Different Groups of Plant Pathogens

Plant pathogens are categorized into various groups based on their characteristics, mode of infection, and host interactions. The major groups include Fungi, Bacteria, and Fastidious Vascular Bacteria (FVB).

1. Fungi

Definition: Fungi are eukaryotic, spore-producing organisms that lack chlorophyll and obtain nutrients through absorption. They are the most significant plant pathogens, causing various diseases like rusts, smuts, blights, and mildews.

Characteristics of Fungi

• Cell Type: Eukaryotic (have a true nucleus)
• Cell Wall Composition: Made of chitin and glucans
• Nutrition: Heterotrophic (absorb nutrients from living or dead tissues)

Reproduction:

• Asexual (spores like conidia, sporangia, zoospores)
• Sexual (ascospores, basidiospores, oospores, zygospores)
  ✔ Spread: Wind, water, insects, soil, seeds, and plant debris

Major Groups of Fungi and Examples

• Ascomycetes (Sac Fungi) – Produce ascospores in sac-like structures (asci). g., Powdery Mildew (Erysiphe spp.), Apple Scab (Venturia inaequalis)
• Basidiomycetes (Club Fungi) – Produce basidiospores on club-shaped structures (basidia). g., Rusts (Puccinia spp.), Smut Fungi (Ustilago spp.)
• Oomycetes (Water Molds) – Resemble fungi but belong to a different group (Chromista). g., Late Blight of Potato (Phytophthora infestans), Damping-Off (Pythium spp.)
• Zygomycetes (Bread Molds) – Produce zygospores in a resistant structure. g., Soft Rot of Vegetables (Rhizopus stolonifer)
• Deuteromycetes (Fungi Imperfecti) – Lack a known sexual stage. g., Wilt Disease (Fusarium oxysporum), Tikka Disease of Groundnut (Cercospora arachidicola)

Fungal Disease Examples

• Rusts (Puccinia graminis – Wheat Rust)
• Blights (Phytophthora infestans – Late Blight of Potato)
• Wilts (Fusarium oxysporum – Fusarium Wilt of Cotton)
• Mildews (Erysiphe graminis – Powdery Mildew of Wheat)

2. Bacteria

Definition: Bacteria are microscopic, unicellular prokaryotic organisms that lack a true nucleus and reproduce by binary fission. They cause plant diseases by producing toxins and enzymes that break down plant tissues.

Characteristics of Bacteria

• Cell Type: Prokaryotic (lack a nucleus)
• Cell Wall Composition: Made of peptidoglycan
• Reproduction: Binary fission (rapid multiplication)
• Movement: Some bacteria have flagella for motility
• Spread: Water splashes, wind, insects, seeds, and contaminated tools

Major Groups of Plant Pathogenic Bacteria

• Gram-Negative Bacteria (Most Plant Pathogens) g., Bacterial Blight of Rice (Xanthomonas oryzae pv. oryzae), Soft Rot of Vegetables (Pectobacterium carotovorum)
• Gram-Positive Bacteria (Less Common in Plants) g., Potato Scab (Streptomyces scabies)
• Mollicutes (Wall-Less Bacteria) Phytoplasma: Cause phloem diseases (g., Little Leaf of Brinjal) Spiroplasma: Spiral-shaped (E.g., Corn Stunt Disease)

Bacterial Disease Examples

• Bacterial Wilt (Ralstonia solanacearum – Affects Tomato, Potato, Banana)
• Citrus Canker (Xanthomonas axonopodis pv. citri)
• Black Rot of Crucifers (Xanthomonas campestris pv. campestris)
• Bacterial Soft Rot (Pectobacterium carotovorum)

3. Fastidious Vascular Bacteria (FVB)

Definition: Fastidious vascular bacteria are specialized bacteria that are difficult to culture in artificial media and live inside the vascular system (xylem or phloem) of plants, causing systemic diseases.

Characteristics of Fastidious Vascular Bacteria

• Difficult to Culture – Require special media or insect vectors for survival
• Spread – Mainly transmitted by insect vectors like leafhoppers, psyllids, and whiteflies
• Live Inside Vascular Tissues – Cause systemic blockage leading to wilting and stunting

Types of Fastidious Vascular Bacteria

• Xylem-Limited Bacteria (Block water transport, causing wilts) g., Pierce’s Disease of Grape (Xylella fastidiosa), Ratoon Stunting Disease of Sugarcane (Leifsonia xyli subsp. xyli)
• Phloem-Limited Bacteria (Affect nutrient transport, causing yellowing and stunting) g., Citrus Greening (Huanglongbing, HLB) (Candidatus Liberibacter spp.), Corn Stunt Disease (Spiroplasma kunkelii)

Fastidious Bacterial Disease Examples

• Citrus Greening (HLB) – Candidatus Liberibacter asiaticus
• Pierce’s Disease of Grapevine – Xylella fastidiosa
• Corn Stunt Disease – Spiroplasma kunkelii

Comparison of Fungi, Bacteria, and Fastidious Vascular Bacteria (FVB)