1. Introduction
Agricultural crops face multiple challenges in the form of biotic stresses, caused by living organisms such as pathogens, insect pests, and weeds. These stresses can cause significant yield and quality losses worldwide.
- Pathogens: fungi, bacteria, viruses, nematodes
- Insect pests: stem borers, aphids, leaf miners, whiteflies
- Weeds: competing plants that reduce crop productivity
Examples of impact: Rice bacterial leaf blight – up to 50% yield loss; wheat rusts – 10–70% loss; cotton bollworm – up to total crop loss.
2. Objectives of Breeding for Biotic Stress
- Develop disease- and pest-resistant crop varieties
- Ensure stable yield and quality under stress conditions
- Reduce dependence on chemical pesticides
- Provide durable resistance that lasts over time
- Enhance economic returns to farmers while ensuring sustainability
3. Types of Resistance
A. Vertical (Qualitative) Resistance
- Controlled by major genes (R genes)
- Provides complete resistance to specific pathogen races
- Mechanism: triggers a hypersensitive response to stop infection
- Examples: Wheat stem rust (Sr genes), Rice blast (Pi genes)
B. Horizontal (Quantitative) Resistance
- Controlled by multiple minor genes (polygenic)
- Provides partial resistance to multiple pathogen strains
- Mechanism: slows pathogen development without complete prevention
- Examples: Powdery mildew in barley, Leaf blight in rice
4. Sources of Resistance
- Cultivated varieties with known resistance
- Wild relatives of crops (novel resistance genes)
- Landraces – locally adapted varieties
- Mutants – induced by radiation or chemicals
5. Breeding Methods for Biotic Stress Resistance
A. Conventional Breeding Methods
1. Hybridization
Cross resistant donor × high-yielding susceptible variety. Select F2 and later generations for resistance.
Example: Crossing rust-resistant wheat with high-yielding wheat.
2. Backcross Breeding
Transfer specific resistance gene to elite variety while retaining its traits.
- F1 = Donor × Recurrent parent
- BC1F1 = F1 × Recurrent parent
- Continue backcrossing and select progeny with resistance
Example: Transfer of Sr24 gene for stem rust resistance in wheat.
3. Selection Methods
- Pedigree selection – track individual plants over generations
- Bulk method – natural selection under stress conditions
- Single seed descent – rapid advancement under controlled conditions
4. Mutation Breeding
- Induce mutations using chemicals or radiation
- Screen large populations for new resistance traits
- Example: Disease-resistant barley and wheat varieties
B. Modern Breeding Methods
1. Marker-Assisted Selection (MAS)
Use DNA markers linked to resistance genes to select plants early. Example: MAS for blast resistance in rice (Pi genes).
2. Gene Pyramiding
Combine multiple resistance genes in a single variety to enhance durability. Example: Rice with Xa21, xa13, xa5 genes.
3. Genetic Engineering
Introduce resistance genes directly. Example: Bt cotton with Cry gene for insect resistance.
4. Genome Editing (CRISPR/Cas9)
Edit susceptibility or resistance genes. Example: CRISPR-edited rice with broad-spectrum bacterial blight resistance.
6. Screening Techniques
- Field screening: Grow crops in disease/pest hotspots
- Artificial/Inoculation screening: Introduce pathogen/pest under controlled conditions
- Greenhouse/Lab screening: For precise evaluation of difficult pathogens
- High-throughput screening: Molecular markers or phenotyping platforms
7. Challenges in Breeding
- Rapid evolution of pathogens
- Complex inheritance of quantitative resistance
- Linkage drag – undesirable traits linked to resistance genes
- Environmental influence on resistance expression
- Time and resource-intensive breeding process
8. Examples of Successful Breeding
| Crop | Biotic Stress | Resistance Strategy | Notes |
|---|---|---|---|
| Rice | Bacterial blight | Gene pyramiding (Xa21, xa13, xa5) | MAS used to combine multiple genes |
| Wheat | Stem rust | Sr gene introgression | Sr24, Sr31 genes widely used |
| Cotton | Bollworm | Bt transgenic gene | Reduces pesticide use |
| Potato | Late blight | Combination of R genes and quantitative resistance | Durable resistance approach |
9. Integrated Approach
- Use resistant varieties
- Cultural practices: crop rotation, spacing, sanitation
- Biological control: predators, parasitoids, microbial agents
- Chemical control: judicious pesticide use
