(a) Discuss about the names of causal organisms, symptoms, disease cycle and management of loose smut of wheat. What are the…

(a) Loose Smut of Wheat; Rust versus Smut Fungi Causal organism: Ustilago tritici (U. nuda f. sp. tritici), a basidiomycete (order Ustilaginales).

Symptoms: the disease shows at ear emergence. The whole inflorescence, including glumes and grain, is replaced by a black powdery mass of teliospores (chlamydospores), covered at first by a thin silvery membrane. The membrane ruptures, the wind scatters the spores, and only the bare rachis remains. Smutted ears emerge a little earlier than healthy ones. Yield loss is usually a few per cent but can reach 10% or more.

Disease cycle: the disease is seed-borne and systemic. Wind-carried teliospores land on open flowers at flowering time and germinate to form a promycelium (basidium) bearing basidiospores (sporidia). Compatible hyphae fuse to form dikaryotic mycelium that enters the ovary, and the fungus lies dormant in the embryo of the infected seed, which looks normal. When the seed is sown, the mycelium grows along with the shoot and finally replaces the ear. No alternate host exists.

Management: use clean, certified seed; solar treatment (soaking seed in water for 4 hours in morning in summer and drying in sun, Luthra's method) or hot water treatment (about 52°C for 10 minutes after pre-soaking); seed treatment with systemic fungicides (carboxin, tebuconazole); resistant varieties; and rogueing of smutted ears before spore release.

Rust versus smut fungi:

• Rusts (Pucciniales, e.g., Puccinia graminis tritici) are obligate parasites with up to five spore stages (pycniospores, aeciospores, urediniospores, teliospores, basidiospores) and often two hosts; smuts are facultative parasites, with only teliospores and basidiospores, and one host, and they can be grown on culture media.
• Rust forms reddish-brown or orange pustules on leaves and stems; smut forms black spore masses, mostly in the ear.
• Rust teliospores are two-celled and stalked; smut teliospores are single-celled and sessile.
• Rust spreads by wind through uredospores in the crop season; loose smut infects through flowers.

(b) Fossil Gymnosperms: Bennettitales and Cordaitales Fossil gymnosperms are broadly grouped into seed ferns (Pteridospermales, e.g., Lyginopteris, Medullosa), Cordaitales, Bennettitales (Cycadeoidales), Glossopteridales and fossil conifers and cycads. Common features: secondary wood, naked seeds, pollen grains, and in many cases fern-like leaves.

Cordaitales (Carboniferous-Permian): tall, branched trees up to 30 m, with a pith with discoid partitions (Artisia), dense pycnoxylic wood (Dadoxylon), and large strap-shaped leaves with parallel venation (Cordaites), up to 1 m long. Reproduction: compound, lax cones, unisexual (separate male and female cones), with ovules in short fertile shoots and platyspermic seeds (Cardiocarpus). They are considered ancestral to conifers.

Bennettitales (Triassic to Cretaceous): short, stout, cycad-like plants (Cycadeoidea) or slender branched forms (Williamsonia), with pinnate leaves (Ptilophyllum, Otozamites) and syndetocheilic (paracytic) stomata, unlike the haplocheilic stomata of cycads. The strobilus was flower-like and bisexual, with microsporophylls around the ovule-bearing receptacle with ovules and interseminal scales. The Jurassic Rajmahal Hills of India are rich in Bennettitales (Williamsonia sewardiana, studied by Birbal Sahni).

Significance: they show gymnosperm evolution and have been compared with the origin of flowers, although they are not ancestors of angiosperms.

(c) Bryophytes: Characteristics and Comparison with Pteridophytes General characteristics: bryophytes are small, non-vascular, green plants of moist habitats. The dominant body is the thalloid or leafy gametophyte, with rhizoids for anchorage and no true roots, stem or leaves. Sex organs are multicellular, antheridia and archegonia, with a jacket. Water is needed for fertilisation. The zygote becomes an embryo, and the sporophyte (foot, seta, capsule) is dependent on the gametophyte.

Resemblances with pteridophytes: heteromorphic alternation of generations; archegonia and antheridia; multicellular jacketed sex organs; spores formed in sporangia; motile, flagellate sperm needing water for fertilisation; and embryo formation.

Differences: the gametophyte is dominant in bryophytes and the sporophyte in pteridophytes; the bryophyte sporophyte is dependent and the pteridophyte one independent; bryophytes lack xylem and phloem and have rhizoids, while pteridophytes have vascular tissue with true roots, stems and leaves; bryophyte sporophytes are unbranched with a single sporangium, while pteridophytes are branched with many.

Examples: Marchantia, Funaria; ferns.