(a) Name the organism that causes stalk rot disease in maize. Describe its symptoms, disease cycle, control and management. (20…

(a) Stalk rot of maize: pathogen, symptoms, disease cycle, control and management (20 marks)

Pathogen. Stalk rot is a disease complex. The chief cause is the fungus Fusarium verticillioides (syn. F. moniliforme; teleomorph Gibberella moniliformis), which causes post-flowering stalk rot. Other causes include Gibberella zeae (Fusarium graminearum), Macrophomina phaseolina (charcoal rot), Stenocarpella (Diplodia) maydis, Pythium aphanidermatum (Pythium stalk rot) and the bacterium Dickeya zeae (bacterial stalk rot).

Symptoms:

• They appear after flowering, from tasselling to grain filling: the plants suddenly wilt and dry, and the leaves turn dull grey-green.
• The lower internodes turn soft and brown, and the pith is discoloured pink to reddish and breaks down, leaving shredded vascular strands, so the stalk is easily pinched and lodges at the nodes.
• White to pink mycelium may be seen at the nodes; the ears are small and light with chaffy grain.
• In charcoal rot, tiny black sclerotia are seen in the pith and beneath the rind. In bacterial stalk rot, the base is water-soaked with a foul smell, and the top may fall. In Pythium stalk rot, the basal internodes twist and the plant falls.

Disease cycle. The fungus survives in crop residues and in the soil as mycelium and spores, and in the seed. Infection occurs through the roots and through wounds made by insects such as the stem borer (Chilo partellus). Stress after silking (drought followed by rain, water logging), excess nitrogen with low potassium, a dense stand and cool, wet weather all favour it. Conidia from the residues spread by wind, rain splash, insects and seed give secondary spread, and the fungus colonises the pith and stops the transport of water and food to the ear.

Control and management:

• Grow resistant or tolerant hybrids and use healthy seed.
• Rotate with non-hosts (legumes) for two to three years, and remove and destroy crop residues by deep ploughing.
• Give balanced fertiliser with adequate potassium, avoid excess nitrogen, keep the recommended plant density, and provide drainage and light irrigation at flowering to avoid water stress.
• Treat the seed with carbendazim or thiram (2 to 2.5 g/kg) or with Trichoderma viride.
• Control stem borer to prevent wounds.
• Drench the base with carbendazim for the fungal rots and with bleaching powder (about 100 ppm chlorine) for the bacterial rot; harvest promptly to limit lodging.

(b) Mycoplasmas: what they are, difference from spheroplast and protoplast, salient features (15 marks)

What they are (5 marks). Mycoplasmas are the smallest free-living prokaryotes, belonging to the class Mollicutes ("soft skin"). They have no cell wall and are bounded only by a unit membrane, which contains sterols in Mycoplasma. They are pleomorphic (0.1 to 0.3 µm), pass through bacterial filters and have a very small genome (about 0.6 to 1.4 Mb). Those that infect plants are called phytoplasmas and cannot be cultured; they cause yellows, little leaf, witches' broom and grassy shoot of sugarcane, and are spread by leafhoppers and planthoppers.

Difference from spheroplast and protoplast (5 marks). A protoplast is a cell whose wall has been completely removed, as by lysozyme in Gram-positive bacteria, and a spheroplast is a Gram-negative cell that retains parts of the wall (outer membrane). Both are artificial products, osmotically fragile, and need an isotonic or hypertonic medium; they can regenerate the wall and revert to normal cells. Mycoplasmas are naturally wall-less, permanently so, because they lack the genes for wall synthesis, and never revert.

Salient features (5 marks):

• Reproduction by binary fission, budding and fragmentation.
• Ribosomes 70S, DNA and RNA both present, and low G+C content.
• Cultivable ones (Mycoplasma) form small "fried-egg" colonies on serum-rich media.
• Insensitive to penicillin and other wall-active agents but sensitive to tetracycline.
• Cause plant yellows and animal diseases such as atypical pneumonia (Mycoplasma pneumoniae) and contagious bovine pleuropneumonia.

(c) Geological time scale and plant life in each era (15 marks)

Geological time scale (7 marks). It is the chronological scheme for the 4.6 billion years of Earth's history, set by fossils, rock strata and radiometric dating. It is divided into eons, eras, periods, epochs and ages. The Precambrian (about 4,600 to 539 million years ago) covers most of Earth's history. The Phanerozoic eon (539 million years ago to now) has three eras: the Palaeozoic (539 to 252 million years ago; Cambrian, Ordovician, Silurian, Devonian, Carboniferous, Permian), the Mesozoic (252 to 66 million years ago; Triassic, Jurassic, Cretaceous) and the Cenozoic (66 million years ago to present; Palaeogene, Neogene, Quaternary). Boundaries mark mass extinctions and major changes in life.

Plant life (8 marks):

• Precambrian: bacteria and cyanobacteria (stromatolites, about 3.5 billion years old), then algae.
• Palaeozoic: marine algae; the first land plants in the Ordovician and Silurian (Cooksonia); the Devonian brought psilophytes (Rhynia), lycopods, horsetails and the first forests and seed plants; the Carboniferous coal forests of Lepidodendron, Sigillaria, Calamites and seed ferns; in the Permian, conifers and the Glossopteris flora of Gondwana, source of Indian coal.
• Mesozoic: the age of gymnosperms, with cycads, cycadeoids (Williamsonia of the Rajmahal Hills), conifers and ginkgos; the first angiosperms appeared in the early Cretaceous and spread quickly.
• Cenozoic: the age of angiosperms, with forests, and grasslands spreading in the Miocene, so that the modern flora had formed by the Pleistocene, when it was reshaped by the ice ages.