Diseases of Rice and Finger Millet
1. Rice Blast (Most Devastating Fungal Disease)
Etiology: Fungus Magnaporthe oryzae (syn. Pyricularia oryzae); anamorph stage produces conidia.
Symptoms:
· Leaf blast: Diamond/spindle-shaped lesions with
gray-white centers and brown margins; lesions enlarge and coalesce, causing
leaf drying.
· Neck blast: Blackened/rotted neck nodes leading to
panicle breakage or chaffy grains.
· Node blast: Black lesions on nodes.
Epidemiology: Favored by high humidity (85-100%), moderate temperatures (20-28°C), cloudy/rainy weather, and excessive nitrogen. Common in tropical/subtropical regions; spreads via wind-borne conidia. Severe in rainfed/lowland rice.
Disease Cycle: Survives in infected seeds, crop debris, or collateral hosts (e.g., grasses). Conidia germinate on wet leaf surfaces, penetrate via appressoria, and produce lesions. Secondary cycles via airborne conidia; multiple generations per season.
Management:
· Resistant varieties (e.g., those with Pi genes).
· Cultural: Balanced NPK fertilization, avoid excess
nitrogen, proper spacing, seed treatment.
· Chemical: Seed treatment with fungicides (e.g.,
carbendazim, tricyclazole); foliar sprays like tricyclazole or isoprothiolane
at tillering/booting.
· Biological: Antagonistic microbes; integrated approaches
recommended.
·
2. Bacterial Leaf Blight (BLB)
Etiology: Bacterium Xanthomonas oryzae pv. oryzae (Xoo).
Symptoms: Water-soaked lesions on leaf margins that turn yellowish-white and extend along veins; "kresek" phase (wilting/rotting of seedlings); severe cases lead to leaf drying and yield loss.
Epidemiology: High temperature (25-30°C), high humidity/rain, wind-driven rain, and flooding favor spread. Common in Asia; enters through wounds/hydathodes.
Disease Cycle: Survives in infected seeds, stubble, and weeds. Bacteria multiply in xylem, ooze from lesions, and spread via rain, irrigation, or tools. Systemic infection possible.
Management:
· Resistant varieties (Xa genes).
· Cultural: Healthy seeds, crop rotation, avoid water
stagnation, balanced fertilizers.
· Chemical: Copper-based bactericides or streptomycin
(limited use); seed treatment.
· Biological and quarantine measures.
·
3. Sheath Blight
Etiology: Fungus Rhizoctonia solani (AG1-IA; teleomorph Thanatephorus cucumeris).
Symptoms: Water-soaked oval/elliptical greenish-gray lesions on leaf sheaths near waterline; centers turn gray-white with brown borders; lesions coalesce, forming "snake-skin" pattern; upper leaves and panicles affected in severe cases; tiller lodging.
Epidemiology: High humidity, dense canopy, high nitrogen, and warm temperatures (28-32°C). Soil-borne, prevalent in intensive irrigated rice.
Disease Cycle: Survives as sclerotia/mycelium in soil, debris, or seeds. Sclerotia float in floodwater, infect sheaths; mycelium spreads plant-to-plant. Secondary spread via contact/humidity.
Management:
· Resistant/tolerant varieties.
· Cultural: Wider spacing, avoid excess N, remove debris,
proper water management.
· Chemical: Fungicides like validamycin, hexaconazole, or
propiconazole.
· Biological: Trichoderma, Pseudomonas spp.;
integrated disease management (IDM).
·
4. Brown Spot
Etiology: Fungus Bipolaris oryzae (teleomorph Cochliobolus miyabeanus).
Symptoms: Small circular/oval dark brown spots with gray centers and yellow halos on leaves/sheaths/glumes; spots coalesce, causing leaf blight; seedling blight; poor grain filling.
Epidemiology: Favored by nutrient deficiencies (esp. N, K, Si), high humidity, and temperatures 20-30°C. Common in rainfed/drought-prone areas; indicator of poor soil/plant health.
Disease Cycle: Survives in seeds and debris. Conidia spread by wind/rain; infects via stomata or direct penetration; multiple cycles.
Management:
· Balanced fertilization (esp. potassium and silicon).
· Resistant varieties, healthy seeds, crop rotation.
· Chemical: Seed treatment and foliar fungicides (e.g., mancozeb, carbendazim).
Blast (Most Important Disease)
Etiology: Fungus Pyricularia grisea (syn. Magnaporthe
grisea).
Symptoms: Spindle-shaped lesions on leaves with gray centers and brown/reddish margins; neck/finger blast causes rotting of neck and poor grain set; seedlings may show blight.
Epidemiology: Rainy season, high humidity, moderate temperatures. Prevalent in India and Africa; airborne spread.
Disease Cycle: Survives in crop residues/seeds. Conidia infect leaves/panicles; secondary spread via air. Similar to rice blast.
Management:
· Resistant varieties.
· Cultural: Timely sowing, balanced nutrients, sanitation
(remove debris).
· Chemical: Seed treatment and foliar sprays (e.g.,
tricyclazole, carbendazim).
· Biological options.
Leaf Spot / Brown Spot / Cercospora
Leaf Spot
Etiology: Various fungi, e.g., Cercospora penniseti or others like Helminthosporium spp.; also associated with grain mold (Curvularia, Fusarium, etc.).
Symptoms: Brown to dark spots on leaves; discoloration of grains; leaf blight in severe cases.
Epidemiology: Humid conditions, poor nutrition; seed- and debris-borne.
Disease Cycle: Survives on debris/seeds; conidia spread by wind/rain.
Management: Resistant cultivars, seed treatment, sanitation, fungicides as needed.
General Management Principles
- Cultural: Use
certified healthy seeds, crop rotation, balanced fertilization (avoid
excess N), proper spacing and irrigation, timely sowing, destroy crop
residues, weed control.
- Host Resistance: Deploy resistant/tolerant varieties (key for
sustainable management).
- Chemical: Seed
treatments and targeted foliar sprays; follow label rates and rotate
fungicides/bactericides to avoid resistance.
- Biological: Biocontrol
agents like Trichoderma, Pseudomonas, Bacillus.
- Integrated Disease Management (IDM): Combine all approaches for best results.
- Monitoring: Scout
fields regularly, especially during humid periods.
A. Rice Blast Disease
 |
| figure 1: Leaf blast Credit: TNAU | Agritech |
One of the most destructive diseases of rice worldwide, caused by the fungus Magnaporthe oryzae (anamorph: Pyricularia oryzae). It can cause yield losses of 10–30% routinely and up to 100% in severe epidemics. It affects all above-ground parts and occurs in most rice-growing regions, especially under humid tropical/subtropical conditions.
Etiology (Causal Agent)
- Pathogen: Magnaporthe
oryzae (Ascomycete fungus).
- It is highly variable with many physiological
races, making resistance breeding challenging.
- Produces pyriform (pear-shaped) two-celled conidia on conidiophores.
Symptoms
Symptoms appear at all growth stages (seedling to
maturity):
- Leaf Blast (most
common early symptom):
- Small, water-soaked lesions that
enlarge into diamond/spindle-shaped spots (0.5–1.5 cm long).
- Gray-white necrotic center with
dark brown margins.
- Lesions coalesce under favorable
conditions, leading to large blighted areas and leaf drying.
- Neck Blast (most
damaging):
- Blackish-brown lesions on the
neck (peduncle) below the panicle.
- Causes panicle rot, breakage, or
"white head" (chaffy, unfilled grains).
Figure2: Neck Blast
Credit: TNAU | Agritech - Node Blast:
- Black lesions on nodes,
weakening stems and causing lodging.
Figure3: Nodal Blast
Credit: TNAU | Agritech - Seedling Blast:
- Damping-off or blighting of
young seedlings.
- Panicle/Grain Blast:
- Discoloration and poor grain
filling.
In severe cases, the field appears scorched or
"burnt."
Epidemiology (Favorable Conditions)
- Temperature: 20–28°C
(optimal for sporulation and infection).
- Humidity: High
relative humidity (>85–90%) and prolonged leaf wetness (dew, rain, or
fog) for 8–12+ hours.
- Weather: Cloudy,
overcast, or rainy periods; low light intensity.
- Crop Factors: Excessive
nitrogen fertilizer, dense planting, susceptible varieties, young
succulent plants.
- Geographic Distribution: Widespread in Asia, Africa, Latin America;
more severe in rainfed upland and lowland rice.
The pathogen spreads rapidly during the tillering to
heading stage.
Disease Cycle
- Survival:
- In infected seeds, crop residues
(straw, stubble), and collateral hosts (grasses like barnyard grass).
- Can survive as mycelium or
conidia; sexual stage (perithecia with ascospores) is rare in fields.
- Primary Infection: Conidia from residues or seeds land on
leaves, germinate in presence of free water, form appressoria, and
penetrate the cuticle/epidermis mechanically and enzymatically.
- Colonization: Fungus
grows intercellularly and produces lesions within 3–7 days.
- Secondary Spread: New conidia produced abundantly on lesions
under humid conditions; dispersed by wind, rain splash, or insects.
Multiple infection cycles (polycyclic disease) occur in one season.
- Overwintering/Off-season: On volunteer plants, weeds, or seeds.
Management Strategies
Effective management requires an Integrated Disease
Management (IDM) approach:
1. Cultural Practices
- Use resistant or tolerant varieties (e.g.,
those with major Pi genes; rotate genes due to pathogen
variability).
- Balanced fertilization: Avoid excess nitrogen;
apply potassium and silicon (silicon strengthens cell walls).
- Proper spacing and planting density to reduce
humidity in canopy.
- Timely sowing and water management (avoid
prolonged flooding or drought stress).
- Destroy crop residues, rogue infected plants,
and control weeds/grass hosts.
- Seed treatment and use of clean, certified
seeds.
2. Chemical Control
- Seed Treatment: Carbendazim, tricyclazole, or thiram.
- Foliar Sprays:
- Tricyclazole (most specific),
isoprothiolane, or combination fungicides.
- Apply at early tillering, late
tillering, and boot/heading stages (2–3 sprays).
- Timing is critical — protect during vulnerable
growth stages under favorable weather.
3. Biological Control
- Antagonistic microbes: Bacillus subtilis,
Pseudomonas fluorescens, Trichoderma spp.
- Some success in reducing inoculum.
4. Host Resistance and Breeding
- Gene-for-gene interaction; many R genes
identified (e.g., Pi-ta, Pi-b).
- Pyramiding multiple resistance genes and
quantitative trait loci (QTLs) for durable resistance.
5. Other Approaches
- Forecasting models based on weather.
- Quarantine to prevent introduction of new
races.
- Transgenic approaches (e.g., RNAi) in research.
Note:
Over-reliance on single fungicides or varieties leads to resistance breakdown.
Combine methods for sustainable control.
Rice blast is a classic example of a polycyclic, airborne
fungal disease in plant pathology. For exams, focus on the disease cycle
diagram (primary inoculum from debris → conidial production → wind dispersal)
and comparison with other foliar diseases like brown spot or sheath blight.
Brown Spot of Rice
(also known as Helminthosporium Leaf Spot or Cochliobolus
Leaf Spot)
 |
| Figure4: Brown Spot of Rice Credit - IRRI |
Brown spot is a
widespread and economically important fungal disease of rice. It is
particularly damaging in rainfed, upland, and nutrient-poor soils. Historically
linked to the Bengal famine of 1943, it can cause significant yield losses (up
to 20–40% or more) by reducing photosynthesis, causing seedling blight, and
affecting grain quality.
Etiology (Causal Agent)
- Pathogen: Bipolaris
oryzae (anamorph); teleomorph (sexual stage) Cochliobolus
miyabeanus.
- It belongs to the Ascomycota group. Produces
dark, multicellular conidia (poroconidia) that are straight or slightly
curved.
Symptoms
- On Leaves:
- Small, circular to oval dark
brown spots (2–5 mm) with a yellow halo.
- Spots enlarge, become
cylindrical/oval with grayish-brown centers and dark brown margins.
- Numerous spots coalesce, leading
to large blighted areas; leaves turn yellow and dry prematurely (scorched
appearance).
- On Seedlings:
- Coleoptile lesions, seedling
blight, and poor stand establishment.
- On Sheaths, Glumes, and Grains:
- Similar dark spots; glume
discoloration leads to shriveled, discolored, or poorly filled grains
(reduced grain weight and quality).
- Overall Plant: Stunting,
reduced tillering, and lower yield in severe cases. Symptoms are more
pronounced under stress.
The disease often starts on older/lower leaves and
progresses upward.
Epidemiology (Favorable Conditions)
- Temperature: 20–30°C
(optimal around 25–28°C).
- Humidity: High
relative humidity and leaf wetness favor spore germination and infection.
- Soil/Plant Factors:
- Nutrient deficiencies
(especially nitrogen, potassium, and silicon).
- Drought stress, poor soil
drainage, and low organic matter.
- High plant density and
susceptible varieties.
- Distribution: Worldwide,
especially in areas with intensive rice cultivation and suboptimal management.
It is often a "stress indicator" disease rather than a primary
aggressive pathogen.
Disease Cycle
- Survival:
- Primarily in infected seeds
(seed-borne), crop residues (straw, stubble), and soil as mycelium or
conidia.
- Can persist for several months
to years in debris.
- Primary Infection:
- Conidia from residues or
contaminated seeds germinate on wet plant surfaces.
- Direct penetration or through
stomata; infection occurs within hours under moist conditions.
- Colonization and Symptom Development:
- Toxins produced by the fungus
contribute to lesion formation and tissue necrosis (3–7 days after
infection).
- Secondary Spread:
- New conidia produced abundantly
on lesions; dispersed by wind, rain splash, or irrigation water.
- Multiple cycles (polycyclic)
during the growing season, especially in humid weather.
- Over-seasoning: In seeds and debris for the next crop.
Management Strategies
Effective control relies on addressing underlying
stresses and using integrated approaches.
1. Cultural Practices (Most Important)
- Balanced Nutrition: Apply adequate nitrogen, potassium, and
silicon fertilizers. Silicon amendments significantly enhance resistance.
- Use healthy, certified, disease-free seeds.
- Crop rotation with non-hosts.
- Proper water management: Avoid drought stress;
maintain good soil fertility and organic matter.
- Timely sowing and optimum plant density.
- Remove and destroy infected crop residues; deep
plowing.
- Resistant or tolerant varieties (partial
resistance is common).
2. Chemical Control
- Seed Treatment: Carbendazim, mancozeb, or thiram to reduce
seed-borne inoculum.
- Foliar Sprays:
- Mancozeb, carbendazim,
propiconazole, or hexaconazole.
- Apply at early appearance of
symptoms or during tillering/booting stages if weather is favorable.
- Avoid unnecessary sprays — chemical control is
more effective when combined with cultural practices.
3. Biological Control
- Antagonistic fungi (Trichoderma spp.)
and bacteria (Pseudomonas, Bacillus).
- Some commercial bio-fungicides available.
4. Other Measures
- Integrated Disease Management (IDM) combining
resistant varieties, nutrition, and timely sprays.
- Monitoring: Regular field scouting, especially
in stressed crops.
Key Difference from Rice Blast: Brown spot lesions are more circular/oval with
prominent halos and are strongly associated with nutrient deficiency/stress,
whereas blast lesions are more spindle-shaped and aggressive under high
humidity regardless of nutrition.
Sheath Blight of Rice
Sheath blight is
one of the most important soil-borne fungal diseases of rice, ranking second
only to rice blast in many intensive rice-growing areas. It causes significant
yield losses (20–50% or more under favorable conditions) by affecting leaf
sheaths, reducing photosynthesis, impairing grain filling, and causing lodging.
It is particularly severe in high-yielding semi-dwarf varieties under dense
planting and high nitrogen inputs.
 |
| Figure4: Blighted Sheath of Rice Credit: aspnet.org |
Etiology (Causal Agent)
- Pathogen: Rhizoctonia
solani Kühn, anastomosis group AG1-IA (anamorph).
- Teleomorph (sexual
stage): Thanatephorus cucumeris (Frank) Donk.
- It is a basidiomycete fungus that produces sclerotia (resting structures) and has a wide host range (many crops and weeds).
Symptoms
- Initial Symptoms: Appear from tillering to heading stage.
- Circular, oval, or ellipsoid water-soaked
greenish-gray lesions on leaf sheaths near the water line.
- Lesions enlarge, centers turn grayish-white
with irregular tan to dark brown or purple-brown borders.
- Progression:
- Lesions coalesce and encircle
the sheath.
- Infection spreads upward to
upper sheaths and leaf blades, producing irregular lesions with a
characteristic "snake-skin" or "rattlesnake skin"
pattern.
- Severe cases: Rotting of
sheaths, drying of leaves, tiller collapse, and lodging.
- Panicles may be affected
indirectly, leading to unfilled grains and poor quality.
- Field Appearance: Circular patches of affected plants (bird’s
nest symptom) in early stages.
All aerial parts except roots are susceptible; roots
remain healthy.
Epidemiology (Favorable Conditions)
- Temperature: 28–32°C
(warm conditions).
- Humidity: High
relative humidity and dense canopy microclimate.
- Crop Factors:
- High nitrogen fertilization
(promotes lush growth).
- Close planting/high tillering.
- Flooded or high-moisture fields.
- Distribution: Worldwide,
especially in tropical and subtropical irrigated rice systems. More severe
in intensive cultivation.
Disease Cycle
- Survival:
- As sclerotia (hard,
resting bodies) in soil, infected debris, or on alternate hosts.
- Mycelium in plant residues.
Sclerotia can survive for several years.
- Primary Infection:
- Sclerotia float to the water
surface in flooded fields.
- They germinate and contact the
lower leaf sheaths, producing mycelium that infects the plant.
- Colonization:
- Mycelium grows intercellularly
and produces enzymes/toxins that kill tissues.
- Lesions develop within days
under humid conditions.
- Secondary Spread:
- Mycelial growth from plant to
plant via contact (leaf-to-leaf or sheath-to-sheath).
- New sclerotia formed on infected
tissues; dispersed by water or farm operations.
- Polycyclic disease with rapid
buildup in favorable weather.
- Over-seasoning: Sclerotia in soil serve as the main source
for the next season.
Management Strategies
Management is challenging due to the soil-borne nature
and wide host range. An Integrated Disease Management (IDM) approach is
essential.
1. Cultural Practices
- Use moderately resistant/tolerant varieties.
- Balanced Fertilization: Avoid excessive nitrogen; apply potassium and
silicon.
- Wider plant spacing to improve air circulation
and reduce humidity.
- Proper water management: Alternate wetting and
drying (AWD) where feasible.
- Remove crop residues and destroy sclerotia
through deep plowing or burning (where permitted).
- Crop rotation with non-host crops (e.g.,
legumes).
- Avoid continuous rice monoculture.
2. Chemical Control
- Fungicides:
Validamycin, hexaconazole, propiconazole, azoxystrobin, or combination
products.
- Timing: First
application at maximum tillering or early symptom appearance; second at
booting/heading if needed.
- Seed treatment has limited effect since the
pathogen is mainly soil-borne.
3. Biological Control
- Antagonists: Trichoderma spp., Pseudomonas
fluorescens, Bacillus subtilis.
- Some commercial bio-products are effective when
applied to soil or sheaths.
4. Other Approaches
- Biological control agents combined with reduced
fungicide doses.
- Forecasting based on weather and crop stage.
- Transgenic resistance and breeding for durable
tolerance are under research.
Comparison with Other Rice Diseases:
- Vs. Blast: Sheath blight is soil-borne
with lesions starting at the base near waterline; blast is airborne with spindle
lesions on leaves/neck.
- Vs. Brown Spot: Sheath blight causes
larger coalescing sheath lesions and snake-skin pattern; brown spot
features smaller circular leaf spots linked to nutrient stress.
Sheath blight exemplifies a soil-borne necrotrophic pathogen with sclerotial survival — a key topic in plant pathology for disease cycle and management.
False Smut of Rice
( False Grain Smut)
False smut has emerged as a significant disease in many rice-growing regions, particularly with the adoption of high-yielding varieties, intensive fertilization, and favorable weather. Unlike true smuts, it does not completely destroy the grain but converts individual grains into smut balls, reducing grain quality, yield, and market value. It also produces mycotoxins that can pose health risks.
 |
 |
| Figure 5: Yellowish and Black chlamydospores in rice panicles |
Etiology (Causal Agent)
- Pathogen: Ustilaginoidea
virens (anamorph); teleomorph Villosiclava virens.
- It is an ascomycete fungus. Produces chlamydospores (dark green to olive-black, powdery) and has both asexual and sexual cycles.
Symptoms
- Symptoms become visible at the booting to
heading stage, usually 2–3 weeks after infection.
- Individual grains in the panicle are
transformed into velvety, yellowish-orange to greenish smut balls
(2–5 times larger than normal grains).
- The smut balls enclose floral parts and are
initially covered by a thin membrane that ruptures to release powdery
spores.
- Infected grains are hollow or filled with spore
masses; only a few grains per panicle may be affected initially, but
incidence can increase.
- No prominent symptoms on leaves or sheaths; the disease is mainly reproductive.
Severe infection leads to reduced grain number, weight,
and quality. The yellow-orange color of young smut balls is a diagnostic feature.
Epidemiology (Favorable Conditions)
- Temperature: 25–30°C.
- Humidity: High
rainfall and humidity during booting and heading stages.
- Crop Factors:
- Excessive nitrogen fertilizer
(especially pre-flood or late applications).
- Late planting.
- Susceptible varieties.
- High humidity and cloudy weather
favor spore germination and infection.
- Distribution: Increasingly important in Asia (including India), the USA, and other rice belts. It has shifted from minor to major status in recent decades.
Disease Cycle
- Survival:
- As chlamydospores in
soil, infected seeds, and crop debris (sclerotia-like structures).
- Can survive for long periods in
soil.
- Primary Infection:
- Spores (chlamydospores)
germinate and infect rice florets during the booting stage (panicle
development inside the sheath).
- The fungus colonizes the floral
organs without killing the spikelet immediately.
- Symptom Development:
- Mycelium grows inside the grain,
converting it into a smut ball. Spores mature as the panicle emerges.
- Secondary Spread:
- Mature spores are released and dispersed
by wind, rain, or insects to healthy panicles.
- Multiple cycles possible under
prolonged humid conditions.
- Over-seasoning: Spores in soil and seeds serve as primary inoculum for the next crop.
The pathogen has a unique infection strategy targeting
young panicles.
 |
| Figure6 : Symptoms and Sign of Ustilaginoidea virens Credit: Sementischlor |
Management Strategies
Integrated management is recommended, focusing on
prevention since the disease appears late in the crop cycle.
1. Cultural Practices
- Use resistant or moderately resistant varieties
(limited options; check local recommendations).
- Avoid late planting.
- Balanced Nitrogen Application: Split doses; avoid excess pre-flood or late
N.
- Timely sowing and proper spacing.
- Use healthy, certified seeds; treat seeds if
needed.
- Destroy crop residues and practice crop
rotation where possible.
- Field sanitation to reduce soil inoculum.
2. Chemical Control
- Fungicides:
Propiconazole, tebuconazole, azoxystrobin, or combinations.
- Timing is Critical: Apply 5–10 days before heading or at early
booting stage (preventive sprays).
- One or two sprays may be needed in high-risk fields.
3. Biological and Other Approaches
- Some biocontrol agents (e.g., Bacillus, Pseudomonas)
are under evaluation.
- Integrated Pest Management (IPM) combining
cultural, chemical, and resistant varieties.
- Monitoring weather forecasts for humid periods
during booting.
Note:
False smut is often associated with high-input systems. Reducing excessive
nitrogen helps manage multiple diseases (sheath blight, blast, false smut).
Comparison with Other Rice Diseases
- Vs. True Smut: False smut produces
large, velvety yellow-green balls on individual grains; true smut affects
more grains with black powdery spores.
- Vs. Blast: False smut is
panicle-specific and appears late; blast affects leaves, neck, and nodes
earlier.
This completes a good overview of major rice diseases
(Blast, Brown Spot, Sheath Blight, False Smut). For your Plant Pathology
course, focus on the unique late-infection strategy and mycotoxin concern of
false smut.
Bacterial Leaf Blight (BLB) of Rice
Bacterial leaf blight is
one of the most destructive bacterial diseases of rice, particularly in Asia.
It can cause yield losses of 20–50% or more in susceptible varieties under
favorable conditions. It is especially severe in lowland irrigated rice and
during the rainy season. The disease affects both seedlings (kresek phase) and
mature plants, leading to leaf drying and reduced grain filling.
Etiology (Causal Agent)
- Pathogen: Xanthomonas
oryzae pv. oryzae (Xoo), a Gram-negative bacterium.
- It is highly variable with many
races/pathotypes, complicating resistance breeding.
- Produces extracellular polysaccharides (xanthan gum) that aid in virulence and symptom development.
 |
| Figure8: Symptoms of Bacterial Leaf Blight Credit: MINDEN Picture |
Symptoms
- Leaf Blight Phase (main phase on older plants):
- Starts as water-soaked,
translucent lesions on leaf margins or tips.
- Lesions enlarge along the veins,
turning yellowish to grayish-white with wavy margins.
- Bacterial ooze (yellowish
droplets) may appear on lesions under high humidity.
- Severe infection leads to
complete leaf drying and "blighting."
- Kresek Phase (seedling
stage):
- Sudden wilting and drying of
leaves (similar to fire damage).
- Young leaves roll, turn pale,
and the whole seedling collapses.
- Pale Yellow Leaf Phase:
- Systemic infection leading to
pale yellow leaves.
- Other: Lesions on
sheaths or panicles in advanced cases; reduced tillering and poor grain
quality.
The disease often appears after heavy rains or storms
that cause wounds.
Epidemiology (Favorable Conditions)
- Temperature: 25–30°C
(optimal).
- Humidity: High
rainfall, flooding, and high relative humidity.
- Spread Factors: Rain splash, wind-driven rain, irrigation
water, and farm tools.
- Crop Factors:
Susceptible varieties, high nitrogen, young succulent plants, and wounds
(from insects, hail, or mechanical injury).
- Distribution: Widespread
in tropical and subtropical Asia; also reported elsewhere. Epidemics occur
during the wet season.
Disease Cycle
- Survival:
- In infected seeds (seed-borne),
crop stubble, volunteer plants, and weeds (e.g., Leersia spp.).
- Can persist in soil for short
periods but mainly in plant debris.
- Primary Infection:
- Bacteria enter through
hydathodes (water pores), wounds, or stomata.
- Multiply in the xylem vessels,
causing vascular blockage.
- Colonization and Symptom Development:
- Systemic movement within the
leaf; toxins and enzymes contribute to tissue damage.
- Symptoms appear 3–10 days after
infection.
- Secondary Spread:
- Bacterial ooze from lesions is
spread by rain, wind, and water.
- Rapid secondary cycles during
rainy periods.
- Over-seasoning: In seeds and stubble for the next crop.
It is a vascular bacterial disease with
rain-dependent dissemination.
Management Strategies
Management emphasizes prevention and host resistance
because bactericides have limited systemic action.
1. Cultural Practices
- Plant resistant varieties (incorporating
Xa genes, e.g., Xa4, Xa5, Xa21).
- Use healthy, certified seeds; treat seeds with
hot water or chemicals (e.g., streptocycline).
- Avoid excessive nitrogen; apply balanced
fertilizers.
- Proper drainage and water management; avoid
flooding young seedlings.
- Destroy crop stubble and weeds; practice crop
rotation.
- Sanitation of farm tools to prevent spread.
2. Chemical Control
- Seed Treatment: Streptocycline, copper oxychloride, or
plantomycin.
- Foliar Sprays:
Copper-based bactericides or antibiotics (limited efficacy once systemic).
- Apply at early symptom appearance or as
prophylactic sprays before rains.
3. Biological Control
- Antagonistic bacteria (Pseudomonas
fluorescens, Bacillus spp.).
- Some success in seed treatment and soil
application.
4. Other Approaches
- Integrated Disease Management (IDM) combining
resistant varieties, cultural practices, and timely sprays.
- Gene pyramiding for durable resistance.
- Quarantine to prevent new races.
Comparison with Other Rice Diseases:
- Vs. Fungal Diseases (Blast, Brown Spot,
Sheath Blight): BLB has water-soaked margins, follows veins, and produces
bacterial ooze; no fungal structures.
- It is favored by similar wet conditions but is
bacterial (not fungal).
Bacterial leaf blight is a classic example of a vascular
bacterial pathogen in plant pathology. For exams, remember the kresek
phase, seed-borne nature, and importance of Xa gene resistance.
Bacterial Leaf Streak (BLS) of Rice
Bacterial leaf streak is an important bacterial disease of rice, often occurring alongside or confused with Bacterial Leaf Blight (BLB). It causes yield losses of 10–30% or more under severe conditions, mainly by reducing photosynthetic area. It is more common in tropical and subtropical regions with high rainfall and is considered a sporadic but emerging threat in intensive rice cultivation.
Etiology (Causal Agent)
- Pathogen: Xanthomonas
oryzae pv. oryzicola (Xoc), a Gram-negative bacterium.
- Closely related to the BLB pathogen (Xanthomonas
oryzae pv. oryzae, Xoo), but distinct in behavior and genetics.
- Intercellular pathogen (unlike Xoo, which is primarily vascular).
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| Figure 9: BLS of rice Credit: IRRI |
Symptoms
- Early Symptoms: Small, dark-green to water-soaked interveinal
streaks (narrow, linear lesions) on leaves, starting from tillering to
booting stage.
- Progression: Streaks
extend lengthwise along the veins (limited by vascular bundles), turning
yellow, orange-brown, or grayish.
- Lesions may coalesce into larger translucent or
necrotic patches.
- Bacterial Ooze: Tiny yellow or amber droplets (exudates)
visible on lesions under humid conditions.
- Severe Cases: Leaves dry up, turn brown/grayish-white; reduced plant vigor and grain quality. Infection can also affect florets and seeds, causing discoloration.
Key Differentiation from Bacterial Leaf Blight (BLB):
- BLS: Narrow, straight-edged, interveinal
streaks; starts between veins.
- BLB: Broader lesions starting from leaf
margins/tips with wavy edges; vascular (xylem) involvement.
- Early stages can look similar, but lesion shape and progression differ.
Epidemiology (Favorable Conditions)
- Temperature: High
temperatures (25–35°C).
- Humidity: High
rainfall, flooding, and prolonged leaf wetness.
- Spread: Rain
splash, wind, irrigation water, and contaminated tools/seeds.
- Crop Factors:
Susceptible varieties, wounds (insects, mechanical), and high humidity
favor rapid spread.
- Distribution: Prevalent in Asia, West Africa, and tropical rice areas; often more sporadic than BLB.
Disease Cycle
- Survival:
- In infected seeds (seed-borne),
crop residues, stubble, and weeds.
- Can persist in plant debris.
- Primary Infection:
- Bacteria enter through stomata,
wounds, or hydathodes.
- Multiply in the intercellular
spaces (mesophyll) between veins.
- Colonization:
- Spread longitudinally between
veins; later stages may invade xylem.
- Produce enzymes and toxins
causing tissue necrosis.
- Secondary Spread:
- Bacterial ooze dispersed by rain
and wind to healthy leaves.
- Rapid polycyclic spread during
wet weather.
- Over-seasoning: Seeds and debris serve as primary inoculum for subsequent crops.
Management Strategies
Management is similar to BLB but emphasizes seed health
and cultural practices due to limited chemical options.
1. Cultural Practices
- Use resistant or tolerant varieties (resistance
genes are being identified).
- Certified, disease-free seeds; seed treatment
(hot water or chemicals).
- Balanced fertilization; avoid excessive
nitrogen.
- Proper spacing and water management to reduce
humidity.
- Destroy crop residues, control weeds, and
sanitize tools.
- Crop rotation where feasible.
2. Chemical Control
- Seed Treatment: Streptocycline, copper-based compounds, or
plantomycin.
- Foliar Sprays: Copper
bactericides or antibiotics at early symptom appearance (preventive sprays
before heavy rains).
- Efficacy is better when applied early.
3. Biological Control
- Antagonistic bacteria (Pseudomonas, Bacillus
spp.) for seed/soil treatment.
4. Integrated Management
- Combine resistant varieties, healthy seeds,
cultural practices, and timely sprays.
- Monitoring during wet seasons is crucial.
Note: BLS
is often less devastating than BLB but can cause notable losses in conducive
environments. Accurate diagnosis (lesion shape + bacterial ooze test) is
important for management.
Zinc Deficiency in Rice
(also known as Khaira Disease in India or Hudda Disease in Pakistan)
Zinc (Zn) deficiency is the most widespread micronutrient disorder in rice worldwide, especially in lowland flooded rice. It significantly reduces tillering, delays maturity, increases spikelet sterility, and can cause yield losses of 20–50% or more if uncorrected. Rice is highly sensitive to Zn deficiency, and symptoms often appear 2–4 weeks after transplanting or flooding.
Causes / Factors Favoring Deficiency
- Soil Factors:
- Alkaline/calcareous soils (high
pH >7.0) — Zn precipitates as insoluble compounds.
- Flooded/anaerobic conditions —
formation of ZnS or Zn(OH)₂; high
bicarbonate (HCO₃⁻) levels.
- Low organic matter or highly
weathered soils.
- High phosphorus (P) levels —
forms insoluble Zn-phosphates.
- High organic soils (peats) or
sodic soils.
- Other Factors: Cold weather after flooding, intensive cropping without Zn replenishment, and varieties with low Zn efficiency.
Zinc availability decreases sharply after soil
submergence due to changes in soil chemistry.
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| Figure 10: Khaira Disease of Rice Credit: Kheti Vyapar and Crop Library |
Symptoms
Symptoms typically appear on young plants (2-leaf to
mid-tillering stage) and are more pronounced in patches:
- Early Symptoms: Chlorosis (yellowing) of older leaves,
starting from the base toward the tip; midrib may remain green longer.
- Characteristic Symptoms: Dusty brown to reddish-brown irregular
blotches or streaks on leaves (bronzing). Leaves lose turgor, become
brittle, and may show white midrib lines.
- Plant Level: Stunted
growth, reduced tillering (or complete stop in severe cases), uneven crop
stand, delayed maturity, and increased empty spikelets.
- Severe Cases: Older leaves turn entirely bronze-brown and die; young leaves remain small and chlorotic; plants may die in patches.
Note:
Symptoms can mimic iron deficiency, sulfur deficiency, or brown spot disease,
but Zn deficiency often shows bronzing and is linked to flooded alkaline soils.
Differentiation from Similar Problems
- Vs. Brown Spot (fungal): Brown spot has distinct circular/oval lesions with halos; Zn deficiency causes diffuse bronzing and stunting without defined spots. Nutrient stress predisposes plants to brown spot.
- Vs. Iron Deficiency: Iron symptoms appear on younger leaves first;
Zn affects older leaves more prominently with bronzing.
Management and Correction
Preventive and corrective measures are highly effective since rice responds quickly to Zn
application.
1. Soil Application (Preventive)
- Apply Zinc Sulfate (ZnSO₄·7H₂O) at 25–50
kg/ha as basal dose before transplanting or flooding.
- Chelated Zn forms (e.g., Zn-EDTA) are effective
but more expensive.
- Incorporate into soil for better availability.
2. Foliar Application (Corrective)
- Spray 0.5–1% ZnSO₄ solution
(with lime or urea to neutralize) at tillering stage.
- Repeat if symptoms persist (1–2 sprays at 7–10
day intervals).
- Quick response — visible greening within days.
3. Cultural and Agronomic Practices
- Use Zn-efficient rice varieties (breeding
efforts are ongoing).
- Apply organic manures (FYM, compost) to improve
Zn availability.
- Avoid excessive P fertilization.
- Alternate wetting and drying (AWD) where
possible to improve Zn uptake.
- Drain floodwater temporarily in severe cases,
apply Zn, and reflood.
4. Integrated Approach
- Soil testing for available Zn (DTPA-extractable
Zn <0.6 ppm indicates deficiency).
- Combine with balanced NPK + other
micronutrients.
- Seed priming or coating with Zn.
Recovery:
Plants often show spontaneous partial recovery as soil conditions change, but
timely Zn application maximizes yield.
Zinc deficiency is a classic example of a micronutrient
disorder in flooded rice systems.
Millet Blast
(Primarily Finger Millet Blast / Ragi Blast)
Blast disease is
the most important and destructive disease of finger millet (Eleusine
coracana), also affecting pearl millet and other small millets to varying
degrees. It is caused by a fungus closely related to the rice blast pathogen
and can cause yield losses of 20–50% or more, especially under humid conditions
during the rainy season. It affects all growth stages, with neck and finger
(head) blast being the most damaging.
Etiology (Causal Agent)
- Pathogen: Pyricularia
grisea (syn. Magnaporthe grisea or Pyricularia oryzae f.
sp. eleusinis).
- Same genus as rice blast pathogen (Magnaporthe
oryzae), but with host-specific adaptations.
- Produces pyriform (pear-shaped) two-celled
conidia.
Symptoms
- Leaf Blast: Spindle or
diamond-shaped lesions with grayish-white centers and dark
brown/reddish-brown margins on leaf blades. Lesions enlarge and coalesce,
causing leaf blight.
- Neck Blast:
Blackish-brown lesions on the neck (peduncle) below the panicle, leading
to rotting and breakage.
- Finger/Head Blast: Infection on fingers (spikelets), causing
shriveled, discolored, or sterile grains. In severe cases, entire fingers
turn black and fail to produce grains.
- Seedling Blast: Damping-off or blight in young plants.
- Overall: Reduced plant vigor, poor grain
filling, and "white head" appearance in fields.
Symptoms can appear at any growth stage but are most
damaging at flowering.
Epidemiology (Favorable Conditions)
- Temperature: 20–28°C.
- Humidity: High
relative humidity (>90%) and prolonged leaf wetness (rain, dew).
- Weather: Rainy,
cloudy, or foggy conditions during tillering to flowering.
- Crop Factors:
Susceptible varieties, excessive nitrogen, dense planting, and late
sowing.
- Distribution: Prevalent
in India, East Africa, and other finger millet-growing regions during the
kharif (rainy) season.
Disease Cycle
- Survival:
- In infected crop residues,
seeds, and collateral hosts (grasses/weeds).
- Conidia and mycelium persist
between seasons.
- Primary Infection:
- Airborne conidia land on leaves
or panicles, germinate in presence of free water, and penetrate via
appressoria.
- Colonization:
- Fungus grows inside tissues,
producing lesions within days.
- Secondary Spread:
- New conidia produced on lesions;
dispersed by wind and rain splash.
- Multiple polycyclic generations
per season under humid conditions.
- Over-seasoning: On debris and seeds.
The cycle is very similar to rice blast.
Management Strategies
Integrated Disease Management (IDM) is essential for
effective control.
1. Cultural Practices
- Use resistant or tolerant varieties (e.g.,
GPU-28, VR-708, or locally recommended ones).
- Timely sowing to avoid peak rainy periods.
- Balanced fertilization (avoid excess N); apply
potassium.
- Proper spacing for better air circulation.
- Destroy crop residues and control weeds/grass
hosts.
- Crop rotation with non-host crops.
2. Chemical Control
- Seed Treatment: Carbendazim, tricyclazole, or thiram.
- Foliar Sprays:
Tricyclazole, carbendazim, or mancozeb at tillering and flowering stages
(1–2 sprays).
- Apply before or at the onset of symptoms during
humid weather.
3. Biological Control
- Antagonistic microbes like Trichoderma
spp. and Pseudomonas for seed treatment and soil application.
4. Other Approaches
- Resistant breeding (major focus in small millet
improvement programs).
- Integrated approaches combining cultural, chemical,
and biological methods yield the best results.
Note:
Finger millet blast shares many similarities with rice blast in etiology and
management, reflecting the close relationship between the pathogens.
Leaf Spot Diseases of Millet (Primarily Finger Millet / Ragi)
Several leaf spot diseases affect millets, with Cercospora
leaf spot, Helminthosporium leaf spot (or Brown spot), and other
fungal spots being common. In finger millet, the most notable ones are Cercospora
leaf spot and Helminthosporium (or Bipolaris) leaf spot.
These diseases reduce photosynthetic area, cause premature leaf senescence, and
can lead to yield losses of 10–30%, especially when combined with other
stresses.
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| Figure12: Leaf spot in millet leaf |
Major Leaf Spot Pathogens in Finger Millet
- Cercospora Leaf Spot
- Causal Agent: Cercospora penniseti or
related Cercospora spp.
- Helminthosporium / Brown Leaf Spot
- Causal Agent: Bipolaris spp. or Helminthosporium
spp. (sometimes associated with grain mold complexes).
Other fungi like Curvularia, Alternaria, or
Phoma can also cause leaf spots.
Symptoms
- Cercospora Leaf Spot:
- Small, circular to oval spots
with dark brown/reddish margins and grayish centers.
- Spots enlarge and may coalesce,
leading to leaf yellowing and drying.
- Lesions often appear on older
leaves first.
- Helminthosporium / Brown Spot:
- Brown to dark brown oval or
irregular spots on leaves.
- Spots may have a yellowish halo;
severe cases cause extensive blighting.
- Similar to brown spot in rice.
- General Symptoms: Reduced leaf area, stunting in severe cases,
and premature defoliation. Spots can also appear on sheaths and glumes.
Symptoms are more severe under humid conditions and
nutrient stress.
Etiology & Disease Cycle
- Pathogens: Fungi that
produce conidia (asexual spores).
- Survival: In crop
residues, seeds, and soil.
- Spread: Wind, rain
splash, and infected planting material.
- Infection: Spores
germinate on wet leaf surfaces, penetrate tissues, and produce new lesions
within 5–10 days.
- Cycles: Multiple
secondary cycles during the rainy season (polycyclic).
Epidemiology (Favorable Conditions)
- High humidity and prolonged leaf wetness (rainy
season).
- Temperatures of 20–30°C.
- Nutrient deficiencies (especially nitrogen and
potassium).
- Susceptible varieties and poor field
sanitation.
Management
- Cultural Practices:
- Resistant or tolerant varieties.
- Balanced fertilization (NPK +
micronutrients like Zn).
- Proper spacing for air
circulation.
- Timely sowing and crop rotation.
- Remove and destroy infected debris; use healthy seeds.
- Chemical Control:
- Seed treatment with carbendazim
or thiram.
- Foliar sprays of mancozeb, carbendazim, or propiconazole at early symptom appearance.
- Biological Control:
- Trichoderma spp. and other antagonists for seed and soil application.
- Integrated Approach: Combine cultural practices with minimal
fungicide use for sustainable management.
Comparison with Blast:
- Blast lesions are typically spindle-shaped with
gray centers and prominent on necks/fingers.
- Leaf spots are more circular/oval and primarily
foliar.
Leaf spot diseases are often secondary but become
important under humid conditions or when blast is managed. For finger millet,
blast remains the priority disease, but good overall plant health (nutrition +
sanitation) helps manage leaf spots effectively.
Source from:
https://www.ars.usda.gov/ARSUserFiles/opmp/Rice%20Bacterial%20Blight%20and%20Streak%20Recovery%20Plan%20Final.pdf
https://bpp.oregonstate.edu/sites/agscid7/files/bpp/attachments/bacterial-leaf-streak-of-rice.pdf
https://apsjournals.apsnet.org/doi/10.1094/PHYTO-06-22-0226-RVW
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