Important plant pathogenic organisms
- Fungal Pathogens
Fungi are the most prominent plant pathogens. They are eukaryotic, spore-producing organisms that derive nutrition from host plants, often causing extensive damage.
Characteristics:
- They reproduce through spores (sexual and asexual).
- Fungi produce structures like mycelium, conidia, and fruiting bodies.
- They infect plants through natural openings (stomata, lenticels) or wounds.
Types and Examples:
Rusts: Cause pustule-like symptoms on leaves. Puccinia graminis: Stem rust of wheat. Uromyces appendiculatus: Bean rust.
Smuts: Replace plant grains with fungal spores. Ustilago maydis: Corn smut. Tilletia indica: Karnal bunt of wheat.
Blights: Rapid killing of tissues. Phytophthora infestans: Late blight of potato. Alternaria solani: Early blight of potato and tomato.
Mildews: Powdery or downy growth on leaf surfaces. Erysiphe necator: Powdery mildew of grapes. Plasmopara viticola: Downy mildew of grapes.
Rots: Decay of plant tissues. Sclerotium rolfsii: Collar rot of groundnut. Rhizoctonia solani: Damping-off in seedlings.
Wilts: Blocking of vascular tissues. Fusarium oxysporum: Fusarium wilt of banana and tomato. Verticillium dahliae: Wilt in cotton and eggplant.
- Bacterial Pathogens
Bacteria are unicellular, prokaryotic organisms that infect plants by entering through wounds or natural openings.
Characteristics: They multiply rapidly in host tissues. Produce toxins, enzymes, or slime that damages plants.
Types and Examples:
Blights: Rapid tissue death. Xanthomonas oryzae pv. oryzae: Bacterial leaf blight of rice. Pseudomonas syringae pv. phaseolicola: Halo blight of beans.
Wilts: Blockage of xylem. Ralstonia solanacearum: Bacterial wilt in tomatoes and bananas.
Cankers: Formation of sunken lesions. Xanthomonas axonopodis pv. citri: Citrus canker.
Soft Rots: Tissue decay. Pectobacterium carotovorum: Soft rot of vegetables.
Leaf Spots: Localized necrotic areas. Xanthomonas campestris pv. campestris: Black rot of crucifers.
- Viral Pathogens
Viruses are non-cellular entities consisting of nucleic acid (DNA or RNA) enclosed in a protein coat. They rely on living cells to replicate.
Characteristics:
- Submicroscopic and obligate parasites.
- Transmitted through vectors (insects, nematodes) or mechanical means.
Types and Examples:
Mosaic Viruses: Tobacco mosaic virus (TMV): Affects tobacco, tomato, and peppers. Cucumber mosaic virus (CMV): Infects cucurbits and other vegetables.
Leaf Curl Viruses: Tomato leaf curl virus (TLCV): Transmitted by whiteflies.
Yellow Vein Viruses: Bhindi yellow vein mosaic virus: Affects okra.
Bunchy Top Viruses: Banana bunchy top virus (BBTV): Transmitted by aphids.
- Phytoplasmas
Phytoplasmas are wall-less, pleomorphic prokaryotes that reside in phloem tissues and are transmitted by insect vectors like leafhoppers.
Characteristics: They lack a cell wall., Cause systemic diseases by affecting plant metabolism.
Examples:
- Little leaf of brinjal.
- Sandalwood spike disease.
- Sesame phyllody.
- Nematodes
Nematodes are microscopic, unsegmented roundworms that infect plant roots, causing damage and predisposing them to other pathogens.
Characteristics:
- Feed on root tissues and disrupt water/nutrient absorption.
- Some nematodes are endoparasitic, while others are ectoparasitic.
Types and Examples:
Root Knot Nematodes: Meloidogyne spp.: Cause gall formation on roots.
Cyst Nematodes: Heterodera spp.: Parasitize cereal and legume crops.
Lesion Nematodes: Pratylenchus spp.: Cause root lesions in bananas and vegetables.
- Parasitic Plants
These are plants that partially or completely depend on their host plants for nutrition.
Types and Examples:
- Obligate Parasites:
- Cuscuta spp. (Dodder): Attacks several crops, including legumes.
- Orobanche spp. (Broomrape): Parasitizes tobacco, tomato, and sunflower.
- Semi-Parasites:
- Loranthus spp.: Parasitic on fruit and forest trees.
- Striga spp.: Parasitizes cereals like maize, sorghum, and millets.
- Algal Pathogens Algae occasionally infect plants, especially in humid tropical climates. Examples: Cephaleuros virescens (Red rust): Affects tea, coffee, and mango leaves.
- Protozoan Pathogens Protozoans are unicellular eukaryotes that occasionally cause plant diseases. Examples: Phytomonas spp.: Infects coconut and oil palm.
- Abiotic Causes Though not pathogens, environmental stressors such as nutrient imbalances, pollution, and unfavorable weather conditions can mimic disease symptoms. Examples: Nutrient deficiencies (e.g., chlorosis due to nitrogen deficiency)., Frost damage, drought stress, and heat injury.
