1Which of the following terms describes an organism that lives primarily as a saprophyte but can live as a parasite when the opportunity arises?
A.Obligate parasite
B.Facultative parasite
C.Facultative saprophyte
D.Biotroph
Correct Answer: Facultative parasite
Explanation:A facultative parasite is primarily a saprophyte (living on dead organic matter) but has the faculty (ability) to become a parasite under certain conditions. Conversely, a facultative saprophyte is primarily a parasite.
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2Which type of pathogen requires a living host to complete its life cycle and cannot be grown on artificial culture media?
A.Hemibiotroph
B.Necrotroph
C.Obligate parasite (Biotroph)
D.Facultative saprophyte
Correct Answer: Obligate parasite (Biotroph)
Explanation:Obligate parasites or biotrophs (e.g., rusts, powdery mildews, viruses) require living host tissue to grow and reproduce. They generally cannot be cultured on non-living media.
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3In the context of variability in fungal pathogens, what is the phenomenon where genetically different nuclei coexist within a common cytoplasm?
A.Mutation
B.Heterokaryosis
C.Hybridization
D.Transformation
Correct Answer: Heterokaryosis
Explanation:Heterokaryosis is the condition where two or more genetically different nuclei are present in the same fungal cell or mycelium. It is a major source of variability in imperfect fungi.
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4Which mechanism of variability is unique to fungi that lack a sexual cycle (Fungi Imperfecti), involving the fusion of haploid nuclei within a heterokaryon followed by haploidization?
A.Conjugation
B.Parasexuality
C.Meiosis
D.Transduction
Correct Answer: Parasexuality
Explanation:Parasexuality (the parasexual cycle) allows for genetic recombination in fungi that do not undergo true sexual reproduction (meiosis). It involves plasmogamy, karyogamy, and haploidization.
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5Which enzyme is primarily responsible for the degradation of the plant cuticle during the initial stages of pathogen penetration?
A.Cellulase
B.Cutinase
C.Pectinase
D.Ligninase
Correct Answer: Cutinase
Explanation:Cutinase is an esterase that breaks down cutin, the main structural component of the plant cuticle, allowing the pathogen to breach the first physical barrier of the host.
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6Soft rot symptoms in vegetables are primarily caused by the action of which group of enzymes that degrade the middle lamella?
A.Amylases
B.Pectinases
C.Lipases
D.Proteases
Correct Answer: Pectinases
Explanation:Pectinases degrade pectin found in the middle lamella of plant cells, leading to cell separation and tissue maceration, characteristic of soft rot diseases caused by pathogens like Pectobacterium.
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7Which of the following is considered a Host-Specific Toxin (HST)?
A.Tabtoxin
B.Phaseolotoxin
C.Victorin (HV-toxin)
D.Tentoxin
Correct Answer: Victorin (HV-toxin)
Explanation:Victorin (produced by Cochliobolus victoriae) is a host-specific toxin that only affects susceptible cultivars of oats (those carrying the Vb gene). The others are non-host-specific toxins.
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8What is the primary function of the enzyme Polygalacturonase (PG) in pathogenesis?
A.It hydrolyzes the glycosidic bonds between galacturonic acid residues in pectin.
B.It removes methoxyl groups from pectin chains.
C.It degrades cellulose microfibrils.
D.It detoxifies phytoalexins.
Correct Answer: It hydrolyzes the glycosidic bonds between galacturonic acid residues in pectin.
Explanation:Polygalacturonase (PG) acts by hydrolyzing the -1,4-glycosidic bonds between galacturonic acid units in pectic substances, breaking the chain into smaller molecules.
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9Which growth regulator is responsible for the 'Bakanae' or 'foolish seedling' disease in rice, characterized by excessive stem elongation?
A.Indole-3-acetic acid (IAA)
B.Gibberellin
C.Ethylene
D.Cytokinin
Correct Answer: Gibberellin
Explanation:The fungus Gibberella fujikuroi produces excessive amounts of Gibberellin, which causes the characteristic internodal elongation seen in Bakanae disease.
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10The formation of 'Green Islands' around infection spots in rust and mildew diseases is attributed to the localized accumulation of which growth regulator?
A.Abscisic Acid
B.Ethylene
C.Cytokinins
D.Gibberellins
Correct Answer: Cytokinins
Explanation:Cytokinins delay senescence and attract nutrients. Their accumulation around infection sites creates 'green islands' of living tissue surrounded by yellowing (senescing) tissue.
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11Which term describes enzymes that are always produced by the pathogen regardless of the presence of the substrate?
A.Inducible enzymes
B.Constitutive enzymes
C.Extracellular enzymes
D.Adaptive enzymes
Correct Answer: Constitutive enzymes
Explanation:Constitutive enzymes are produced at a constant low level regardless of the substrate's presence. In contrast, inducible enzymes are synthesized only when the specific substrate or an inducer is present.
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12Wildfire disease of tobacco is associated with which non-host-specific toxin?
A.Tabtoxin
B.Amylovorin
C.HC-toxin
D.T-toxin
Correct Answer: Tabtoxin
Explanation:Tabtoxin is a non-host-specific toxin produced by Pseudomonas syringae pv. tabaci. It causes chlorotic halos (wildfire symptoms) by inhibiting the enzyme glutamine synthetase.
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13Hypertrophy in plant tissues, often seen in tumorous diseases like Crown Gall, refers to:
A.Increase in cell division (number)
B.Increase in cell size
C.Death of cells
D.Fusion of cells
Correct Answer: Increase in cell size
Explanation:Hypertrophy is the abnormal enlargement of cell size. Hyperplasia refers to the increase in cell number (excessive division).
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14Agrobacterium tumefaciens induces crown gall tumors by transferring T-DNA that codes for the synthesis of which plant hormones?
A.Ethylene and ABA
B.Auxin (IAA) and Cytokinin
C.Gibberellin and Ethylene
D.Brassinosteroids and Salicylic Acid
Correct Answer: Auxin (IAA) and Cytokinin
Explanation:The T-DNA of Agrobacterium tumefaciens contains genes (iaaM, iaaH, and ipt) that code for the biosynthesis of Auxins and Cytokinins, leading to uncontrolled cell proliferation (tumors).
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15Which enzyme degrades the -1,4-glucosidic linkages in cellulose?
A.Amylase
B.Cellulase
C.Xylanase
D.Ligninase
Correct Answer: Cellulase
Explanation:Cellulase enzymes (specifically endo-glucanases and cellobiohydrolases) break down the -1,4-glucosidic bonds of cellulose, converting it into glucose.
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16Pathogens that kill host tissue in advance of penetration and live on the dead matter are known as:
A.Biotrophs
B.Necrotrophs (Perthotrophs)
C.Symbionts
D.Endophytes
Correct Answer: Necrotrophs (Perthotrophs)
Explanation:Necrotrophs secrete toxins or enzymes to kill host cells before colonizing them, feeding on the dead tissue.
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17The 'shot-hole' symptom in leaves involves the formation of an abscission layer, often triggered by the production of:
A.Gibberellins
B.Cytokinins
C.Ethylene
D.Polysaccharides
Correct Answer: Ethylene
Explanation:Ethylene is a stress hormone often produced in response to infection. It promotes the formation of an abscission layer around the infected spot, causing the dead tissue to fall out (shot-hole effect).
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18Which of the following is a definition of a Vivotoxin?
A.A toxin produced only in culture media.
B.A substance produced in the infected host which is not the pathogen itself but causes disease symptoms.
C.A toxin that kills the pathogen.
D.An enzyme involved in respiration.
Correct Answer: A substance produced in the infected host which is not the pathogen itself but causes disease symptoms.
Explanation:According to Dimond and Waggoner, a Vivotoxin is a substance produced in the infected host by the pathogen and/or the host that functions in the production of disease, but is not the pathogen itself.
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19T-toxin, produced by Bipolaris maydis race T, specifically affects maize plants with:
Explanation:T-toxin is a host-specific toxin that specifically targets the mitochondria of maize with Texas male-sterile cytoplasm (CMS-T), causing Southern Corn Leaf Blight.
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20Lignin, a complex polymer conferring rigidity to wood, is primarily degraded by which group of organisms?
A.Viruses
B.White rot fungi (Basidiomycetes)
C.Nematodes
D.Fastidious vascular bacteria
Correct Answer: White rot fungi (Basidiomycetes)
Explanation:White rot fungi produce oxidative enzymes like lignin peroxidases and laccases capable of degrading the complex aromatic structure of lignin.
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21What is the role of Pectin Methyl Esterase (PME) in cell wall degradation?
A.It breaks the pectin chain into galacturonic acid.
B.It removes the methoxyl group () from pectin, producing pectic acid.
C.It degrades the cellulose network.
D.It inhibits host defense enzymes.
Correct Answer: It removes the methoxyl group () from pectin, producing pectic acid.
Explanation:Pectin Methyl Esterase (PME) removes the methyl group esterified to the galacturonan chain. This makes the pectin more susceptible to degradation by chain-splitting enzymes like polygalacturonases.
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22Which mechanism creates variability in bacteria through the uptake of naked DNA from the environment?
A.Conjugation
B.Transduction
C.Transformation
D.Transfection
Correct Answer: Transformation
Explanation:Transformation is the process where bacteria take up free genetic material (naked DNA) from their surroundings and incorporate it into their own genome.
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23The phenomenon of Saltation (or sectoral mutation) in fungal cultures refers to:
A.The complete death of the culture.
B.The appearance of sectors with distinct morphological characters due to mutation or heterokaryosis.
C.The sexual reproduction stage.
D.The production of liquid exudates.
Correct Answer: The appearance of sectors with distinct morphological characters due to mutation or heterokaryosis.
Explanation:Saltation appears as a wedge-shaped sector in a fungal colony that differs morphologically (color, growth rate) from the rest of the parent colony, often resulting from mutation or segregation of heterokaryons.
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24Phaseolotoxin, produced by Pseudomonas syringae pv. phaseolicola, causes halo blight in beans by inhibiting:
Explanation:Phaseolotoxin inhibits the enzyme Ornithine carbamoyltransferase, leading to the accumulation of ornithine and a deficiency in arginine, causing chlorosis.
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25Fusicoccin, a toxin produced by Fusicoccum amygdali, affects plants by:
A.Inhibiting protein synthesis
B.Stimulating uncontrolled opening of stomata
C.Blocking xylem vessels
D.Degrading chlorophyll directly
Correct Answer: Stimulating uncontrolled opening of stomata
Explanation:Fusicoccin activates the plasma membrane -ATPase, leading to potassium influx and the irreversible opening of stomata, which causes excessive water loss and wilting.
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26What is the primary effect of Indole-3-Acetic Acid (IAA) when produced in excess by pathogens like Pseudomonas savastanoi?
A.Leaf abscission
B.Hyperplasia (gall formation)
C.Dwarfing
D.Root rot
Correct Answer: Hyperplasia (gall formation)
Explanation:Excess IAA (Auxin) stimulates cell division (hyperplasia) and cell enlargement (hypertrophy), leading to the formation of galls and knots on the host plant.
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27Which of the following is NOT a cell wall degrading enzyme?
A.Cellulase
B.Hemicellulase
C.Protease
D.Pectinase
Correct Answer: Protease
Explanation:While Proteases degrade proteins (cell membrane constituents or cytoplasmic enzymes), they do not degrade the carbohydrate polymers that make up the structural cell wall (cellulose, hemicellulose, pectin).
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28The 'Gene-for-Gene' hypothesis, explaining variability and host-pathogen interaction, was proposed by:
A.H.H. Flor
B.E.J. Butler
C.Anton de Bary
D.Robert Koch
Correct Answer: H.H. Flor
Explanation:H.H. Flor proposed the gene-for-gene hypothesis while working with Flax Rust (Melampsora lini), stating that for every resistance gene in the host, there is a corresponding avirulence gene in the pathogen.
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29HC-toxin affects maize by inhibiting which specific enzyme activity?
A.Histone deacetylase
B.Glutamine synthetase
C.Plasma membrane ATPase
D.RNA polymerase
Correct Answer: Histone deacetylase
Explanation:HC-toxin (produced by Cochliobolus carbonum) inhibits histone deacetylase, which affects gene regulation and defense responses in susceptible maize varieties (HM1).
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30Witches' broom symptoms are typically associated with an imbalance of which plant hormone?
A.Gibberellins
B.Cytokinins
C.Abscisic Acid
D.Ethylene
Correct Answer: Cytokinins
Explanation:Witches' broom involves the release of lateral buds from apical dominance, leading to a proliferation of shoots. This is primarily caused by elevated levels of Cytokinins.
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31Which enzyme breaks down hemicellulose (e.g., xylan) in the plant cell wall?
A.Xylanase
B.Lipase
C.Amylase
D.Cutinase
Correct Answer: Xylanase
Explanation:Xylanase is a hemicellulase that degrades xylan, a major component of hemicellulose in plant cell walls.
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32In the context of bacterial variability, Conjugation requires:
A.A bacteriophage vector
B.Physical contact between two bacteria via a pilus
C.Naked DNA in the solution
D.Formation of spores
Correct Answer: Physical contact between two bacteria via a pilus
Explanation:Conjugation involves the direct transfer of genetic material (usually plasmids) from a donor to a recipient bacterium through a conjugation pilus.
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33Phytotoxins that are not specific to a host and can affect a wide range of plants are termed:
A.Host-selective toxins
B.Non-host-selective toxins
C.Phytoalexins
D.Antibodies
Correct Answer: Non-host-selective toxins
Explanation:Non-host-selective toxins (e.g., Tabtoxin, Tentoxin) produce symptoms on a wide variety of plant species, not just the natural host of the pathogen.
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34Which molecule acts as the 'cement' holding plant cells together and is the first target of enzymatic attack by soft-rot pathogens?
A.Lignin
B.Pectin
C.Suberin
D.Cellulose
Correct Answer: Pectin
Explanation:Pectin makes up the middle lamella, which cements adjacent plant cells together. Pectinases degrade this layer, causing tissue disintegration.
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35Abscisic Acid (ABA) generally promotes disease susceptibility by:
A.Strengthening the cell wall
B.Suppressing defense responses like Salicylic Acid pathways
C.Killing the pathogen directly
D.Increasing photosynthesis
Correct Answer: Suppressing defense responses like Salicylic Acid pathways
Explanation:ABA is often antagonistic to defense hormones like Salicylic Acid (SA) and Jasmonic Acid (JA). By suppressing these pathways, ABA can compromise plant immunity.
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36Tentoxin, produced by Alternaria alternata, induces chlorosis by interfering with:
A.Chloroplast development and energy transfer
B.DNA replication
C.Water uptake
D.Cell division
Correct Answer: Chloroplast development and energy transfer
Explanation:Tentoxin binds to the chloroplast coupling factor 1 (CF1-ATPase), inhibiting photophosphorylation and interfering with chloroplast development, leading to chlorosis.
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37The conversion of indole-3-acetamide (IAM) to indole-3-acetic acid (IAA) in bacteria like Pseudomonas savastanoi is catalyzed by:
A.Tryptophan monooxygenase
B.IAA hydrolase
C.Indoleacetamide hydrolase
D.Pectin lyase
Correct Answer: Indoleacetamide hydrolase
Explanation:The pathway involves Tryptophan IAM (via Tryptophan monooxygenase) IAA. The second step is catalyzed by Indoleacetamide hydrolase.
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38What is the primary role of phospholipases in pathogenesis?
A.Degrading the plant cell membrane
B.Degrading the cell wall
C.Degrading the nucleus
D.Degrading starch reserves
Correct Answer: Degrading the plant cell membrane
Explanation:Phospholipases degrade phospholipids, which are the primary structural components of cell membranes, leading to loss of membrane integrity and leakage of cellular contents.
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39Amylases are enzymes that degrade:
A.Proteins
B.Lipids
C.Starch
D.Nucleic acids
Correct Answer: Starch
Explanation:Amylases degrade starch (the main reserve polysaccharide in plants) into glucose. While useful for pathogen nutrition, they are generally less critical for penetration than cell wall degrading enzymes.
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40Root knot nematodes (Meloidogyne spp.) induce the formation of giant cells. This is an example of pathogen-induced changes in:
A.Toxin levels
B.Growth regulators
C.Lignification
D.Transpiration
Correct Answer: Growth regulators
Explanation:Nematodes secrete effectors that manipulate plant growth regulators (Auxin/Cytokinin balance) to induce the formation of giant nutritive cells (hypertrophy) and galls (hyperplasia).
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41Transduction as a mechanism of variability involves:
A.Direct DNA uptake
B.Transfer of DNA via a bacteriophage (virus)
C.Sexual fusion
D.Mutation by UV light
Correct Answer: Transfer of DNA via a bacteriophage (virus)
Explanation:Transduction is the process where a bacteriophage (virus) transfers genetic material from one bacterium to another.
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42Which of the following is an example of a Hemibiotroph?
A.Phytophthora infestans
B.Tobacco Mosaic Virus
C.Botrytis cinerea (Strict Necrotroph)
D.Powdery Mildew fungi
Correct Answer: Phytophthora infestans
Explanation:Hemibiotrophs (like Phytophthora infestans or Colletotrichum) start their infection cycle as biotrophs (living on live tissue) and later switch to a necrotrophic phase (killing tissue).
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43The 'Epinasty' (downward bending of leaves) symptom seen in some wilt diseases is primarily caused by:
A.Lack of water
B.Excessive Ethylene
C.Excessive Auxin
D.Deficiency of Cytokinin
Correct Answer: Excessive Ethylene
Explanation:While sometimes linked to auxin, Ethylene is the primary hormone that induces epinasty, often produced in response to vascular plugging or toxins in wilt diseases.
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44What is the function of Cutinases during the pre-penetration and penetration phase?
A.It hydrolyzes the lipid polyester cutin to allow hyphal entry.
B.It kills the epidermal cells.
C.It creates a nutrient sink.
D.It masks the pathogen from the host immune system.
Correct Answer: It hydrolyzes the lipid polyester cutin to allow hyphal entry.
Explanation:Cutinase breaks down cutin, the structural polymer of the cuticle, creating a physical opening for the germ tube or infection peg.
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45Which enzyme splits pectin chains by 'elimination' (trans-elimination) rather than hydrolysis?
A.Polygalacturonase
B.Pectin Lyase (PL) / Pectate Lyase
C.Pectin Methyl Esterase
D.Cellulase
Correct Answer: Pectin Lyase (PL) / Pectate Lyase
Explanation:Lyases (like Pectin Lyase or Pectate Lyase) cleave the glycosidic bond through -elimination, resulting in a product with a double bond, unlike hydrolases (PG) which add water.
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46A pathogen strain that differs from others in pathogenicity (ability to infect specific host varieties) is called a:
A.Morphotype
B.Physiologic Race
C.Species
D.Genus
Correct Answer: Physiologic Race
Explanation:A Physiologic Race is a subgroup within a pathogen species that is morphologically similar to other races but differs in its ability to infect specific varieties or cultivars of the host.
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47Which toxin causes 'fire blight' symptoms but is actually a virulence factor rather than a strict phytotoxin?
A.Amylovorin
B.Victorin
C.T-toxin
D.HC-toxin
Correct Answer: Amylovorin
Explanation:Amylovorin is an acidic polysaccharide produced by Erwinia amylovora. It interferes with water transport in xylem, causing wilt, acting as a virulence factor.
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48In fungal pathogenesis, oxalic acid acts synergistically with which enzymes?
A.Proteases
B.Pectinases
C.Lipases
D.Nitrogenases
Correct Answer: Pectinases
Explanation:Oxalic acid (produced by pathogens like Sclerotinia) sequesters Calcium () from the cell wall. This prevents the formation of calcium pectate, making the pectin more accessible to Pectinases.
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49General resistance (Horizontal resistance) is usually effective against:
A.Only one specific race of the pathogen
B.All races of the pathogen
C.Only viruses
D.None of the above
Correct Answer: All races of the pathogen
Explanation:General (Horizontal) resistance is typically polygenic and provides partial resistance against all races of a pathogen, unlike specific (Vertical) resistance which is race-specific.
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50The degradation of the secondary cell wall typically involves the breakdown of:
A.Only Pectin
B.Cellulose, Hemicellulose, and Lignin
C.Starch and Lipid
D.Cutin and Wax
Correct Answer: Cellulose, Hemicellulose, and Lignin
Explanation:Secondary cell walls are lignified and contain cellulose and hemicellulose. Pathogens need enzymes like cellulases, xylanases, and ligninases to degrade them.