Types of Parasitism and Variability in Plant Pathogens

1. Types of Parasitism in Plant Pathogens

Parasitism is a relationship in which one organism (parasite) derives nutrients from another living organism (host), often causing harm. Plant pathogens exhibit different types of parasitism based on their host dependency and mode of infection.

• Based on Host Dependency

(a) Obligate Parasites (Biotrophs)

• These parasites can survive and reproduce only on living host tissues.
• They do not kill the host immediately but extract nutrients over a prolonged period.
• Examples:
  • Puccinia spp. (Rust fungi)
  • Powdery mildew fungi (Erysiphe spp.)
  • Plasmopara viticola (Downy mildew of grapevine)

(b) Facultative Parasites (Necrotrophs)

• These pathogens can live as parasites but can also survive as saprophytes (on dead organic matter).
• They kill host cells and feed on the decayed tissue.
• Examples: Alternaria spp. (Causes leaf spots), Botrytis cinerea (Grey mold of fruits), Rhizoctonia solani (Damping-off in seedlings)

(c) Facultative Saprophytes

• Normally saprophytic but can become parasitic under favorable conditions.
• Examples:Fusarium oxysporum (Wilt pathogen). Sclerotinia sclerotiorum (White mold in various crops)

(d) Obligate Saprophytes They live only on dead organic matter and do not infect living plants. Examples:  Trichoderma spp. (Used as a biocontrol agent)

2. Based on Infection Mode

(a) Localized Parasites Confined to a particular plant organ (leaves, stems, roots). Examples: Alternaria solani (Early blight of potato – leaf spots), Colletotrichum spp. (Anthracnose of fruits)

(b) Systemic Parasites Spread throughout the plant via vascular tissues (xylem or phloem). Examples: Fusarium oxysporum (Wilt pathogen – xylem-invading), Xanthomonas oryzae (Bacterial leaf blight of rice – phloem-associated)

3. Based on Host Range

(a) Monophagous Parasites (Host-Specific) Infect only one species or a closely related group of plants. Examples:  Puccinia graminis (Wheat rust – specific to wheat and barberry)

(b) Polyphagous Parasites (Broad-Host Range) Infect a wide variety of plant species. Examples: Botrytis cinerea (Affects grapes, tomatoes, strawberries), Rhizoctonia solani (Affects rice, potato, vegetables)

• Variability in Plant Pathogens Plant pathogens show genetic variability, which allows them to adapt to different environments, overcome host resistance, and develop resistance to fungicides.
• Types of Variability in Pathogens

(a) Genetic Variability

• Arises due to mutations, recombination, and hybridization.
• Leads to new pathogen races or biotypes.
• Examples: Different races of Puccinia striiformis (Stripe rust of wheat).

(b) Physiological Variability Differences in virulence and pathogenicity among isolates of the same species. Some strains are more aggressive than others. Examples: Phytophthora infestans (Late blight pathogen with different virulence levels).

(c) Morphological Variability Differences in size, shape, and structure of spores or reproductive structures. Examples:  Variation in Fusarium spp. conidial size and shape.

(d) Pathogenic Variability Some strains of the same pathogen can infect different plant varieties, while others cannot. Examples:  Magnaporthe oryzae (Rice blast pathogen has multiple races affecting different rice varieties).

(e) Cultural Variability Variation in growth rate, colony color, and texture when grown in artificial media. Examples:  Different strains of Colletotrichum species have varying growth patterns.

2. Factors Contributing to Variability in Pathogens

• Mutation Sudden changes in the genetic material that lead to new strains. Example: Puccinia graminis (Wheat rust evolves new virulent strains).
• Genetic Recombination Sexual reproduction (meiotic recombination) results in new genotypes. Example: Phytophthora infestans (Late blight pathogen undergoes sexual recombination).
• Heterokaryosis The presence of genetically different nuclei within the same fungal cell. Example: Rhizoctonia solani (Soilborne pathogen exhibits heterokaryosis).
• Parasexuality Exchange of genetic material without meiosis, common in fungi. Example: Fusarium oxysporum (Wilt pathogen shows parasexual recombination).
• Hybridization Crossbreeding of two different strains leading to new pathogenic forms. Example: Hybrid strains of Zymoseptoria tritici (Septoria leaf blotch).

3. Impact of Variability in Plant Pathogens

• Leads to breakdown of host resistance (e.g., new rust races in wheat).
• Causes reduced fungicide effectiveness (e.g., Pythium resistance to metalaxyl).
• Results in development of new biotypes affecting different crops.

III. Management Strategies for Variability in Pathogens

1. Use of Resistant Crop Varieties Breeding for durable resistance using multiple genes. Example: Rust-resistant wheat variety HD 2967.
2. Crop Rotation and Sanitation Breaks the disease cycle and prevents pathogen adaptation.
3. Integrated Disease Management (IDM) Combining biological, cultural, and chemical control.
4. Fungicide Rotation Prevents resistance development in fungal pathogens.
5. Molecular Breeding and Biotechnological Approaches Use of CRISPR and marker-assisted selection (MAS) for disease-resistant crops.