Plant and Fungal Parasites: Pathogens, Defenses & Nematodes
Lesson Overview
- What Is a Plant Disease? The Disease Triangle
- Types of Plant Pathogens and Parasitic Plants
- Nematodes and Their Effect on Plants
- How Do Pathogens Enter Plants?
- Abiotic Stressors That Weaken Plants
- Preformed and Induced Plant Defenses
- Chemical Compounds in Plant Defense
- Transpiration Stream and Pathogen Spread
- Dwarf Mistletoe and Timber Destruction
- Conclusion
Plant health is constantly challenged by invaders like fungi, nematodes, and parasitic plants. This detailed lesson on plant and fungal parasites explores how pathogens infect plants, how plants defend themselves, and how environmental factors affect disease. It equips students to understand plant-pathogen relationships, resistance, and coevolution.
What Is a Plant Disease? The Disease Triangle
A plant disease occurs only when three factors are present together:
- A pathogen (organism capable of causing disease),
- A susceptible host plant, and
- Favorable environmental conditions.
This framework is called the disease triangle and helps scientists understand disease outbreaks.
Key Outcomes:
- Pathogen + Susceptible Host = Disease
- Pathogen + Resistant Host = No Disease
- Pathogen + Non-Host = No Disease
- Mutualist + Host = No Disease
For example, a fungus cannot infect a corn plant if the plant has resistance genes or if humidity is too low for spore germination.
Types of Plant Pathogens and Parasitic Plants
A. Common Pathogens
- Fungi: Most widespread; cause blights, rusts, smuts, and rots.
- Bacteria: Cause soft rots and wilts; enter through wounds or stomata.
- Viruses: Spread by insects; cause mosaic patterns, yellowing.
- Nematodes: Microscopic roundworms that feed on roots and disrupt growth.
B. Parasitic Plants
Parasitic plants derive water and nutrients from other plants using a specialized organ called a haustorium.
1. Holoparasites
- Fully dependent on host; lack chlorophyll; cannot photosynthesize.
- Tap into both xylem and phloem for water, sugars, and nutrients.
- Examples: Dodder (Cuscuta spp.), Broomrape (Orobanche spp.)
2. Hemiparasites
- Have chlorophyll; can photosynthesize
- Rely on host for water and minerals (via xylem)
- Example: Witchweed (Striga spp.)
3. Obligate Parasites
- Cannot grow independently or in artificial media
- Must live in direct contact with a host
- Example: Downy mildew fungi
These parasitic strategies reduce a host's productivity and survival by draining its resources.
Nematodes and Their Effect on Plants
Nematodes are microscopic roundworms that can be free-living or parasitic.
A. Nematode Characteristics
- Lack a circulatory system
- Have a complete digestive, nervous, and reproductive system
- Reproduce rapidly in soil and roots
- Have a stylet: a spear-like mouthpart used to puncture plant cells
B. Types of Plant-Parasitic Nematodes
- Ectoparasites:
- Live on the root surface
- Feed from outside the plant
- Example: Dagger nematodes
- Endoparasites:
- Enter root tissues
- Can be:
- Migratory (move through roots)
- Sedentary (stay in one place, like soybean cyst or root-knot nematodes)
C. Symptoms of Nematode Infection
- Cysts or galls on roots
- Stunted growth, chlorosis (yellowing), wilting
- Entry points for secondary pathogens like fungi or bacteria
D. Control Methods
- Crop rotation with non-hosts (wheat, corn)
- Resistant cultivars
- Biological control using predatory microbes
How Do Pathogens Enter Plants?
Pathogens go through several stages to cause disease:
A. Stages of the Disease Cycle
- Dissemination: Pathogen spreads via wind, water, insects, or tools.
- Inoculation: Initial contact between the pathogen and plant.
- Pre-penetration: Pathogen attaches, recognizes host, and prepares enzymes.
- Penetration: Entry through stomata, root tips, wounds, or insect feeding sites (called infection courts).
Entry is often passive for fungi (e.g., spore germination) but active for nematodes (stylet puncture).
Abiotic Stressors That Weaken Plants
Abiotic (non-living) environmental stresses can predispose plants to disease by damaging tissues or disrupting metabolism.
A. Stressors That Damage Cell Membranes:
- High temperatures: Increase membrane fluidity; causes leakage.
- Low temperatures: Cause rigidity; damages structure.
- Drought (desiccation): Reduces turgor pressure and integrity.
- Salt stress: Dehydrates cells and causes ion toxicity.
B. Conditions That Worsen Salt Toxicity:
- Prolonged drought
- High temperatures
- Low humidity (increases evaporation)
- Alkaline soils (high pH makes some ions more toxic)
Damaged cell membranes allow easier pathogen entry and disrupt plant signaling.
Preformed and Induced Plant Defenses
A. Preformed Defenses
These exist before infection and are always present.
1. Mechanical Barriers:
- Cuticle: Waxy layer that blocks pathogens
- Trichomes: Hair-like structures that deter insects
- Small stomata: Fewer openings for pathogens to enter
2. Chemical Barriers:
- Phenolic acids: Antimicrobial
- Saponins: Damage pathogen membranes
- Cyanogenic glycosides: Release toxins on cell damage
- Lactones: Inhibit spore germination
These are passive defenses-like a "plant security system."
B. Induced Defenses
Triggered after infection begins.
- Lignitubers: Woody swellings that isolate infection
- Tyloses: Block xylem vessels to halt pathogen movement
- Callose: Sugar polymer deposited in cell walls to reinforce them
Successful induced defenses can limit or stop disease progression.
Chemical Compounds in Plant Defense
Plants produce a wide range of biochemicals in response to stress and infection:
- Saponins: Detergent-like molecules that break fungal membranes
- Cinnamic acid: Found in cinnamon; antimicrobial and antioxidant
- Benzoic acid: Occurs naturally in fruit; used as a preservative
- Gamma lactones: Involved in signaling and antimicrobial defense
- Flavonoids: Involved in UV protection and attracting pollinators
These compounds are produced either constitutively or on demand.
Transpiration Stream and Pathogen Spread
The transpiration stream is the flow of water through a plant, from roots to leaves:
- Water uptake from soil
- Moves into root apoplast
- Travels through xylem
- Exits through stomata as vapor
Some pathogens exploit this stream to move within the plant, especially vascular pathogens like Verticillium and Fusarium.
Dwarf Mistletoe and Timber Destruction
A. Dwarf Mistletoe (Arceuthobium spp.)
- Found in the Western U.S., Great Lakes, and Canada
- Parasitizes conifers like pine and fir
- Causes severe growth deformities, galls, and tree death
- Responsible for losses equating to 100,000 homes worth of timber per year
Control is challenging due to high seed dispersal and long infection cycles.
Conclusion
Understanding plant and fungal parasites means recognizing the complexity of life-and-death interactions between plants and their invaders. From microscopic nematodes to full-blown mistletoe, from pre-built barriers to sophisticated chemical warfare, plants are not passive. They fight infections, adapt to stress, and evolve defenses over generations. This lesson reveals how plant biology, chemistry, and ecology come together to safeguard life from parasites and pathogens.
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