Plant pathogens

are the expression of disease by a plant as a response to a pathogen.

are structures or products of a pathogen on or in diseased plants.

derives its food from a living host

causes disease in a living host

• • infectious
• • transmissible

• • noninfectious
• • nontransmissible

• 1. The suspected pathogen must be consistently associated with diseased plants.

• 2. The suspected pathogen must be isolated in a pure culture and its characteristics noted.
• 3. The disease must be reproduced in a healthy plant inoculated with the isolated organism.
• 4. The same pathogen characterized in step 2 must be isolated from the inoculated plant.

Identify the species and cultivar of the plant, when possible. Determine what a healthy specimen should look like, so that you can recognize abnormalities of the plant in question. Carefully observe aboveground symptoms and also belowground symptoms, if necessary. In addition to observing symptoms expressed by a single plant, look for disease patterns in the plant population. Note the apparent host range (i.e., what plant species are affected).

Look for signs, evidence of a pathogen or its parts, both above ground and below ground, if necessary. A hand lens is often helpful, but laboratory evaluation is commonly necessary to check for signs. Signs are often more obvious under moist conditions (for example, before dew has dried in the early morning or after rain or irrigation). Abiotic factors produce no signs, and there may be no visible signs of some biotic pathogens, such as viruses. Abiotic factors may affect nearly every plant species in an area to some degree, but biotic pathogens tend to be restricted to a single species or members of the same family of plants. Some signs may be evidence of secondary invaders, which are not the initial cause of the disease.

Disease will develop in susceptible plants in the presence of a pathogen only if the environment is favorable. Several environmental factors are important, including natural factors and those imposed by human activities. Determine the recent conditions for plant growth, including chemical and fertilizer applications, characteristics of the soil or growing medium, and environmental factors, such as temperature and water supply. Look for factors that commonly affect disease development, such as the timing and amount of irrigation or rainfall, air movement, and the nutrient status of the plant.

live in or on another living organism and obtain nutrients (food) from it.

obtain nutrients from dead organic matter.

• (obligate parasites)

• have narrow host ranges
• cannot grow as saprophytes
• attack healthy host tissue at
• any stage
• kill host cells slowly
• penetrate directly or
• via natural openings

• (facultative parasites/facultative saprophytes)

• have wide host ranges
• can grow as saprophytes
• attack young, weak, or senescent tissues
• kill host cells rapidly by producing toxins or enzymes
• penetrate through wounds or natural openings

• nematode diseases
• • phytoplasma diseases
• • virus diseases
• • downy mildews
• • powdery mildews
• • rusts

• • anthracnoses
• • cankers
• • fruit rots
• • leaf spots and blights
• • root rots
• • vascular wilt

• Dutch elm disease
• Stinking smut
• Verticillium wilt

• Apple scab
• Coffee rust
• Late blight of potato and tomato

• plant development and the ability of the plant to mount defenses against invasion,
• dispersal of inoculum, both primary and secondary,
• the ability of the parasite to penetrate the plant, and
• the survival of the parasite in the absence of the host plant

allows parasites to move to infection courts and penetrate host tissue. Fungal spores landing on a plant surface commonly require water for germination and the production of infection structures. Zoospores are motile and can swim to plants or on plant surfaces. Nematodes can move toward plants and on plant surfaces in a thin film of water. Bacteria require water for multiplication and infection. Seeds of parasitic plants require water for germination.

• Focused on the plant
• protection

• Focused on the pathogen
• avoidance
• exclusion
• eradication

• Focused on the environment
• protection

hyphae, which form mycelia

predominantly chitin with glucan (a complex sugar)

glycogen

eukaryotic (with haploid nuclei)

heterotrophic, nutrients absorbed through cell walls

spores (sexual and asexual)

• ascomycetes
• basidiomycetes
• zygomycetes

septate hyphae

septate hyphae; some but not all species have clamp connections

nonseptate (coenocytic, aseptate) hyphae

• a genus name (capitalized) and a species name, or specific epithet (not capitalized).
• Example Armillaria gallica

• the genus can then be abbreviated to its first letter if the name is used again.
• Example A. gallica

Example Armillaria sp.

• is a plural form that refers to more than one species belonging to that genus.
• Example Armillaria spp.

• capitalized but not italicized.
• Example Armillaria root disease

• perfect stage
• • sexual spores only

• • imperfect stage
• • asexual spores only

• spores in all stages, sexual and asexual

• septate mycelium
• sexual spores: ascospores in an ascus (sac)
• asexual spores: conidia

• septate hyphae; some species have clamp connections at some septa
• sexual spores: basidiospores, generally produced in a group of four external to the basidium (the cell that produces them)
• asexual spores: vary considerably within this group

• nonseptate hyphae
• sexual spores: large, dark zygospores
• asexual spores: sporangiospores, produced in a sporangium

• nonseptate hyphae
• cell wall containing cellulose (no chitin)
• diploid nuclei
• sexual spores: oospores, produced after contact between an oogonium and an antheridium
• asexual spores: zoospores, with two flagella, produced in a sporangium

• Choose planting sites and planting times to avoid environmental
• conditions favoring disease.

• Impose quarantines (local and international).
• Plant pathogen-free seed or stock.

• Rotate to nonhost crop.
• Remove weed hosts.
• Destroy infested plant debris.
• Apply fungicides.

• Apply fungicides.
• Minimize leaf wetness.
• Plant resistant hosts.

• • are haploid
• • infect first host species to form pycnia

• • combine with receptive hyphae in pycnia
• • result in formation of dikaryotic mycelium (plasmogamy)

• • are dikaryotic spores produced in an aecium
• • infect alternate host

• • are dikaryotic spores produced in uredinia
• • infect the host species on which they are produced (repeating stage)

• • are diploid spores produced in telia (dikaryotic, becoming diploid)
• • produce haploid basidiospores (after karyogamy and meiosis)
• • serve as the survival stage

a single host species to complete their life cycle (similar to most fungal pathogens).

two unrelated plant species to their complete life cycle.

• Nonseptate hyphae
• Sexual spores: oospores; paragynous antheridia, often more than one antheridium per oogonium
• Asexual spores: zoospores released from vesicles produced by lumpy to round sporangia
• Diseases: seedling damping-off; stem, crown, and root rots; diseases of lower tissues reached by water splashing up from the soil (the "splash zone")

• Nonseptate hyphae
• Sexual spores: oospores; paragynous or amphigynous antheridium; usually only one antheridium per oogonium
• Asexual spores: zoospores produced by typically lemon-shaped sporangia
• Diseases: stem, crown, and root rots; diseases of lower tissues reached by water splashing up from the soil (the "splash zone").
• Phytophthora infestans causes late blight of potato and tomato, the disease that led to the Irish potato famine in the 1840s and the beginning of the science of plant pathology.
• Phytophthora ramorum causes the devastating ramorum blight (sudden oak death) on the U.S. west coast and was probably introduced into the United States on rhododendron nursery stock.
• Phytophthora cinnamomi is a widespread, destructive root and crown pathogen throughout the tropical world.

• Plasmopara, Peronospora, Pseudoperonospora, Bremia, and other genera

• Nonseptate hyphae
• Sexual spores: oospores, produced in infected aboveground plant tissue
• Asexual spores: sporangia, formed on treelike sporangiophores, which typically emerge through stomata on the lower leaf surface; some sporangia produce zoospores, and others germinate directly
• Diseases: blue mold of tobacco; crazy top of corn; yellow tuft of turfgrasses; downy mildews of brassicas, curcurbits, grape, hops, onion, snapdragon, and other plants

acervulus

amphigynous

apothecium

cleistothecium

paragynous

perithecium

pycnidium

sporodochium

synnema

Hypha with a septum and a clamp connection (arrow).

Septate (A) and nonseptate hyphae (B). Note the cross-walls in the septate hypha.

Fruiting body (conk) of a wood decay fungus, a sign of the pathogen. Ganoderma applanatum.

Examples of asci and ascospores. The shape and size of the ascus and ascospores vary greatly.

Reproduction of a basidiomycete. A, Fruiting body. B, The lower surface of a fruiting body, containing numerous pores. C, Formation of basidiospores following meiosis. D, Basidia and basidiospores emerging from pores in a fruiting body.

Corn smut, caused by Ustilago maydis, a basidiomycete.

Clamp connection and dikaryotic hypha formed during mitosis.

Conidiomata. A, Sporodochium. (Myrothecium sp.) B, Synnema. (Didymostilbe sp.) C, Acervulus. (Marsonina sp.) D, Pycnidium. (Phyllostica sp.). Note that the conidia are not in asci.

Ascomata with bitunicate asci. A, Ascostroma with multiple locules filled with asci. (Apiosporina morbosa) B, Pseudothecium. Note the double wall of the bitunicate asci.

Generalized process of ascus formation. A, Male structure (often an antheridium) and female structure (ascogonium). B, Plasmogamy: the male structure contributes nuclei to the female structure. C, A limited dikaryotic mycelium develops. D–F, Through a complex mechanism, the ascus mother cell forms. G, Karyogamy: the two nuclei fuse to form a single diploid nucleus. H, Meiosis produces four haploid nuclei. I, In many ascomycetes, mitosis produces eight haploid nuclei and the ascus elongates. J, A spore wall forms around each nucleus to produce eight ascospores.

• aerobes, peritrichous flagella, abundant EPS
• Example: Agrobacterium tumefaciens

• facultative anaerobes, peritrichous flagella; pectolytic enzymes (some species)
• Examples: Erwinia amylovora (no pectolytic enzymes), Pectobacterium carotovorum (pectolytic enzymes)

• aerobes, polar flagella; some fluoresce under ultraviolet light and chelate (remove) iron from their environment
• Example: Pseudomonas syringae (many pathovars)

• aerobes, one polar flagellum, yellow colonies; source of "xanthan gums"
• Examples: Xanthomonas axonopodis pv. citri, X. axonopodis pv. vesicatoria

• aerobes, irregularly shaped rods, nonmotile
• Examples: Clavibacter michiganensis subsp. michiganensis, C. michiganensis subsp. sepedonicus

• aerobes, branched filaments, spore-forming (actinomycete)
• Example: Streptomyces scabies

• aerobes, pleomorphic
• Example: aster yellows phytoplasma

• aerobes, helical shape
• Example: Spiroplasma citri

• Fastidious Vascular-Colonizing Plant-Pathogenic Bacteria
• • Symptoms: water-stress symptoms including wilt, stunting, scorch on leaves

• • Pathogen: Xylella fastidiosa
• • Shape: bacilliform (rod-shaped)
• • Vectors: insects that feed in xylem, e.g., sharpshooters and spittlebugs
• • Example diseases: bacterial leaf scorch of shade trees, citrus variegated chlorosis, Pierce's disease of grapevine

• Symptoms: yellowing, stunting, witches' brooms

• Fastidious Vascular-Colonizing Plant-Pathogenic Bacteria

• Phytoplasmas

• Spiroplasmas

• Phloem-colonizing walled bacteria

s the most common xylem-limited pathogen.

• Impose quarantines (local and international).

• Grow plants in dry environments.
• Plant pathogen-free seed or stock.

• Rotate to nonhosts.
• Rogue weed hosts.
• Destroy infested plant debris.
• Use heat treatment of seeds or propagative material.
• Apply antibiotics.
• Apply insecticides (to kill infective vectors).

• Apply copper chemicals.
• Apply antibiotics.
• Apply bacterial antagonists.
• Minimize leaf wetness.
• Plant resistant hosts.

fusion of cells , bringing two sexually compatible haploid nuclei together in one cell

nuclear fusion forming a diploid nucleus

formation of four genetically recombined haploid nulei from a single diploid nucleus

conidium (conidia, pl)

a divider hyphae with crossed walls

the sexual stage of fungi

asexual stage of fungi

ascospores, ascus

asexual spores often produced by anamorphs

sexual fruiting body

sexual fruiting body of basidiomycete

asexual spores of rust

dust like spores of smut

oospores

small asexual swimming (flegella) spores of chytriomycota and oomycete produced in sporangium

sexual spores of zygomycota

asexual spores of zygomycota

aeciospores - dikaryotic

urediniospores -dikaryotic (repeating stage)

teliospores - diploid (karyogamy meiosis)

basidiospores -haploid

pycniospores + receptive hyphae- dikaryotic (plastogamy)