Types of Parasitism in Plant Pathogens

Parasitism in plant pathogens refers to the relationship where a pathogen derives nutrients from a living host plant, often causing disease. Based on their dependency and interaction with the host, plant pathogens exhibit different types of parasitism:

1. Obligate Parasites (Biotrophs)

• These parasites can only survive and reproduce on living host tissues.
• They do not kill the host immediately but establish a long-term association.
• They obtain nutrients without destroying the host cells.
• Examples:
  • Puccinia graminis (Wheat rust)
  • Plasmopara viticola (Downy mildew)
  • Erysiphe spp. (Powdery mildew)

2. Facultative Parasites (Necrotrophs)

• These pathogens are primarily saprophytes (live on dead organic matter) but can attack living plants under favorable conditions.
• They cause rapid destruction of host tissues, producing toxins and enzymes.
• Examples:
  • Alternaria solani (Early blight of potato)
  • Botrytis cinerea (Gray mold)
  • Rhizoctonia solani (Damping-off disease)

3. Hemibiotrophic Parasites

• These pathogens start as biotrophs (living on living host tissue) and later switch to necrotrophs (killing host tissue).
• Initially, they establish a relationship without harming the plant but eventually cause tissue death.
• Examples:
  • Colletotrichum spp. (Anthracnose)
  • Phytophthora infestans (Late blight of potato)
  • Magnaporthe oryzae (Rice blast)

4. Endoparasites and Ectoparasites

• Endoparasites: These live inside the host tissues (e.g., nematodes like Meloidogyne spp.).
• Ectoparasites: These remain outside the host but derive nutrients from it (e.g., some fungi and mistletoe).

Variability in Plant Pathogens

Variability in plant pathogens refers to the differences in their genetic makeup, virulence, morphology, or behavior, leading to the evolution of new races or strains. This variability affects disease development, resistance, and management strategies.

1. Genetic Variability

• Occurs due to mutations, genetic recombination, or horizontal gene transfer.
• Leads to the emergence of new virulent strains that can overcome host resistance.
• Examples:
  • Puccinia graminis f. sp. tritici (New races like Ug99 in wheat rust)
  • Phytophthora infestans (Different strains affecting potato)

2. Pathogenic Variability

• Pathogens differ in their ability to infect different plant varieties or species.
• Can result in the breakdown of host resistance.
• Examples:
  • Different races of Fusarium oxysporum affecting different crops.
  • Strain variation in Xanthomonas oryzae (Bacterial leaf blight in rice).

3. Morphological and Physiological Variability

• Changes in size, shape, spore formation, or enzymatic activity.
• Affects how pathogens survive, infect, and spread.
• Examples:
  • Variation in Alternaria spp. spore morphology.
  • Physiological adaptation in Erwinia amylovora (Fire blight).

4. Environmental Variability

• Changes in temperature, humidity, soil, and climate influence pathogen behavior.
• Affects pathogen survival, sporulation, and infection severity.
• Examples:
  • Rhizoctonia solani is more severe in warm, wet conditions.
  • Phytophthora infestans spreads rapidly in cool, moist climates.