rice Blast
5. Disease Predication of Rice Blast
6. Forecast of Rice Blast
7. Factors Influencing Epidemiology of Rice Blast

1. Introduction to Rice Blast:

Rice (Oryza sativa L.) is the major staple food for nearly one half of the world’s population. It occupies an area of 156.7 million hectare, with a total production of 650.2 million tonnes in 2007. India has an area of over 44.0 million hectare under rice, producing -144.1 million tonnes of paddy in 2007.

Rice cultivation is the principal activity and source of income for about 100 million household in Asia and Africa (FAO, 2004). It is primarily a tropical and subtropical crop, but the best grain yields are obtained in temperate regions.

The rice crop suffer from a number of diseases among them rice blast caused by one of the most devastating agricultural pathogens in the world, a fungus called Magnapor the grisea (Hebert) Barr [anomorph: Pyricularia grisea (Cooke) Sacc.], is one of the most important, causing significant losses in yield. Rice blast was probably first recorded as rice fever disease in China in 1637.

It was later described as imochi-byo in Japan in 1704, and as brusone in Italy in 1828. The fungus is currently reported to be present in at least 85 countries. In 1996 rice blast was found in rice in California, and has since been found in grasses on golf courses in the mid-western United States.

It can also infect a number of other agriculturally important cereals including wheat, rye, Barley, and Pearl Millet causing diseases called blast disease or blight disease. M. grisea causes economically significant crop losses annually, each year it is estimated to destroy enough rice to feed more than 60 million people.

2. Economic Importance of Rice Blast:

Rice blast is an economically important disease and has received attention in all major rice growing countries because of its severe destruction. Several epiphytotics of the disease have been recorded in different parts of the world resulting in serious losses in yield. In Korea, it is the only rice disease that has ever caused serious losses in yield.

The loss in yield during 1953, an epidemic year was estimated at about 800,000 tones in Japan. In India epiphytotics have been reported in the Tanjore area of Madras state in 1919. In the hills, blast may cause more than 65% loss in yield. Its incidence on a large scale has been reported from the plains and also peninsular India.

3. Symptoms of Rice Blast:

Rice blast is caused by Magnaporthe grisea (Hebert) Barr. All the above ground parts of the plant can be attacked by the fungus at any growth stages. However, Seedling stage, rapid tillering stage after transplanting and flower emergence stage were identified as the most susceptible ones to blast.

The disease can be described based on the part of the plant infected as follow:

i. Leaf Blast:

On the leaves the lesion/ spots first appear as minute brown specks, and then grow to become spindle-shaped, pointed at both ends. The center of the spots is usually gray or whitish with brown or reddish-brown margin. Fully developed lesions reach 1-1.5 cm long, 0.3-0.5 broad. Under favorable conditions, lesions enlarge and coalesce; eventually kill the leaves.

ii. Collar Rot:

Infection at the junction of the leaf blade and sheath in the typical brown “collar rot” symptom. A severe collar rot can cause the leaf to die completely. When collar rot kill the flag or penultimate leaf it may have a significant impact on yield.

iii. Neck Blast:

This occur when the pathogen infect the neck of the panicle to cause a typical “neck rot” or rotten neck blast symptom. The infected neck is griddled by a grayish brown lesion and the panicle falls down if the infection is severe. If the neck blast occurs before the milk stage, the entire panicle may die prematurely, leaving it white and completely unfilled.

iv. Panicle Blast:

The pathogen also causes brown lesions on the branches on the panicles and on the spikelets pedicles, resulting in “panicle blast”. Infection of the neck, panicle branches, and spikelets pedicles may occur together or may occur separately.

v. Node Blast:

The fungus may also attack the stem at nodes, node blast in which the stem bend and break at the node causing spikelets sterility.

Blast Symptoms of seeds themselves consist of brown spots, blotches.

4. Models for Rice Blast:

For example, at IRRI, Torres and Teng (1993) derived an equation linking leaf and panicle blast, Y = 0.21 + 1.012X₁ + 0.51 X₂ (R² = 0.8); where Y = per cent yield loss, X₁ = percent disease leaf area at, tillering and X₂ = percent panicle blast incident at harvest.

The equation suggests that leaf blast is approximately twice that of panicle blast.

Models are simplified representation of an object, system or idea in some form which can be describe quantatively by means of equations and its completeness and validity are related to the objective of building the model.

5. Disease Predication of Rice Blast:

To minimize the threat of epidemics, disease prediction of fore warning is an ideal approach, which permits prior actions, so that economize the use of pesticides, there by reduces in the environmental hazards. Predicting an outbreak or increase in disease intensity is based on weather conditions, host and pathogen identifying past prediction.

A quantitative disease prediction is possible through simulation models. Forecast weather chart are useful tools in prediction of disease weather model. Reliable forecasting formulae can be formed with number of spore tapped and the severity of disease accounting meteorological data and host resistance.

Kiyosawa (1972) develop an equation based on cumulative spore number of P. oryzae.

Y=Y₀ e^rt; where t = time in days, Y = the number of lesions or cumulative spores, r = rate of disease increase or multiplication of rate of pathogens and Y₀ = number of lesions or cumulative spore number at the time of infection.

6. Forecast of Rice Blast:

Linearity is easier to deal and has greater application in prediction approaches. For example, Yield prediction (Y) in various genotypes possessing different level of blast resistance were regressed linearly by using, Y= 5510.85 – 579.50X (r= 0.9124), where X = blast severity.

Further, a linear model to predict percentage yield loss from leaf blast severity based on the equation:

Y = 8.57 + 1.274X (r=0.885), where X = leaf blast

Multiple Regression Analysis (MRA):

It has been used to define independent biological and climatological variables to describe progress and of an epidemic,For example, a forecaster of Aomori prefecture used the equation Y = 5.05X₁+0.7X₂+67.9X₃ + 54.3X₄ – 128.6; Where Y = number of leaf blast lesion one week later, X₁ = Number of developing lesion at present, X₂ = total number of lesion; X₃ = wind velocity at 9.0 am; X₅ = mycelia growth index (10 g), by the sheath inoculation method.

Prediction for Leaf Blast:

In India, Padmanabhan (1965) reported the following forecast rules:

(i) seed bed infection occur if minimum temperature is less than or equal to 24-26°C for 4-7 days;

(ii) leaf blast occur during tillering if temperature if minimum temperature is below 24°C for 5 days during post transplanting and tillering, and if RH is greater than or equal to 90%; and

(iii) neck infection occur if condition in September and October favor leaf infection and temperature are 20-24°C for a number coincides with RH is greater than or equal to 90%. Severe leaf blast is conditional for neck blast to occur.

7. Factors Influencing Epidemiology of Rice Blast:

i. Cause of Epidemic:

The development of the blast in epidemic proportion is influenced by the presence of inoculums (i.e., virulent pathogen), susceptible stage of the host, and period of favorable environmental conditions. It is obvious that there are other secondary causes that is determine by a net-work of conditions, circumstances and factors which change over time.

For example being, when a major part of the pathogen population become insensitive to fungicides, failure of disease control may occur which leads to an epidemic.

ii. Factors Influencing/Favoring Severe Blast Epidemic:

The disease is favoured by long period of free moisture, high humidity, little or no wind at night and night temperature between 17.2-22.7°C. Leaf wetness from dew or other sources is required for infection. Spores are produced and released under high relative humidity (RH) conditions, with no spore production below 89 % RH. Sporulation increased with increasing RH above 93%.

The optimum temperature for spore germination, lesion formation and Sporulation is 25-27.7°C. Lesion produce spores for longer periods at 16.1-25°C than 27.7°C. Excessive nitrogen fertilizer, aerobic soils and drought stress favour blast. High nitrogen rate increase rice susceptibility to the disease.

The comparison of thirteen years (1984-1996 from 25^th July to 7^th September) meteorological date reveal that the number or rainy days in a week with > 90% RH were probably the critical factors in the development of rice blast epidemic during 1984 and 1992.

Mean max., min. temperature (26-27 and 19-10°C, respectively) and high humidity (RH87%) or high rain fall (2766mm) during the past years favour the rapid blast development (Table 1.1). In general, most of the years, temperature, RH, Rain fall and wind velocity were more or less conducive /favourable for rice blast disease but not adequate for epidemic development.

iii. Overwintering:

The primary source of inoculum to cause an epidemic is Overwintering of the fungus or from diseases seedlings. Overwintering can either be in rice straw, seeds, weeds, planting debris, soil, bamboo or bamboo grass or on alternate hosts in the form of mycelium which retains it variability up to the new growing season.

The blast fungus survives as mycelia in plant residues, conidia or in living plant tissue; in tropical and subtropical areas, all three modes are considered important as sources of initial inoculum.

Conidia may also be transported by air, water or seed. Mycelia surviving in rice straw have been found to remain viable for up to 3 years at 18-32°C and to produce conidia when moistened, while conidia are reportedly viable for 1 year at 8°C and 20% RH. Although M. grisea conidia are associated with seed, there is no evidence that seed infection plays a role in initiating epidemic.

In paddy rice ecosystems, puddling greatly reduces survival of conidia in rice refuse or seeds. Long distance dispersal as a source of initial inoculum is not well documented. In irrigated rice areas such those of Tamil Nadu, India, it is common to find germinated rice seedling around threshing flats in villages which grow two rice crops a year.

The storage of rice straw for cattle feed and the use of the straw as thatching in many South Asian villages also provides other sources. In intensively cropped irrigated rice areas, such as the triple-cropped Mekong Delta in Vietnam, the turnaround period between successive rice crops is as short as 15 days in some provinces.

Not all fields are fully synchronized in each province and it is common to find live rice tissue throughout the year.

iv. Host Phenology:

In general, susceptibility to facultative saprophytes increases with the age, while susceptibility to obligate parasites decreases with age; with increasing age of the plant there was progressive decrease in susceptibility to infection show by many studies and the blast fungus prefers to attack the younger upper leaves of rice plants where as, the mature leaves become resistant.