Fungal classification and life cycles
The main groups of plant-pathogenic fungi and fungus-like organisms, their spores and fruiting bodies, sexual and asexual cycles, and a spore count sum.
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Fungi and fungus-like organisms cause most of the world's serious crop diseases: rice blast, wheat rusts, potato late blight, Fusarium wilts and head blight, powdery and downy mildews, and many more. Knowing which group a pathogen belongs to tells a plant pathologist how it spreads, how it survives between seasons, which fungicides will work, and how quickly it may overcome resistant varieties. That is why fungal classification and life cycles are a core part of every plant pathology course.
What fungi are
Fungi are eukaryotic organisms that absorb food from their surroundings. Most grow as threads called hyphae, which together form a mycelium; their walls contain chitin. They reproduce by spores, which may be formed asexually or sexually. Fungi are more closely related to animals than to plants.
Several important plant pathogens look like fungi but are not:
- Oomycetes (water moulds), such as Phytophthora, Pythium, downy mildews and white rusts, belong to the Stramenopiles, related to brown algae and diatoms. Their walls are mainly cellulose, their mycelium is diploid, and many make swimming zoospores. This is why some fungicides that control true fungi do not control oomycetes, and vice versa.
- Plasmodiophorids, such as Plasmodiophora brassicae (clubroot of brassicas) and Spongospora (powdery scab of potato), are obligate parasites in the Rhizaria.
The main groups
| Group | Key features | Important plant pathogens |
|---|---|---|
| Chytridiomycota | simple, zoospores with one flagellum | Synchytrium endobioticum (potato wart) |
| Mucoromycota (formerly Zygomycota) | non-septate hyphae; sexual zygospores | Rhizopus (soft rots of fruit and stored produce) |
| Glomeromycota | arbuscular mycorrhizal fungi, beneficial | none; partners of most crop roots |
| Ascomycota (sac fungi) | septate hyphae; sexual ascospores in sacs called asci | powdery mildews, Pyricularia oryzae (rice blast), Fusarium, Botrytis, Venturia (apple scab), Claviceps (ergot), Alternaria, Colletotrichum |
| Basidiomycota | septate hyphae with clamp connections; sexual basidiospores on basidia | rusts (Puccinia), smuts (Ustilago), Rhizoctonia solani, wood-rotting and root-rot fungi such as Armillaria and Ganoderma |
| Oomycota (not true fungi) | diploid, cellulose walls; oospores and zoospores | Phytophthora infestans (late blight), Pythium (damping-off), Plasmopara viticola (grape downy mildew), Albugo (white rust) |
Many fungi were long known only from their asexual state and were grouped as Deuteromycota or "Fungi Imperfecti". DNA sequencing has placed almost all of them in the Ascomycota or Basidiomycota. Since 2013, the rule of "one fungus, one name" has meant that a fungus with separate names for its sexual and asexual forms (such as Gibberella and Fusarium) should have a single name.
Spores and fruiting bodies
Asexual spores
- Conidia: borne on hyphae called conidiophores, sometimes inside fruiting bodies: pycnidia (flask-shaped), acervuli (cushion-like, as in Colletotrichum anthracnose), or sporodochia.
- Sporangiospores: formed inside a sporangium, as in Rhizopus.
- Zoospores: swimming spores of oomycetes and chytrids, spread by water.
- Chlamydospores: thick-walled resting spores, as in Fusarium oxysporum, that survive years in soil.
- Sclerotia: hard, compact masses of hyphae that survive adverse conditions, as in Sclerotinia and Sclerotium rolfsii.
Sexual spores and structures
- Zygospores (Mucoromycota).
- Ascospores in asci, usually eight per ascus, often inside fruiting bodies: chasmothecia (formerly cleistothecia, closed, in powdery mildews), perithecia (flask-shaped, as in Fusarium and Claviceps), apothecia (cup-shaped, as in Sclerotinia) and pseudothecia (as in Venturia).
- Basidiospores on basidia, usually four per basidium; in rusts and smuts they form after the thick-walled teliospores germinate.
- Oospores (Oomycota), from the fusion of an antheridium and an oogonium.
The life cycle
Most plant-pathogenic fungi have two cycles:
- The asexual cycle, repeated many times in a season, produces huge numbers of genetically identical spores. Diseases with many asexual cycles a season, such as rice blast, late blight, rusts and powdery mildews, are polycyclic and can build up explosively.
- The sexual cycle, often once a year, goes through three steps: plasmogamy (fusion of cytoplasm of compatible hyphae), karyogamy (fusion of nuclei), and meiosis. In Ascomycota, the dikaryotic stage is short; in Basidiomycota, especially rusts and smuts, the dikaryon (two separate nuclei per cell) is the main stage of the life cycle.
Sexual reproduction matters for disease control because it creates new genetic combinations, so pathogens can adapt faster to resistant varieties and fungicides. It also often produces the survival stage: ascospores of apple scab overwinter in fallen leaves; oospores of Phytophthora infestans, formed where both mating types (A1 and A2) meet, can survive in soil for years.
Fungi may be homothallic (self-fertile) or heterothallic (needing a compatible partner of another mating type).
Survival and spread
- Survival: in infected crop residues, seed, soil (chlamydospores, sclerotia, oospores), alternate or weed hosts, and perennial hosts.
- Spread: wind (rust urediniospores can travel thousands of kilometres), rain splash, irrigation water, soil and machinery, insects, infected seed and planting material.
Making a spore suspension for inoculation
A pathologist needs 50 mL of a Fusarium spore suspension at 1 × 106 spores per mL. She counts spores in a haemocytometer, whose small counting squares are 0.2 mm × 0.2 mm with a depth of 0.1 mm.
- Volume of one square = 0.2 × 0.2 × 0.1 = 0.004 mm3 = 4 × 10−6 mL.
- She counts 120 spores in 5 squares: an average of 24 per square.
- Concentration of the stock = 24 ÷ (4 × 10−6) = 6 × 106 spores per mL.
- To dilute: stock volume needed = 50 mL × (1 × 106) ÷ (6 × 106) ≈ 8.3 mL, made up to 50 mL with 41.7 mL of sterile water.
Key points
- Fungi are chitin-walled, absorptive eukaryotes growing as hyphae; oomycetes and plasmodiophorids are fungus-like but unrelated.
- Main groups: Chytridiomycota, Mucoromycota (zygospores), Glomeromycota, Ascomycota (ascospores in asci), Basidiomycota (basidiospores on basidia); Oomycota (oospores, zoospores).
- Asexual spores (conidia, sporangiospores, zoospores, chlamydospores) spread disease; sexual spores create variation and often survive between seasons.
- The sexual cycle runs plasmogamy → karyogamy → meiosis; the dikaryon dominates in rusts and smuts.
- "One fungus, one name" replaced dual names for sexual and asexual forms.
What the exams ask
- School-leaving and entrance papers: what fungi are, examples of fungal diseases, and differences between fungi and oomycetes.
- Degree and research-fellowship entrances: classification with examples, spore types and fruiting bodies, life cycles, homothallism and heterothallism, survival structures, and the significance of the sexual cycle.
- Professional examinations: linking pathogen biology to fungicide choice, resistance breeding and quarantine.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Why does it matter whether a pathogen is an oomycete?
Oomycetes differ in biochemistry from true fungi, so some fungicide groups work on one and not the other. Metalaxyl and related products, for example, act on oomycetes, not true fungi.
Do all fungi have a sexual stage?
Many have one, but some appear to reproduce only asexually, or their sexual stage is rare or unknown. They still generate some variation through mutation and other mechanisms.
Why do some fungal diseases come back every year in the same field?
Their survival structures, such as sclerotia, chlamydospores and oospores, persist in soil or residues for years, ready to infect the next susceptible crop.
Sources
- Agrios, G. N. Plant Pathology, 5th edition. Elsevier Academic Press, 2005. Chapter 11.
- Webster, J. and Weber, R. W. S. Introduction to Fungi, 3rd edition. Cambridge University Press, 2007.
- Hibbett, D. S. et al. "A higher-level phylogenetic classification of the Fungi." Mycological Research 111, 509–547 (2007).
- Taylor, J. W. "One Fungus = One Name: DNA and fungal nomenclature twenty years after PCR." IMA Fungus 2, 113–120 (2011).
- Dean, R. et al. "The Top 10 fungal pathogens in molecular plant pathology." Molecular Plant Pathology 13, 414–430 (2012).
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