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

Smut diseases represent one of the most economically significant fungal diseases affecting sorghum (Sorghum bicolor) cultivation worldwide. These diseases can cause substantial yield losses and significantly reduce grain quality, making their understanding and management crucial for sustainable sorghum production.

1. Introduction

Sorghum, a vital cereal crop cultivated across tropical and subtropical regions, serves as a staple food source for millions and provides valuable livestock feed and industrial raw materials. However, its production faces constant threats from various smut diseases. Three primary types of smut affect sorghum: covered kernel smut, loose kernel smut, and head smut, each caused by distinct fungal pathogens with unique characteristics and epidemiological patterns.

Economic Importance: Smut diseases can cause yield losses ranging from 10% to 100% in severely infected fields, depending on the type of smut, environmental conditions, and cultivar susceptibility. Beyond direct yield reduction, these diseases also compromise grain quality, making it unsuitable for human consumption or commercial use.

2. Symptoms of Smut Diseases

2.1 Covered Kernel Smut

Covered kernel smut, also known as grain smut, manifests primarily on individual kernels within the panicle. The symptoms become most apparent at grain maturity:

image source - TNAU Agritech Portal

• Individual kernel infection: Only some kernels within a head are typically infected, while others remain healthy
• Sorus formation: Infected kernels are completely replaced by a dark brown to black, oval or spherical mass of spores (sorus) covered by a thin, grayish membrane
• Kernel size: Infected kernels are often slightly larger than healthy ones and have a more rounded appearance
• Membrane rupture: The covering membrane may rupture during handling or harvesting, releasing masses of dark, powdery spores (teliospores)
• Odor: A characteristic fishy or trimethylamine odor may be present when infected heads are crushed

2.2 Loose Kernel Smut

Loose kernel smut presents distinct symptoms that differentiate it from the covered form:

image source-- CABI 

• Early spore exposure: The fungal spore mass lacks a persistent membrane covering, making spores visible and loose at an early stage
• Complete kernel destruction: Infected kernels are entirely transformed into a powdery mass of black spores
• Spore dispersal: Wind and rain readily disperse the loose spores from infected heads during grain development
• Vascular bundles: Only the fibrovascular tissue of the kernel may remain after spore dispersal, creating a skeletal appearance
• Partial head infection: Like covered smut, typically only portions of the panicle show infection

2.3 Head Smut (Long Smut)

Head smut represents the most devastating form, with systemic infection leading to dramatic symptoms:

images source - Plantix

• Systemic infection: The entire inflorescence is transformed into a long, cylindrical, whip-like structure covered with a grayish-white to brown membrane
• Premature emergence: Infected heads emerge earlier than healthy ones and protrude conspicuously above the canopy
• Whip-like appearance: The infected head appears as an elongated, curved structure that can extend 30-90 cm in length
• Membrane breakdown: As the infection matures, the membrane ruptures, releasing enormous quantities of dark brown to black teliospores
• Plant stunting: Infected plants often show reduced vigor and may be slightly stunted
• Leaf symptoms: Chlorotic streaking or pale green stripes may appear on leaves of systemically infected plants
• No grain production: Infected heads produce no viable grain whatsoever

Field Identification: Head smut can be identified from considerable distances due to the conspicuous whip-like structures that stand above the crop canopy. Early detection is crucial for implementing roguing strategies to prevent further spore dissemination.

3. Etiology and Causal Organisms

3.1 Covered Kernel Smut

Causal organism: Sporisorium sorghi (formerly Sphacelotheca sorghi)

Taxonomic Classification:

• Kingdom: Fungi
• Phylum: Basidiomycota
• Class: Ustilaginomycetes
• Order: Ustilaginales
• Family: Ustilaginaceae

Morphological Characteristics:

• Teliospores: Dark brown, spherical to subspherical, 5-9 μm in diameter, with echinulate (spiny) surfaces
• Spore germination: Teliospores germinate to produce a promycelium (basidium) bearing 4-8 basidiospores
• Infection structures: Basidiospores conjugate to form infection hyphae that penetrate the ovarian tissue

3.2 Loose Kernel Smut

Causal organism: Sporisorium cruentum (synonym: Sphacelotheca cruenta)

Morphological Characteristics:

• Teliospores: Reddish-brown to dark brown, spherical, 5-7 μm in diameter, smooth to finely echinulate
• Spore balls: Teliospores often occur in small clusters or balls surrounded by sterile cells
• Color distinction: Fresh spore masses have a distinctive reddish-brown color compared to the darker covered smut

3.3 Head Smut

Causal organism: Sporisorium reilianum (synonym: Sphacelotheca reiliana)

Morphological Characteristics:

• Teliospores: Light to dark brown, spherical to oval, 9-14 μm in diameter, larger than kernel smut spores
• Surface ornamentation: Spores have a reticulate (net-like) pattern on the surface
• Columella: A central core of sterile tissue (columella) extends through the sorus

Pathogen Specialization: Different formae speciales or races of these pathogens exist, showing varying degrees of virulence on different sorghum genotypes. This genetic variability in both host and pathogen creates a dynamic co-evolutionary relationship that influences disease management strategies.

4. Disease Cycle and Epidemiology

4.1 Covered and Loose Kernel Smut Disease Cycle

1. Survival and Primary Inoculum:

The disease cycle begins with teliospores that serve as the survival and primary inoculum structures. These spores possess remarkable longevity:

• Teliospores survive on or in seeds as surface contamination or within the seed coat
• Spores can remain viable in soil for 1-3 years, though viability decreases over time
• Spores can persist on plant debris, equipment, and storage facilities
• Soil-borne inoculum becomes significant in fields with a history of severe infection

2. Spore Germination and Infection:

Infection occurs during a narrow window of host development:

• Timing: Infection takes place during flowering when ovaries are receptive
• Germination: Teliospores germinate in response to moisture and suitable temperatures (20-30°C optimal)
• Basidiospore production: The promycelium produces basidiospores that serve as the actual infection propagules
• Conjugation: Compatible basidiospores must conjugate (fuse) to form dikaryotic infection hyphae
• Penetration: Infection hyphae penetrate the ovarian wall and establish within developing kernels
• Critical period: The ovary remains susceptible for only 5-7 days after pollination

3. Colonization and Sorus Development:

• The fungus grows intercellularly within the developing kernel tissues
• Fungal mycelium completely replaces kernel contents as it develops
• Sorus formation begins 2-3 weeks after infection
• Teliospore production continues until kernel maturity
• Millions of teliospores are produced within each infected kernel

4. Spore Dispersal and Cycle Completion:

• At maturity, sori rupture during harvesting, threshing, or handling
• Released spores contaminate healthy seeds in the same head or during processing
• Wind, rain, insects, and harvesting equipment spread spores to other plants and fields
• Contaminated seed serves as the primary means of long-distance pathogen dissemination

4.2 Head Smut Disease Cycle

Head smut follows a fundamentally different infection strategy involving systemic colonization:

1. Survival and Soil-borne Inoculum:

• Teliospores survive primarily in soil for 3-10 years or longer
• Survival period depends on soil temperature, moisture, microbial activity, and spore depth
• Surface teliospores degrade faster than those incorporated into soil
• Seed-borne inoculum is rare but possible

2. Seedling Infection:

• Critical timing: Infection occurs during seedling emergence and early growth stages
• Temperature sensitivity: Soil temperatures of 25-30°C favor infection; temperatures above 35°C inhibit it
• Germination: Teliospores near seedlings germinate and produce basidiospores
• Root/shoot penetration: Basidiospores or their conjugation products penetrate coleoptiles, mesocotyls, or young roots
• Depth effect: Shallow planting (< 2.5 cm) increases infection risk

3. Systemic Colonization:

• After penetration, the fungus grows systemically through the plant
• Mycelium advances intercellularly through the vascular system
• The fungus reaches the apical meristem and growing points
• The pathogen remains dormant in vegetative tissues until reproductive development
• No external symptoms appear until flowering initiation

4. Sorus Formation and Sporulation:

• When the host initiates flowering, fungal growth accelerates dramatically
• The entire inflorescence meristem is transformed into fungal tissue
• The characteristic whip-like sorus develops, enclosed initially in a membrane
• Sori emerge 5-10 days earlier than healthy heads
• Membrane ruptures at maturity, releasing billions of teliospores

5. Dissemination:

• Wind disperses vast quantities of teliospores to soil surfaces
• Rain splash and irrigation water move spores into soil
• Contaminated soil serves as inoculum for subsequent crops
• Farm equipment and soil movement spread the pathogen between fields

4.3 Environmental Factors Influencing Disease Development

Factor	Kernel Smuts	Head Smut
Temperature	Optimal 20-30°C during flowering; high temperatures (>35°C) reduce infection	Optimal soil temperature 25-30°C; temperatures >35°C inhibit seedling infection
Moisture	High humidity and rainfall during flowering favor infection; dry conditions reduce disease	Adequate soil moisture required for spore germination and seedling infection
Planting Date	Extended flowering periods increase infection opportunity	Early planting into cool soil reduces infection; late planting into warm soil increases risk
Planting Depth	Minor effect	Shallow planting (<2 .5="" cm="" increases="" infection="" risk="" significantly="" td="">
Soil Type	Limited direct effect	Sandy soils warm faster, potentially increasing early-season infection

5. Disease Management

Integrated management strategies combining multiple approaches provide the most effective and sustainable control of sorghum smut diseases. Management tactics must be tailored to the specific smut type and local conditions.

5.1 Genetic Resistance

Host plant resistance represents the most economical and environmentally sustainable management strategy:

Resistance Mechanisms:

• Kernel smuts: Resistance often involves physical barriers (thick ovary walls), biochemical defenses, and rapid wound healing
• Head smut: Resistance may involve barriers to seedling penetration, hypersensitive responses, or inhibition of systemic colonization
• Both qualitative (major gene) and quantitative (polygenic) resistance have been identified

Breeding and Cultivar Selection:

• Modern sorghum breeding programs actively screen for smut resistance
• Resistant hybrids and cultivars are available for most growing regions
• Farmers should select cultivars with documented resistance to prevalent local smut types
• Genetic diversity should be maintained to avoid vulnerability to new pathogen races
• Resistance stability across environments and years should be considered

Resistance Durability: While resistant cultivars provide excellent control, pathogen populations can evolve to overcome resistance genes. Deploying cultivars with different resistance genes in rotation or as mixtures can extend resistance durability.

5.2 Cultural Practices

Crop Rotation:

• Rotate sorghum with non-host crops for 2-3 years in fields with head smut history
• Deep-rooted crops and forages can help reduce soil-borne teliospore populations
• Rotation effectiveness varies with smut type: highly effective for head smut, less so for seed-borne kernel smuts

Planting Practices:

• Planting date: Adjust planting times to avoid conditions favoring infection (e.g., delay planting until soil warms to reduce head smut)
• Planting depth: Plant seeds 3.5-5 cm deep to reduce head smut infection
• Plant density: Maintain proper spacing to promote air circulation and reduce humidity
• Uniform planting: Ensure uniform crop development to minimize extended infection periods

Field Sanitation:

• Immediately rogue (remove) and destroy infected plants, especially head smut whips, before spore release
• Collect rogued material in sealed bags to prevent spore dispersal
• Deep plowing or burial of infected residues can reduce surface inoculum
• Clean equipment between fields to prevent mechanical transmission
• Avoid grazing or feeding animals with infected heads, as viable spores can pass through digestive systems

Water Management:

• Avoid excessive irrigation during flowering to reduce kernel smut infection
• Ensure adequate but not excessive soil moisture during seedling emergence for head smut
• Drip or furrow irrigation may reduce spore splash compared to overhead irrigation

5.3 Seed Treatment and Seed Health

Seed Selection and Certification:

• Purchase certified seed from reputable sources with documented smut testing
• Visually inspect seed lots and reject those with visible smut contamination
• Seed certification programs typically enforce maximum allowable smut contamination levels
• Use seed saved from fields with minimal or no smut incidence

Seed Treatments:

Chemical seed treatments effectively control seed-borne inoculum of kernel smuts:

• Systemic fungicides:
  • Carboxin + thiram: Highly effective against kernel smuts; applied at 2-3 g a.i./kg seed
  • Mancozeb: Provides good surface disinfection at 2.5-3 g/kg seed
  • Metalaxyl + mancozeb: Broad-spectrum protection including seed rots
  • Azoxystrobin: Newer strobilurin fungicides with systemic activity
• Application: Use commercial seed treatment equipment or slurry methods ensuring uniform coverage
• Limitations: Seed treatments are ineffective against soil-borne head smut inoculum
• Seed priming: Combining fungicide treatment with seed priming can improve germination and seedling vigor

Hot Water Treatment:

• Immerse seeds in hot water at 52-54°C for 10-12 minutes
• Effective against surface-borne teliospores without chemical use
• Requires precise temperature control to avoid seed damage
• Dry treated seed immediately to prevent deterioration
• Primarily used for small-scale or organic production

5.4 Chemical Control

Foliar Fungicides:

Foliar fungicide applications have limited effectiveness against smut diseases:

• Difficult to achieve adequate ovary coverage during the brief infection period for kernel smuts
• Generally not economically justified for smut control alone
• May provide incidental protection if applied for other disease management
• Strobilurin and triazole fungicides show some suppressive activity

Soil Treatments:

• Soil fumigation or drenches with fungicides can reduce head smut inoculum
• Generally not economical except in high-value seed production or severely infested fields
• Environmental concerns limit use of many soil fumigants

5.5 Biological Control

Emerging research explores biological control approaches:

• Antagonistic microorganisms: Certain bacteria and fungi (e.g., Trichoderma spp., Bacillus spp.) can suppress teliospore germination or colonization
• Seed biopriming: Coating seeds with beneficial microorganisms may provide protection during germination
• Current status: Biocontrol products show promise in research but limited commercial availability for sorghum smuts
• Integration: Biological approaches work best when integrated with other management practices

5.6 Integrated Disease Management (IDM) Program

A comprehensive IDM program for sorghum smut should incorporate:

1. Pre-planting:
   • Select resistant cultivars appropriate for local conditions
   • Use certified, treated seed from disease-free sources
   • Rotate crops in fields with head smut history
   • Clean and disinfect equipment
2. Planting:
   • Plant at optimal depth (3.5-5 cm) and date for local conditions
   • Maintain proper plant density and uniform stands
   • Ensure good seed-to-soil contact for uniform emergence
3. Growing Season:
   • Scout fields regularly for smut symptoms, especially head smut whips
   • Implement immediate roguing of infected plants before spore release
   • Manage irrigation to avoid excessive moisture during flowering
   • Maintain good crop nutrition for plant vigor
4. Post-harvest:
   • Remove and destroy infected crop residues
   • Plow infected fields deeply to bury surface inoculum
   • Clean harvesting and processing equipment thoroughly
   • Store grain properly to prevent quality deterioration
5. Monitoring and Record-keeping:
   • Document disease incidence and severity by field and year
   • Track cultivar performance for resistance
   • Adjust management strategies based on historical disease patterns
   • Participate in regional disease monitoring networks

Economic Considerations: The most cost-effective management strategy emphasizes prevention through resistant cultivars and cultural practices rather than reactive chemical control. The investment in certified resistant seed and proper cultural practices typically provides excellent economic returns through reduced disease losses.

6. Conclusion

Smut diseases continue to pose significant threats to sorghum production globally. However, understanding the biology, epidemiology, and management of these pathogens enables farmers and agronomists to implement effective control strategies. The key to successful management lies in integrating multiple approaches—especially deploying resistant cultivars, maintaining seed health, practicing good field sanitation, and implementing appropriate cultural controls.

As climate patterns shift and agricultural practices evolve, continued research into sorghum smut diseases remains essential. Future advances in molecular breeding, precision agriculture, and biological control promise to enhance our ability to manage these diseases sustainably. Meanwhile, farmers should adopt currently available integrated management strategies tailored to their specific conditions and smut types to minimize losses and ensure profitable sorghum production.

The success of any management program ultimately depends on timely implementation, consistent monitoring, and adaptation to local conditions. By combining scientific knowledge with practical experience, sorghum producers can effectively manage smut diseases and protect their crops' yield and quality.