Disease of Rice | Diseases of Field & Horticultural Crops and their Management-I Notes

Rice (Oryza sativa L.) is one of the most important cereal crops globally, feeding more than half of the world's population. However, rice production faces significant challenges from various diseases that can cause substantial yield losses. This chapter provides a comprehensive overview of seven major rice diseases: Blast, Brown Spot, Bacterial Blight, Sheath Blight, False Smut, Khaira disease, and Tungro. Understanding the symptoms, causative agents, disease cycles, and management strategies is essential for effective disease control and sustainable rice production.

1. Rice Blast

Symptoms

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Rice blast is one of the most destructive diseases of rice worldwide. The symptoms vary depending on the plant part affected and the growth stage. Leaf blast appears as diamond-shaped or spindle-shaped lesions with gray-white centers and brown margins. These lesions may coalesce, causing complete blighting of leaves. Node blast causes blackening and rotting of nodes, leading to easy breakage of stems. Neck blast, the most damaging form, occurs at the panicle neck just below the panicle, causing the panicle to fall over and remain unfilled or partially filled, resulting in chaffy grains.

Etiology

The causal organism is Magnaporthe oryzae (formerly Pyricularia oryzae), a filamentous ascomycete fungus. The pathogen produces abundant conidia (asexual spores) that serve as the primary means of dissemination. The fungus exhibits high genetic variability, resulting in numerous races that can overcome host resistance.

Disease Cycle

The fungus survives on infected plant debris, seeds, and volunteer rice plants. Conidia are produced on infected tissues and are disseminated by wind and rain splash. Under favorable conditions of high humidity (above 90%), moderate temperatures (25-28°C), and extended leaf wetness periods, conidia germinate and penetrate the host through stomata or directly through the cuticle using appressoria. The incubation period ranges from 4-7 days. The disease is particularly severe when plants receive excessive nitrogen fertilization or during prolonged periods of cloudy, humid weather.

Management

Integrated disease management includes cultural, chemical, and host resistance strategies. Use resistant or moderately resistant varieties suited to the local environment. Practice proper water management by avoiding continuous flooding in nurseries and maintaining proper drainage. Apply balanced fertilization, avoiding excessive nitrogen which predisposes plants to infection. Split application of nitrogen is recommended. Seed treatment with fungicides such as tricyclazole or carbendazim (2g/kg seed) is effective. Foliar application of systemic fungicides like tricyclazole (0.06%), or kasugamycin (2ml/L) during the active tillering stage and before panicle emergence provides good control. Remove and destroy infected plant debris after harvest to reduce inoculum sources.

2. Brown Spot

Symptoms

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Brown spot affects all above-ground parts of the rice plant. On leaves, the disease appears as circular to oval spots, varying in size from small specks to large lesions up to 1 cm in diameter. The spots are initially brown, later developing a gray center surrounded by a reddish-brown margin. In severe infections, numerous spots coalesce, causing extensive leaf area damage and premature drying. On the panicle, dark brown to black spots appear on glumes, causing grain discoloration and reducing grain quality. Seedlings from infected seeds show brown lesions on coleoptiles and primary leaves.

Etiology

The causal agent is Bipolaris oryzae (synonym: Helminthosporium oryzae), a deuteromycete fungus. The fungus produces dark, multi-septate conidia characteristic of the genus. Brown spot is often considered a disease of stressed plants and becomes severe under nutrient-deficient conditions, particularly nitrogen and silicon deficiency.

Disease Cycle

The fungus overwinters on infected seeds, plant debris, and weed hosts. Both seed-borne and soil-borne inoculum serve as primary sources of infection. Conidia are produced on infected tissues and are disseminated by wind currents. Infection occurs through stomata or direct penetration. The disease develops optimally at temperatures between 25-30°C with high humidity. The incubation period is typically 8-12 days. The disease is particularly severe in fields with nutrient deficiency, especially silicon and nitrogen, poor soil conditions, and drought stress.

Management

Use certified disease-free seeds and treat seeds with fungicides such as carbendazim (2g/kg) or mancozeb (2.5g/kg) before sowing. Ensure balanced fertilization with adequate nitrogen, phosphorus, potassium, and silicon. Foliar application of potassium chloride (2%) or soluble silicon can reduce disease severity. Maintain proper field sanitation by removing infected stubbles and weeds. Application of fungicides such as mancozeb (0.2%) or edifenphos (0.1%) at the tillering and heading stages provides effective control. Grow resistant varieties where available and ensure adequate spacing between plants to reduce humidity within the canopy.

3.Bacterial Blight

Symptoms

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Bacterial blight is characterized by water-soaked lesions that start at the leaf tips or margins and progress downward, eventually affecting the entire leaf. The lesions turn yellow to white and have a wavy margin. In severe cases, systemic infection leads to kresek, a wilting of seedlings or young plants resulting in death. On older plants, the disease causes leaf blight where leaves dry from the tips downward. Yellow bacterial ooze (exudate) can be observed early in the morning on young lesions. The disease can also affect the grain, causing discoloration and reduced quality.

Etiology

The causal organism is Xanthomonas oryzae pv. oryzae, a gram-negative, rod-shaped bacterium. The bacterium enters the plant through wounds or natural openings (hydathodes and stomata) and multiplies in the xylem vessels, causing systemic infection. The pathogen exhibits considerable variability with multiple races or pathotypes that can overcome resistance in certain rice varieties.

Disease Cycle

The bacterium survives on infected plant debris, volunteer rice plants, weed hosts, and infected seeds. Primary infection occurs from contaminated seeds or soil-borne inoculum. The bacteria enter through hydathodes at leaf tips during guttation or through wounds caused by wind, insects, or mechanical damage. Secondary spread occurs through irrigation water, rain splash, and wind-driven rain. The disease is favored by warm temperatures (25-34°C), high humidity, and flooding. Kresek is most severe when young plants (2-4 weeks old) are subjected to deep water or heavy rains following transplanting.

Management

Plant resistant varieties carrying major resistance genes (Xa genes). Use disease-free, certified seeds or treat seeds with bleaching powder (100g/10L water for 8 hours). Avoid mechanical injury to plants during transplanting and field operations. Maintain balanced fertilization, avoiding excessive nitrogen which increases susceptibility. Drain fields periodically to reduce favorable conditions for bacterial spread. Application of copper-based bactericides such as copper oxychloride (0.2%) or copper hydroxide during early infection stages can reduce disease spread. Remove and destroy infected plants and stubbles after harvest. Maintain proper plant spacing to reduce humidity and facilitate air circulation.

4. Sheath Blight

Symptoms

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Sheath blight typically begins near the water line on leaf sheaths during the tillering to heading stages. Initial symptoms appear as water-soaked, elliptical or oval greenish-gray lesions on the leaf sheath. These lesions gradually enlarge and develop irregular, tan-colored patches with brown margins. As the disease progresses, lesions coalesce and move upward, affecting upper sheaths, leaves, and occasionally panicles. Severely infected plants show premature drying of leaves and reduced grain filling. In humid conditions, white mycelial growth and small, round, brown sclerotia (survival structures) can be observed on infected tissues.

Etiology

The causal organism is Rhizoctonia solani Kühn (teleomorph: Thanatephorus cucumeris), a soil-borne basidiomycete fungus. The pathogen has a wide host range and can infect numerous crop species. The anastomosis group AG-1 IA is specifically associated with rice sheath blight.

Disease Cycle

The fungus survives as sclerotia in soil and on infected plant debris. Sclerotia can remain viable in soil for several months to years. Under favorable conditions (high humidity, temperatures of 28-32°C, and dense plant canopy), sclerotia germinate and produce mycelium that initiates infection. The fungus first colonizes the leaf sheath near the waterline and then spreads upward. Mycelial growth and lesion development are rapid under warm, humid conditions. Sclerotia formed on infected tissues fall into the soil or water and serve as inoculum for subsequent crops. The disease is particularly severe in fields with high plant density, excessive nitrogen fertilization, and prolonged flooding.

Management

Since highly resistant varieties are not available, management relies on cultural practices and chemical control. Maintain proper plant spacing to improve air circulation and reduce humidity within the canopy. Practice balanced fertilization with judicious use of nitrogen and adequate potassium. Drain fields periodically to reduce humidity and interrupt disease development. Remove infected plant debris and deep plow after harvest to bury sclerotia. Avoid use of infected straw as compost. Application of systemic fungicides such as validamycin (0.2%), hexaconazole (0.2%), or azoxystrobin (0.1%) at the early tillering stage and repeated 2-3 weeks later provides effective control. Biological control using Trichoderma spp. or Pseudomonas fluorescens can suppress disease development when applied to the soil or as foliar sprays.

5. False Smut

Symptoms

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False smut is characterized by the transformation of individual rice grains into large, velvety, yellowish-green to greenish-black spore balls (smut balls). These spore balls are much larger than normal grains, typically 3-10 times their size. Initially, the smut balls are covered by a thin membrane that later ruptures, exposing the powdery spore mass. Not all grains in a panicle are affected; usually only a few grains per panicle are converted into smut balls. The disease is most visible at the milk to dough stage of grain development. While false smut rarely causes significant yield loss, severely infected grains produce mycotoxins that can affect grain quality and marketability.

Etiology

The causal organism is Ustilaginoidea virens (teleomorph: Villosiclava virens), an ascomycete fungus. The pathogen produces both sexual (ascospores) and asexual (chlamydospores) spores. The characteristic greenish-black spore balls consist of masses of chlamydospores.

Disease Cycle

The fungus survives as chlamydospores on infected plant debris and in soil. Sclerotia may also form and serve as survival structures. Under favorable conditions during the flowering stage, chlamydospores germinate and produce conidia or directly form mycelium. The fungus infects flowers at the booting to flowering stage when spikelets are most susceptible. Infection occurs through the gap between lemma and palea or through the stigma. The mycelium colonizes the developing grain, transforming it into a smut ball. The disease is favored by high humidity (above 90%), warm temperatures (25-28°C), overcast conditions during flowering, and excessive nitrogen fertilization. High rainfall or heavy dew during flowering significantly increases disease incidence.

Management

Use disease-free seeds and resistant or tolerant varieties. Avoid excessive nitrogen fertilization, particularly during the reproductive stage. Maintain balanced nutrition with adequate potassium and micronutrients. Adjust planting dates to avoid flowering during prolonged rainy periods when possible. Remove and destroy infected plant debris and volunteer rice plants to reduce inoculum. Application of copper-based fungicides such as copper oxychloride (0.25%) or propiconazole (0.1%) at the late booting stage (5-7 days before flowering) can significantly reduce disease incidence. Proper field sanitation and crop rotation with non-host crops help reduce soil-borne inoculum.

6. Khaira Disease (Zinc Deficiency)

Symptoms

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Khaira disease, primarily a physiological disorder caused by zinc deficiency, typically appears 2-4 weeks after transplanting. The characteristic symptoms include the development of brown blotches and spots on older leaves, particularly near the base and midrib. Leaves may develop rusty brown discoloration, and growth becomes stunted with reduced tillering. Affected plants show shortened internodes and delayed maturity. In severe cases, new leaves emerge with a characteristic pale yellow to white discoloration, and plants may die. The symptoms are more pronounced in alkaline soils, submerged conditions, and fields with high phosphorus levels that interfere with zinc uptake.

Etiology

Khaira disease is a nutritional disorder caused by zinc (Zn) deficiency in rice plants. While not caused by a pathogen, zinc deficiency weakens plants and predisposes them to secondary infections by fungi such as Fusarium spp. and Helminthosporium spp. The disorder is particularly common in soils with pH above 7.5, high bicarbonate content, flooded conditions that reduce zinc availability, and soils with low organic matter.

Disease Cycle

Zinc deficiency develops when soil zinc levels are insufficient or when soil conditions render zinc unavailable for plant uptake. Factors that exacerbate zinc deficiency include waterlogged conditions that promote the formation of insoluble zinc compounds, high soil pH that reduces zinc solubility, excessive phosphorus fertilization that interferes with zinc uptake, and low soil organic matter content. The symptoms typically appear during the active vegetative growth stage when zinc demand is high. Secondary fungal infections may occur on zinc-deficient plants, complicating symptom diagnosis.

Management

Soil application of zinc sulfate (25 kg/ha) or zinc oxide before transplanting or at the tillering stage corrects deficiency. Foliar application of zinc sulfate (0.5% solution containing 0.25% lime to prevent leaf scorching) is effective for quick correction of deficiency symptoms. Spray 2-3 times at 10-day intervals. Root dipping of seedlings in 2% zinc oxide suspension before transplanting provides early protection. Incorporate organic matter into soil to improve zinc availability and chelation. Avoid excessive phosphorus fertilization as high phosphorus levels interfere with zinc uptake. In severely deficient soils, combine soil and foliar applications. Maintain proper water management, alternating wetting and drying rather than continuous flooding, to improve zinc availability. Use zinc-enriched fertilizers in zinc-deficient areas.

7. Tungro Disease

Symptoms

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Tungro is one of the most destructive viral diseases of rice in Southeast and South Asia. Symptoms vary with the rice variety, plant age at infection, and viral strain. Early infection (2-3 weeks after transplanting) causes severe stunting, reduced tillering, and yellowing or orange-yellow discoloration of leaves, beginning from the tip and spreading downward. Infected plants remain stunted with short, narrow, twisted, and mottled leaves showing interveinal chlorosis. Late infection results in mild stunting and yellow or orange-yellow discoloration of upper leaves. Infected plants produce fewer tillers, delayed flowering, and chaffy or partially filled grains, resulting in significant yield loss (up to 100% in severely affected plants).

Etiology

Tungro disease is caused by a complex of two viruses: Rice tungro bacilliform virus (RTBV), a pararetrovirus, and Rice tungro spherical virus (RTSV), a picorna-like virus. RTBV is responsible for most symptoms, while RTSV is required for efficient transmission of RTBV by the vector. Both viruses are transmitted exclusively by the green leafhopper, Nephotettix virescens, and related species in a semi-persistent manner. The viruses are not seed-transmitted and do not survive in plant debris.

Disease Cycle

The viruses survive in infected rice plants, ratoon crops, and volunteer rice plants. The green leafhopper acquires the viruses by feeding on infected plants for 30 minutes to several hours. After an incubation period of a few hours, the viruliferous leafhoppers can transmit the viruses for several days but do not pass them to their progeny. Transmission occurs when infected vectors feed on healthy plants, injecting the viruses during feeding. The incubation period in plants is typically 10-25 days depending on temperature and plant age. Young plants (2-4 weeks old) are most susceptible. The disease is favored by high leafhopper populations, presence of infected plants serving as virus reservoirs, continuous cropping of rice without a fallow period, and optimal temperatures (25-30°C).

Management

Management focuses on controlling the leafhopper vector, eliminating virus sources, and using resistant varieties. Plant tungro-resistant or tolerant varieties. Remove and destroy infected plants, volunteer rice plants, and ratoon crops that serve as virus sources. Synchronize planting to avoid overlapping of susceptible crops and virus reservoirs. Apply insecticides to control leafhopper populations, particularly during the early growth stages. Systemic insecticides such as imidacloprid (0.4ml/L) or thiamethoxam (0.2g/L) applied at transplanting and at the tillering stage reduce vector populations. Use yellow sticky traps in fields to monitor and reduce leafhopper populations. Practice crop rotation with non-host crops when possible. Maintain proper field sanitation by destroying stubbles and plant debris after harvest. Apply balanced fertilization, avoiding excessive nitrogen that attracts leafhoppers. Use light traps to monitor and control adult leafhopper populations.

Conclusion

The diseases described in this chapter represent major constraints to rice production worldwide. Successful disease management requires an integrated approach combining resistant varieties, cultural practices, chemical control when necessary, and proper field sanitation. Early detection and prompt intervention are critical for minimizing yield losses. As rice production intensifies and climatic conditions change, continuous monitoring for disease outbreaks and development of new resistant varieties remain essential for sustainable rice production. Understanding the biology, epidemiology, and management of these diseases enables farmers and agricultural professionals to implement effective control strategies tailored to local conditions, ultimately contributing to food security and agricultural sustainability.