Practical 5: Higher Fungi-Taxonomy and Pathological Importance (Phylum Ascomycota)

The phylum Ascomycota are well-developed fungi. The Hemiascomycetes comprising the yeasts are unicellular, but all other classes produced hyphae that are septate. Most are identified from the type of conidium (asexual spore) that are commonly produced. They may be openly attached on conidiosphores, or are contained within fruiting structures such as pycnidium (closed) or the acervulus. Where sexual reproduction occurs, several types of fruiting body (called the ascocarps) are formed, that may be closed and rounded (cleisthothecium), closed with ostiole (perithecium), saucer shaped (apothecium) or cushioned shaped with locules (ascostroma). The ascocarps contain ascus bearing the ascospores (sexual spores) that are usually 8 in number. Order argonomycetales in the class Deuteromycetes consists of fungi bearing neither asexual nor sexual spores. The hyphae are sterile but are known to produce sclerotium ( small and roundish hyphal aggregations)

Fungal Culture

A pure culture of Penicillium sp, Chaetomium sp, Fusarium sp and Curvularia sp are provided. Make temporary slides and examine for the presence of asexual stage e.g conidium or sexual stage.

Plant Disease Specimens

        I.            Powdery mildew of rubber

Caused by Onidium heveae. Infects young leaves of rubber causing massive leaf fall. Fungus also lives on mature leaves throughout the year.

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     II.            South American Leaf Blight (SALB) of Rubber.

Caused by Mycrocylus ulei. This disease occurs only in South America and is unknown in rubber grown in Asia (Malaysia, Thailand and Indonesia) and therefore, of quarantine importance. Study the pictures of the disease symptoms observed on rubber in Brazil.

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  III.            Sooty mould disease.

Caused by more than one fungi: Tripospermum sp., Caonodium sp., Limancinula sp. Infect various crops such as fruit trees ( mango, citrus, rambutan, etc). Symptoms mainly of blackish layer of fungal growths on leaves and other plants parts. Serious infections will affect photosynthesis in plants. The fungi grow on honeydew (secretions) produced by insects such as aphids, mealy bugs or thrips.

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  IV.            Black mildew

Caused by  Meliola sp. Similar with sooty mould disease but the fungus forms scattered, blackish growths instead of layers mainly on leaves of mango and other fruit trees.

    V.            Shealth blight of paddy

Caused by Rhizoctonia solani. Cause blackish lesions on leaf sheaths and stems. Fungus forms sclerotia.

  VI.            Foot rot

Caused by Sclerotium rolfsii. Infects many vegetables crops such as brinjal, chilli, tomato, and groundnuts.  Causes rots at the base of stems identified by the whitish growths (Rhizmoprhs) of the fungus. Fungus forms sclerotia.

VII.            Bird eye spot of rubber

Caused by Drechslera heveae. Symptoms on leaves as suggested by name of disease.

VIII.            Panama disease or banana wilt

Caused by  Fusarium oxyporum f.sp. cubensis. Major disease of banana world wide causing wilt and death of plants.

Slides

        I.            Saccharomyces cerevisiae

Unicellilar, yeast cells in Class Hemiascomycetes.

     II.            Penicillium sp, Aspergillus sp.

The different forms of conidia formed by both species. Generally, Penicillium forms phiallides which are brushlike while conidia are produced on sterigmata on conidial heads in Aspergillus.

  III.            Erysiphe cichorachearum.

Infects cucuibits e.g cucumber. The conidium (odium) formed in chains on erect conidiospores (asexual stage). The sexual stage produce cleistothecium(closed ascocarps bearing the ascus and ascospores)

  IV.            Morchella sp.

Example of class Discomycetes that produce apothecium.

    V.            Class Deuteromycetes

Order Moniliales:

Fungus based on different shapes of conidia of different genus such as Alternaria, Culvularia, Cercospora, Drechslera, Fusarium sp. etc.

  VI.            Order Melanconiales:

Conidia formed within an acervulus.

Example: Collectotrichum capsica (sickle shaped conidia)

Collectotrichum gloeosporioides (rod shaped conidia). Presence of setae( black, sterile hairs). The genus  Pestalotiopsis is also in this group.

VII.            Order Sphaeropsidales:

Conidia formed within a pycnidium.

Example: Phoma, Botryodiplodia sp.

Disease Cycle

The diseases cycle of various Ascomycetes as shown in figure.

        I.            Mycosphaerella musicola (Fig. 11-47)

     II.            Bipolaris (Helminthosporium) maydis (Fig. 11-51)

  III.            Rhizoctonia solani (Fig. 11-104)

  IV.            Sphaerotheca pannosa (Fig. 33)

    V.            Fusarium oxysporium f.sp. lycopersici (Fig. 45)